cm. The average length of 12 leaves from a single branch 7 inches

in length was found to be 2.82 cm.; the average breadth was 0.35 cm. On another branch, 23 cm. long, were 35 mature leaves which averaged almost exactly the size of those just given. Since these leaves are rather numerous and of good size, it will appear at once that the species has a relatively large leaf-surface, even if the area can not be stated more definitely. (Fig. 30.)

The evergreen shrub _Capparis spinosa_ has the largest leaves of any plant native to the Ghardaia region, and probably of southern Algeria. Its leaves are bilateral and nearly round and are placed upright on the branches; a series of 29 leaves from one branch 33 cm. long varied from 2 to 3.2 cm. in length and slightly less in width, the average being, length 2.9 cm., breadth 2.2 cm.

_Euphorbia guyoniana_ has a deciduous leaf-habit. Its leaves are rather small but numerous. A shoot 22 cm. long bore 40 leaves varying in length from 1.5 to 2.1 cm. and in breadth from 1.2 to 2.0 mm., with the average length and average breadth 1.7 cm. and 1.6 mm. respectively.

It is worth noting that the surface of the leaves, as shown by inspection, is, possibly, most often smooth, as, for example, in _Capparis_, _Henophyton_, and _Haloxylon_. Dense hairiness is an exception among the species seen, _Lithospermum callosum_ being the only plant striking in this regard.

GROWTH AND FLOWERING HABITS IN THE GHARDAIA REGION.

Upon arriving at Ghardaia in November, it was surprising to find several species of perennials, under strictly desert conditions, putting on new growth, forming fresh leaves, or coming into flower. Especially was this unexpected in view of the fact that no rain had fallen in the region for twelve months. A study of the roots in relation to the depth to water showed also that most of the native plants, during the dry season, could not have penetrated to a depth anywhere near that of the water-table of the valley, and the water relations of the plants growing on the plain above the M’Zab Valley were even more severe. Further, most of the species do not possess water-storage faculty. It should be noted that the only apparent difference in the water relation between November and in early autumn, or summer, was that of raising the relative humidity of the air through the lower temperature. In a preceding section it has been stated that little or no evaporation took place at night in November at Ghardaia, and possibly less in the daytime than would have been supposed. The leading environmental changes were, of course, the lower daily temperature and the really cool nights.

It is not uncommon in the Arizona desert for a species to form flowers or leaves, following a change in temperature, the moisture conditions being not otherwise changed, but, so far as I know, the temperature changes bringing about this result are always from a cooler to a warmer condition and not the reverse. It would not be expected, consequently, that in the present instance a renewal of vegetative activities would follow as a _direct_ result of a lower temperature, although analogous changes are necessary before certain species, after rest, will start development. Whatever may be the immediate cause of the renewal of activity on the part of the plants at Ghardaia as noted, the following species were seen to have formed new leaves: _Henophyton deserti_ and _Zilla macroptera_; also the following fresh flowers with or without shoot growth: _Fagonia bruguieri_, _Haloxylon articulatum_, _Henophyton deserti_, _Ononis polyclada_, and _Zollikoferia resedifolia_, and three other species not determined.

=GHARDAIA TO TOUGGOURT.=

From Ghardaia the route, consisting of camel trails only, pursued a course south of east to Ouargla, and from Ouargla a direction east of north to Touggourt, over 400 kilometers, Ouargla being about half-way. There are no villages between Ghardaia and Ouargla and none between this place and a point 20 kilometers south of Touggourt, so that in accounts of the vegetation or the topography, lacking convenient points around which to center descriptions, the device will be adopted of using distance estimations to or from the three chief towns.

The trail followed the valley of the Oued M’Zab, or kept close to it, for 63 to 73 kilometers before finally leaving it. It passed the sister towns of Ghardaia and crossed short intervals of plain, descending occasionally to the oued. The walls of the M’Zab Valley, 60 meters more or less at Ghardaia, become lower and less precipitous as one goes down the drainage, until at length they become little more than rounded banks. The low, flat-topped mountains, which are a feature of the topography about Ghardaia, were soon left behind, and nothing similar was encountered until the vicinity of Ouargla was reached. Between Ghardaia and El Ateuf are small dunes in the valley and sand is drifted along the base of the walls at various places. Near and immediately east of this town the sand is especially abundant and, being shifted by the winds, constitutes an ever-present menace to the small gardens belonging to the inhabitants of the place; in order to control its drifting, fences of palm leaves are made or the sand is removed when it becomes too abundant. Often the gardens are abandoned, leaving the palm fences to mark their sites after the sand has gone beyond. Where the air-currents are most powerful or most consistent, or the walls are broken down, the sand may be carried in small quantities onto the plain, where it constitutes a mulch, influencing in a striking manner the character of the vegetation (fig. 42).

Finally leaving the valley of the M’Zab about 73 kilometers from Ghardaia, the trails wound upwards through low rounded hills to the hamada. This is the northern edge of the region of the Gantara, 100 by 150 kilometers or more in extent, reaching from the valley of the M’Zab on the north to the region of the dunes to the south. It slopes towards the Oued Igharghar, or the drainage depression connected with this great oued. The Gantara has a few chotts and is crossed by three oueds in the southern portion. It is probably the most arid part of southern Algeria. About 60 kilometers of the plain were crossed and here it was gently rolling and stretched without a break to the horizon. The surface resembles that of the hamada at Ghardaia, that is, stones of various sizes, usually small, lie on its surface, but never forming a continuous cover, as in some portions of the Arizona desert. The soil is brown, of fine grain, and with little or no addition of sand. In the innumerable little hollows the soil is deeper than on the slight rounded ridges. Wind is apparently the most potent erosive agent.

Two chotts were encountered between Ghardaia and Ouargla, one unimportant, the other large and with many features of interest. The latter, the Chott Mellala, is about 10 by 15 kilometers in size. The trail descends from the plain, winding through a zone of rounded, cone-shaped hills or mamelons, to the floor of the chott, which lies about 60 meters lower than the general level of the plain. The chott was quite dry in November when we visited it, but at rare intervals water is said to flood the central portion. Toward the outer edges the floor is thrown into waves, where the heavy incrustation of salts is broken. In the center the salt crust forms an unbroken and level surface. Gypsum (calcium sulphate) is the predominant salt. On the eastern side a long and high ridge of sand rears its uneven summits. The height of this ridge was estimated to be 250 meters, and was said by Massart to be the largest seen by him in the Sahara. This dune we had seen lying on the eastern horizon for one or two days before reaching the chott.

The relation of the Chott Mellala to the country to the north or the south was not seen, but between it and Ouargla there lies a succession of smaller and more irregular chotts, which together form a fairly well-connected chain. These chotts are separated by low passes and flat-topped hills whose summits are on a level with the neighboring plain. Many of the hills are cone-shaped and in other topographic features the region shows the eroding action of wind. About 7 kilometers from Ouargla an opening in the mamelons gives a view of a plain extending on a lower level to the horizon. This is the reg, or fluvial desert. The Ouargla plain, or reg, is connected with the drainage of the great Oued Ighaghar and has a character which in many ways is different from the Gantara, over which we had just passed.

With the descent to the reg desert a more monotonous region is encountered. In the vicinity of Ouargla and for some kilometers to the north the topography is quite flat and gives the impression of a flood-plain. To the east it stretches unbroken to the horizon, but to the west it is bounded by a fairly abrupt wall, the Gantara escarpment. At intervals of several kilometers low sand ridges cross the route over the plain, and on the second day somewhat higher ground was traversed and a sand ridge about 4 kilometers across was encountered. The country then becomes somewhat more broken and presents the appearance of being the remains of an ancient and more elevated plain. About 56 kilometers from Ouargla are the largest dunes crossed; where traversed, these were 10 kilometers from north to south and extended beyond our vision both to the east and the west. This is apparently the edge of extensive dune regions which lie mainly east of Touggourt and of the Oued Rirh. For possibly the last 30 kilometers of the journey to Touggourt there are dunes and chotts in alternation.

A word should be said regarding the hydrography of the region whose surface features have been sketched above. Between the cities of Beni M’Zab and Ouargla two wells were passed, although a route could have been taken which could have included three wells. The wells are 125 meters or less in depth and are maintained by the government for the benefit of the caravans, as well as to provide water for the large number of goats and sheep pastured in the neighborhood. The situations of the wells are always in depressions, either along the Oued M’Zab or in similar although smaller drainage areas, and none are on the Gantara. At Ouargla and on the reg desert to the north of the town the water lies very close to the surface of the ground. It can be dipped with buckets and the roots of the palms reach to the water-table. The water from the shallow wells is strongly impregnated with salts. Before reaching Touggourt standing water was seen where the trail crossed certain chotts. Numerous artesian wells have been made by the government which penetrate the ground several hundred meters and give a large and continuous supply of sweet water.

GHARDAIA TO OUARGLA—VEGETATION.

The plants seen during the first day’s march from Ghardaia were such as have already been observed to be characteristic of the valley of the Oued M’Zab or of the neighboring hamada. The vegetation of the hamada, usually very sparse, was noticeably more abundant wherever the sand had been drifted over it from the dunes of the valley, even if the thickness of the sand was so slight as to be little more than a mulch. Here low grasses, much eaten, were the prevailing forms. On the dunes in the valley of the M’Zab, 20 kilometers from Ghardaia, the number of species and individuals is relatively large, the most abundant species being drinn (_Aristida pungens_), although _Deverra scoparia_ is also fairly numerous. Somewhat farther on the route, and in a sandy flat, besides these two species, there is much _Ephedra alata_. On the slopes leading from this flat and on the plain above, there is an almost pure stand of _Rhantherium adpressum_.

Crossing relatively small portions of the plain, in place of always following the bends of the Oued M’Zab, about noon of the second day we reached the bordj Zolfana, in the valley of the Oued amidst low and narrow dunes, which are moving slowly across the flats. (Fig. 44.) No vegetation appears on the dunes, but on the fixed sand between them, or on the stationary dunes at the border of the flats, some plants are to be found. Among these the most abundant, but really not numerous, are _Euphorbia guyoniana_ and retam (_Retama retam_), which was often seen later along the line of march as well as in the vicinity of Biskra. Retam superficially resembles _Ephedra alata_ in having rudimentary leaves and green, reed-like branches; it is carefully avoided by animals, although _Genista saharæ_, a very similar plant, which grows in like situations between Ghardaia and Ouargla, is said by Massart (_loc. cit._) to be eaten greedily by them. (Fig. 43.)

A short distance beyond the bordj the bottoms suddenly widened, the dunes disappeared, and for the remainder of the day’s march we passed through the richest vegetation we had seen since reaching the M’Zab region. (Figs. 45, 46, and 47.) Here the shrubs were of fair size and of sufficient abundance to give character to the landscape. About the 63 kilometers camp the leading species are _Retama retam_, _Ephedra alata_, and _Haloxylon schmidtianum_. On the hamada adjoining the flat are several species, including _Aristida ciliata_, _Artemisia herba-alba_, _Farsetia ægyptiaca_, _Farsetia linearis_, _Gymnocarpon fruticosum_, _Helianthemum eremophilum_, _Henophyton deserti_, _Marrubium deserti_, _Salsola vermiculata_, _Teucrium polium_, _Thymelæa microphylla_, and _Zollikoferia mucronata_. On the hamada just adjoining the place of our camp, however, there appeared to be _Haloxylon schmidtianum_, to the exclusion of other species.

After crossing the bottoms of the oued the trail climbed up to the hamada and we did not see the M’Zab Valley afterward. The vegetation of the hamada soon becomes very sparse and as far as the eye can reach the appearance is that of entire bareness. But, as was found to be the case near Ghardaia, close examination revealed the presence of many living perennials as well as the remains of the previous annual flora. This plain, the Gantara, of wide extent, is the most arid region seen in southern Algeria. Massart estimates that there is an average precipitation of 15 cm. on the desert, but, from data previously cited, it would appear that this amount is rarely attained; indeed, several months, or even two or more years, may pass without any rainfall whatever. When one searches the hollows he finds a few small perennials; plants are almost wholly absent on the low ridges. But in some areas on the plain, where superficial examination does not show any plants, a surprising number were found. For example, on the level hamada, 96 kilometers from Ghardaia, on an area 16 meters square, 389 living plants were found, but only 24 were so large as to be seen from a distance; all were either eaten badly or trampled to the ground, so that it could not be learned what the flora of the area might have been had no animals interfered with its full development.

After leaving the 96 kilometer station the perennials were seen to diminish rapidly in numbers and to decrease in size, until the ridges of the low undulations were absolutely without plants, and there were but few in the hollows. There was no apparent change in the character of the hamada or in that of the soil. This sterile condition persisted for 26 kilometers, when vegetation similar in character to that previously seen was again encountered. The zone of better vegetation lasted for 3 kilometers, when the country became barren once more, which condition lasted for 10 kilometers. The presence of barren belts on the Gantara, where the plants at the best are insignificant in size as well as number and without change in topography or soil, points to an especially arid belt. This conclusion is further strengthened by the observation that the plants found were relatively of very small size.[28]

The flora was also sparse upon the route followed by Massart across the Gantara, which was apparently somewhat farther to the north than the one now being described. He mentions having found _Argyrolobium uniflorum_, _Asteriscus graveolens_, _Fagonia microphylla_, _Deverra chlorantha_, _Fagonia glutinosa_, _Halogeton alopecuroides_, _Helianthemum_ sp., and _Herniaria fruticosa_. He says that _Deverra_ is one of the rare glabrous forms on the hamada. It is said to have the odor of parsley, and the Arabs have a belief that camels which eat it become blind, but Massart’s camels were not injured by eating the plant.

As we drew near the Chott Mellala, on the eastern edge of the barren zone mentioned in the preceding paragraph, we suddenly encountered a belt of _Ephedra alenda_, stretching to the north and to the south as far as could be seen (fig. 49). On one side was the barren zone, on the other the immense plantation. Upon examination it was found that the _Ephedra_ was the only species of perennial. It had several points of interest, but the short time at our disposal precluded more than a superficial examination. Usually keen about the desert species, it is curious to note that the Arabs appear not to recognize this, called by them “alenda,” as being related to the larger species of _Ephedra_ with which they are well acquainted. It is a small species, growing from 30 to 40 cm. high. It does not occur singly, but has the habit of growing in groups of a half dozen or more. Between the _Ephedra_ colonies were only the dried remains of the annual flora of the last rainy season. An examination of the root-system of the species showed that, like other species of the genus, it has a well-developed tap-root (fig. 50).

_Alenda_, however, has a root-habit which, although not peculiar to it, is at least very striking and of great importance to its survival. From the root-crown a stolon arises which extends away from the parent for a distance of about a meter. From this stolon there arise shoots which develop into daughter plants. Through this method of reproduction the small colonies of the species are formed and possibly the species mainly multiplied. From the stolons small roots arise, giving aid to the mother root in providing the offspring with moisture. This habit is very like that of _Kœberlinia spinosa_ of the Arizona desert, which has a similar type of root-system and which reproduces vegetatively in a similar manner. _Alenda_ continued to be the dominant species until near the edge of the big chott (Mellala), a few kilometers east of where it was first seen.

Practically no plants were seen when descending through the eroded portions of the hamada to the floor of the chott (fig. 52), but on reaching the bottom of the great chott a surprisingly large number was observed, including _Anabasis articulata_, _Aristida pungens_, _Ephedra alata_, _Limoniastrum guyonianum_, _Retama retam_, and _Traganum nudatum_ (fig. 51). In the wide central part of the chott, where the salts are perhaps most dense, there are no plants; but on the eastern side are _Euphorbia guyoniana_, _Anabasis articulata_, and _Zygophyllum_ sp. These small species were growing far apart and were badly injured, either by being trodden under foot or by being eaten by the passing animals. To the south of where we crossed the chott and also to the north the sand mountains arise. After leaving Chott Mellala we ascended gradually to go over a low pass separating it from a small chott to the east. Here we obtained a backward view of the sand mountains, low as seen from the pass, with sharp, wind-made ridges, and bearing a few specimens of _Aristida pungens_. Descending somewhat, another but smaller chott was crossed. Between the latter chott and the Ouargla plain the plants were very few and confined to the slopes and the higher ground, avoiding almost wholly the depressions. Of those recognized, _Traganum nudatum_ was the most abundant. This is the “vamran” of the Arabs and is an inconspicuous shrub frequently seen later on the way to Touggourt.

=OUARGLA TO TOUGGOURT—VEGETATION.=

Ouargla is an ancient, rambling town, somewhat in decay, set in the midst of extensive palm gardens. There are said to be 500,000 date palms at the oasis. It was founded in the tenth century by the M’Zabites and later taken possession of by the Arabs. The town is peculiar in its situation and its gardens, lying, as before described, in the flood-plain of an ancient river. The plants cultivated in the town as ornamentals are fewer than at Ghardaia, but of the same kinds, and (besides dates) the gardens contain fewer fruits and apparently fewer kinds of vegetables. Between the gardens one finds _Tamarix_ in some abundance.

Leaving Ouargla by the western gate and turning north, we soon passed through the crooked streets and reached the reg desert to the north. There is little vegetation near Ouargla, but on some low dunes extending over the reg we found _Aristida pungens_ and _Phragmites_ sp., growing in a hollow. On the flood-plain there were a large number of individuals and probably a large number of species. Among the most conspicuous of the shrubs were _Retama retam_ and _Limoniastrum guyonianum_, the “zaita” of the Arabs. (See figs. 56 and 57.) Zaita is a handsome shrub with cylindrical leaves often covered with a fairly heavy salt incrustation. So abundant is the exudation that in localities where the species is especially abundant, as at our camp 32 kilometers south of Touggourt, the plants have the appearance of being covered with snow. Although, like many other species between Touggourt and Ouargla, zaita can live where there are salts in excess, it appears not to be an extreme type of halophyte and does not occur where the salts are most dense. It was seen both on the dunes and on the low lands. Other common halophytes are “souid” (_Salsola tetragona_), “belbel” (_Anabasis_ sp.), and _Halocnemon strobilaceum_, which appears especially resistant. (See figs. 59, 60, 61, and 62.)

Other forms appear on the higher ground and on the plain about 50 kilometers from Ouargla, where _Ephedra alata_ especially is common. There are no dunes on the plain, but some sand swept across it from the fluvial desert to the west passes eastward and augments the dunes farther east. The larger specimens of _Ephedra_ are rather effective sand-binders and bring about the formation of diminutive dunes. The effect on the growth of the species by the piling sand is peculiar. As the sand accumulates it covers the lower branches of the shoot; these are stimulated to unusual growth and new branches may spring from them. Thus the effect is similar to that habitually occurring in _E. alenda_, but in _E. alata_ the habit is not a fixed one. As an instance of the length which such submerged branches may attain, it may be mentioned that one, 2 to 3 cm. in diameter, was over 4 meters long.

About 75 kilometers from Ouargla, and on a plain similar to that just referred to, “dhamran” (_Traganum nudatum_) was the most common species, but was by no means abundant. The census of an area 16 by 16 meters, taken in a locality where dhamran was dominant, resulted in finding 31 living and a few dead plants, probably all _Traganum_. It is to be understood that the vegetation of the reg desert not far distant is much richer, not only in species but especially in individuals, and also that the plants are much larger.

From 56 kilometers to about 71 kilometers from Ouargla the route lay over more or less continuous dunes, where the leading species seen were _Aristida_ sp., _Ephedra_ sp., _Euphorbia guyoniana_, _Limoniastrum guyonianum_, and _Traganum nudatum_.

In or about the edges of the chotts, which are the leading topographical features of the region immediately south of Touggourt, the most common species are _Haloxylon_ sp., _Salsola tetragona_, _Limoniastrum guyonianum_, and _Arabis aphyla_. Besides these forms, about 28 kilometers south of Touggourt fine specimens of _Halocnemon strobilaceum_ were seen growing in an extremely salty situation, to the total exclusion of other species. _Tamarix_ also is to be found in and about salt spots near Touggourt. (See fig. 64.)

Both Massart and Doumet-Adanson (Bull. Soc. Bot. France, 39, 1892) have discussed the flora of the Touggourt-Ouargla region. The following exhaustive list is given by the latter author as having been collected between the two desert towns:

Henophyton deserti. Matthiola livida. Malcolmia ægyptica var. linearis. Moricandia cinerea. Sisymbrium pendulum. Savignya longistyla. Reseda stricta. R. arabica. Helianthemum sessiliflorum. H. ellipticum. Randonia africana. Frankenia pulverulenta. Silene nicoeensis. Monsonia nivea. Erodium glaucophylum. Fagonia glutinosa. F. frutescens. Polycarpæa fragilis. Gymnocarpus decandrus. Retama retam. Genista saharæ. Astragalus gumbo. Anthyllis henoniana. Neurada procumbens. Ammodaucus leucotrichus. Mesembryanthemum sp. Deverra chlorantha. Nolletia chrysocomoides. Senecio coronopifolius. Anthemis monilicostata. Tanacetum cinereum. Ifloga fontanesii. Centaurea purpuracea. Amberboa omphalodes. Rhantherium adpressum. Atractylis flava. A. prolifera. A. microcephala. Tourneuxia variifolia. Catananche arenaria. Spitzelia saharæ. Zollikoferia chondrilloides. Z. squamosa. Scorzonera undulata. Echium humile. Echiochilon fruticosum. Arnebia decumbens. Statice pruinosa. Limoniastrum guyonianum. Plantago ciliata. P. psyllium. Scrophularia saharæ. Linaria fruticosa. Euphorbia guyoniana. Atriplex dimorphostegius. Caroxylon tetragonum. Echinopsilon muricatus. Calligonum comosum. Haloxylon articulatum. Anabasis articulata. Cornulacea monocantha. Thymelea hirsuta. Th. microphylla. Aristida flaccosa. A. pungens. Cyperus conglomeratus. Ephedra fragilis. Erythrostictus punctatus. Scilla sp. Dipcadi serotinum. Asphodelus pendulinus. Ruppia maritimus. Chara fœtida.

=TOUGGOURT TO BISKRA—PHYSICAL FEATURES AND VEGETATION.=

From Ouargla to Touggourt we have found that the country gradually descends, the former place being 124 meters and the latter 77 meters above the level of the sea. From Touggourt, also, for a distance of about 120 kilometers, the descent along the route continues until at the Chott Merouan a level of 6 meters is attained. The lowest places in every case are of course the chotts and the connecting oueds. The most important chott of this series is Melrirh, 11 meters or more below sea-level. This drainage system is the northern culmination of the vast one of which the Oued Igharghar is the most important part. In an earlier age water came north in the oued from the highlands of central Sahara and poured into the Chott Melrirh, having passed successively through the lesser chotts farther south. At that time, also, the Chott Melrirh probably was connected with the Gulf of Gabes.

From Touggourt to Chott Merouan, the lowest portion of the route to Biskra, the topography is that of a region of chotts; that is, there are salt spots surrounded on every side by higher ground, which in many cases is of sand. From Chott Merouan the route passes over a higher desert of a different character, which in part bears a remote resemblance to the Gantara and in part to the Ouargla plain. It is a vast plain, with little topographical diversity, which rises to meet the Atlas Mountains to the west and north. In the eastern portion it is somewhat rolling, stones are strewn plentifully on its surface (hamada), and there has been considerable erosion, so that gullies are formed. In the portion nearer Biskra the surface is more level, the soil is fine (reg), and there has been comparatively little erosion.

The soil of the chott region is largely of sand; on the hamada there is much clay, while on the reg it is fine and easily blown by the wind, and this in spite of the fact that the most vegetation seen in the Sahara was in this region. A slight breeze picks up the dust and carries it long distances in dark clouds. It fills the throat, nose, and eyes of the traveler and makes crossing the reg exceedingly disagreeable.

The plant life as seen along the portion of the route through the chott region consists almost wholly of halophytes, as would be expected, in addition to which, where for short distances the hamada or dunes were crossed, there were forms characteristic of such areas. The _Tamarix_ is especially common in the region and _Limoniastrum guyonianum_ is also often met.

The date gardens of the Oued Rirh are justly famous. One passes numberless plantations where the date is cultivated, and in the neighborhood of each group of gardens one sees squalid Arab villages. Over 19,000 tons of dates are said to be carried each year by camel from the Oued Rirh to Biskra, whence they go to the markets of the world. One day we passed 700 camels laden with dates going to Biskra.

As soon as the chott region is left and the higher ground is reached a different as well as a richer flora is encountered. Here diversity of topography favors diversity of plant life. On the reg near Biskra vegetation is especially abundant. Here, in fact, one passes through large thickets of _Tamarix_ and other species, and sees that the desert is much less intense than that in the south, especially in the region of the M’Zab, where in many respects the topography is similar. The kinds of plants also, as the list below will indicate, are different in the main from those farther to the south. Where the surface is most rolling we find _Tamarix_ sp. on the heights, _Zizyphus lotus_ in the hollows, and the following grasses: _Stipa tortilis_, _Hordeum maritimum_, and _Phalaris minor_. A salsolaceous shrub (_Arthrocnemon macrostachyum_) may be found in washes, _Nitraria_ and _Limoniastrum guyonianum_ on sandy places, and _Odontospermum pygmæum_ and _Anastatica hierochuntica_ occur between rocks. On clay flats one finds _Halocnemon strobilaceum_ and _Suæda vermiculata_, indicating the presence of salts.

The preceding notes are in part from Massart. The following species are given by Doumet-Adanson as having been collected by him between Biskra and Touggourt:

Savignya longistyla. Eremobium lineare. Lonchophora capiomontana. Monsonia nivea. Erodium pulverulentum. Fagonia sinaica. Haplophyllum tuberculatum. Lœfflingia hispanica. Paronychia nivea. P. cossoniana. Herniaria fruticosa. Zygophyllum album. Z. cornutum. Peganum harmala. Nitraria tridentata. Astragalus gyzensis. Ononis serrata. Ammodaucus leucotrichus. Cyrtolepis alexandrina. Anacyclus clavatus. Pyrethrum fuscatum. Nolletia chrysocomoides. Tanacetum cinereum. Ifloga fontanesi. Artemisia herba-alba. Anvillæa radiata. Atractylis flava. A. prolifera. Nonnæa micrantha. Lithospermum callosum. Heliotropium undulatum. Plantago ciliata. P. albicans. Limoniastrum guyonianum. Statice pruinosa. Echinopsilon muricatus. Traganum nudatum. Ephedra alata. Cutandia memphitica. Erythrostictus punctatus. Asparagus albus. Aristida pungens.

=THE BISKRA REGION.=

TOPOGRAPHY.[29]

Biskra lies immediately south of the Atlas Mountains, in the Department of Constantine, 220 kilometers from the Mediterranean and about 400 kilometers from Ouargla. To the northeast of the oasis lie the Aurés and to the west the beginning of the Saharan Atlas, which run south of west across Algeria into Morocco. Just north of the place are small hills and low, jagged mountains—detached spurs from the main ranges. These are the Djebel Bou Rhezal, running nearly northeast and southwest. The highest of them, directly west of Biskra and about 8 kilometers distant, has an altitude of 463 meters. The Bou Rhezal Mountains have a precipitous southern face, but fall away more gradually to the north, where the slope joins a wide and undulating plain. The latter extends to the base of the main Atlas ranges. Southwest of the oasis, and about 2 kilometers distant, a range of rocky hills extends for a distance of about 6 kilometers, or until they join the mountain range of the Saharan Atlas. These hills are called Ed Delouatt. To the south of Biskra, as has already been stated, there extends a vast plain, the reg, which dips gently to the south and drains into the Chott Melrirh, 50 kilometers or more distant. The situation of Biskra relative to the mountains on the one side and the desert on the other, together with its altitude, governs the climate of the place. Except Laghouat no vicinity in the desert proper was seen with so great an amount of precipitation (about 200 mm.) as Biskra, which is indicated by the relatively rich flora. One who has seen only Biskra can not draw conclusions regarding the vegetation or the conditions of plant life of those portions of the Sahara that lie farther to the south, where much more intense conditions of aridity obtain.

The soils of the vicinity of Biskra are various. That of the low hills between the town and the Djebel Bou Rhezal is only a few centimeters in thickness, but in the washes from the hills it is a meter or more. Here the soil is a sandy loam with an admixture of small stones and pebbles. On the flat ground to the north and to the south of these hills it is of a finer texture, approaching the adobe of the southwestern United States. On the reg to the south of the oasis the soil is also fine, and in some places, if not underlying the plain as a whole, there are strata of gravel at varying depths beneath the surface. This soil in places carries considerable salts. It dries to a powder during the long dry seasons and is easily blown by the winds. Owing to outcropping rock, the south face of the Bou Rhezal Mountains has but scant soil, but that of the northern side resembles the soil of the low hills to the south, which has already been characterized. Near the town are dunes of good size. Especially to the southwest the sand banks against Ed Delouatt hills is in large amount. In the opposite direction, but farther from the oasis, the dunes are fairly extensive.

The Oued Biskra is of great importance to the oasis, since it carries water for several weeks of the year and furnishes water for irrigation. Its channel lies about 3 meters, or possibly more, below the general level of the oasis, and possibly in earlier times may not have been so well defined as at present, spreading its waters over its flood-plain during high water. The oued is made up of several tributary oueds which cross the plain north of Djebel Bou Rhezal, unite where there is a pass in these mountains, and finally debouch on the reg to the south of the town, where the channel becomes continually less well defined. Another oued takes its origin in the Bou Rhezal Mountains in several independent branches which unite at a pass in Ed Delouatt hills and extend for a distance of 15 kilometers or more into the reg. One of the feeders of this oued is from hot springs, Hamman es Salahine, about 8 kilometers northwest of Biskra.

PLANT HABITATS OF THE BISKRA REGION.

From the preceding sketch of the leading topographical conditions of the vicinity of Biskra it will be seen that the plant habitats are more diverse than at any other place visited. For the present purpose the habitats may be distinguished as follows: The alluvial desert (reg), which lies on every side of the oasis save the north; low hills adjoining the oasis on the north; Ed Delouatt hills, southwest; the oueds and their flood-plains; the Dj. Bou Rhezal; the hamada (?) lying both to the north and to the south of the latter.

VEGETATION OF THE BISKRA REGION.

A glance over the list of plants which grow naturally in the vicinity of Biskra[30] shows that many are the same as occur farther south, with many unlike these, having affinities outside of the desert proper; also, the number of plants as well as species is greater at Biskra than farther south. This would be expected from the greater rainfall and more diverse topography.

The flora of the Biskra oasis, according to the authors referred to above, consists of 175 or more species. Of cultivated plants there are 25 or more species, the most conspicuous being the date. The other species are mostly the same as have already been noted at other Algerian oases, except that both the peach and the apricot are wanting at Biskra, although cultivated at Laghouat, Ghardaia, etc. On the outskirts of the town the fairly extensive flood-plain is given over mainly to the cultivation of grain, barley predominating.

The hills and mountains, and the bajada at their base, as well as certain oueds with water relations, exposure, and soils different from those of the oasis, have also a very different flora, which, for the most part, is desertic in character. For purposes of comparison, some of the leading characteristics of the plants growing in a half-dozen localities will be given.

As stated above, to the southwest of Biskra there runs a range of hills, Ed Delouatt, to the south and the north of which may be found interesting plants and plant conditions. On the south side the slope (bajada) descends gradually to the great reg, and near the base of the range, in the vicinity of the place where tradition says a Roman town formerly existed, there is a wide, sandy plain, reaching from the Oued Melah, which pierces Ed Delouatt hills, nearly or quite to the western extension of the oasis. There are no large dunes here, but sand billows about a meter in height and sand hillocks diminutive in size. Between these the plain is fairly level. Over this whole tract there seemed to be only one species (_Euphorbia guyoniana_), but this was fairly abundant. (Fig. 65.) As at Ghardaia and elsewhere, this species grows in small colonies because of its suckering habit, and acts to a small degree as a sand-binder, each group being situated on a sandy hillock. It will be remembered that this species at Ghardaia, as well as at a certain bordj east of that place, had roots which were somewhat fleshy as well as roots which were fibrous, on one and the same plant. I was interested to learn whether similar conditions should obtain at Biskra, since it had been learned, in the case of two species of _Opuntia_ in the Tucson region, that a different habit followed certain differences in habitat. Several specimens of _Euphorbia_ were removed with care from the soil and in no case was it found that the roots were fleshy, but in every instance they were entirely fibrous. It is supposed that the reason for the variation in behavior may possibly be traced to differences in the water relation, as was found to be the case of the variation in cactus roots cited, but no experiments have been made on _Euphorbia_ to prove this.[31]

There is a variety of habitats on the opposite (north) side of the hills, ranging from the large dune (which reaches the summit to the west of the pass through which the oued passes and which must be 100 to 150 meters above the oued) to the flood-plain, with water very near the surface, during rainy season at least. There are rocky slopes, also, and sandy slopes apart from the dunes referred to, as well as a fine clay with sand admixture on the flood-plain. In all of these habitats, except the large dune, the vegetation is actually or relatively abundant. (See fig. 67.) Among the rocks are small shrubs or half-shrubs, and also on the plain below. Here one may find, among other species, _Echiochilon fruticosum_, _Helianthemum_ sp., _Atractylis serratuloides_, _Gymnocarpos fruticosum_, _Thymelæa microphylla_, _Nitraria tridentata_, and _Acanthyllis tragacanthoides_. On the sandy slope was growing a very numerous population of liliaceous forms, mainly, perhaps wholly, _Asphodelus fistulosus_ (fig. 72).

It has been noted that the oued which pierces Ed Delouatt is made up of the confluence of all of the small oueds lying between the Bou Rhezal Mountains to the north of Biskra and the hills lying directly north as well as those (Ed Delouatt) to the southwest. The united oueds reach the base of the hills nearly 1 kilometer east of the pass, and turning abruptly follow the base the remainder of the distance. At the place where the oued touches the range the soil is moist nearly to the surface of the plain (reg) and for several meters back from the oued. Here, then, the water relations of the plants are such as to favor, probably most of the year, the growth of mesophytes (?) or even of hydrophytes, but for some reason there is not much vegetation there; whether large species have been removed as for fuel, have been destroyed by animals, or never existed, was not learned. The most interesting plant found was the well-known parasite _Phelypæa violacea_, which grows, according to Mobius, on a salsolaceous host.[32] Only a few specimens were seen at the time of the second visit to Biskra, in March, and they were just appearing above the ground. (See figs. 69, 70, and 71.) When removed from the soil the longest specimen was found to have penetrated over 59 cm.; it was 6.5 cm. in diameter at a point 50 cm. from the tip. The plants were exceedingly heavy, being gorged with sap, and appeared to be able to absorb moisture from the wet soil through the white and delicate epidermis of the entire portion submerged. If this is the case the parasitical relation is of especial interest, as the species is wholly dependent on its host for organized foods, but not for water—a condition opposite that found in such semi-parasites as the mistletoes, which obtain water and unorganized foods in solution from the host, but which, save for the fact of attachment, are otherwise independent of it.

To the northeast of Ed Delouatt hills and north of Biskra, but immediately adjoining the town, is an irregular group of low, rounded hills, mostly very arid, which support a scant vegetation of typical desert plants. These hills, without a name, are eroded to a degree and have shallow washes leading from them in every direction. The soil appears to be thin, except in the washes, where it has accumulated to the depth of a meter or more. On the rounded summits it is rather fine, but in the washes there is much gravel and larger stones. Here one finds the greatest range in exposure, and probably also an accompanying difference in the temperature of the air and soil.

In considering the vegetation of the hills it must be remembered that no plants suitable for forage or large enough for fuel would be left untouched, as the biotic factor is quite as much in evidence in modifying the Biskra flora as that of the other regions visited. As the plants are at present, however, and for whatever definitive causes, there is a considerable variation in numbers and apparently also in kinds. An examination of different parts of the hills by which various exposures as well as other conditions are seen bears out the hypothesis. On the southern slopes, particularly the upper portions, the plant covering is especially poor. The population of an area 16 by 16 meters in the upper south slope included 134 perennials and numerous annuals. Of the species, _Haloxylon scoparium_ was the most numerous; there were also _Dæmia cordata_, _Thymelæa microphylla_, and _Fagonia sinaica_ (figs. 78 and 80). All of these species were small, so that from superficial examination the area appeared fairly barren (fig. 73).

When one crosses the hills to the north, so that northern and northwestern exposures are seen, a striking difference is immediately apparent. The northern slopes are much richer in perennials than the opposite facing. The population of an area of the size before taken shows this observation to be correct. On a square 16 by 16 meters, with north exposure, 536 living perennials and numerous annuals were found, with _Haloxylon scoparium_ again the dominant species. The species second in number was _Ferula vesceritensis_, which is to be found chiefly on the north exposure (figs. 74, 75, and 77). There were also a few specimens of _Acanthyllis numidica_, and many annuals, which, unlike those on the south facing, were not in flower.

On the lower slopes, in the small and open gulches and the lower portions of the washes and their flood-plains, the plants were most numerous and include a variety of forms, with _Peganum harmala_ the most conspicuous and most numerous.

The roots of several species growing on the hills were studied with the following as perhaps the leading results: The root-system of _Haloxylon scoparium_, as has already been seen to be the case with an allied species in the country of the M’Zab, may be said to be a modification of the generalized type. One specimen, partly exposed by erosion in a wash, had a tap-root over 113 cm. in length (fig. 76). As this root was 4 mm. in diameter where left, and was 8 mm. in diameter at the crown, it may have penetrated much beyond the point where it was left, provided the soil conditions continued favorable. The main root gave off two large laterals, of which one left the parent root 8 cm. and the other 18 cm. beneath the surface of the ground, but there were also numerous filamentous roots about 2 cm. in length, which were borne in tufts. (See fig. 79.) These resembled the deciduous rootlets found on many perennials in the Tucson region and doubtless function quite as they, namely, they are organized at the beginning of the rainy season, operate to increase the water-absorbing area of the plant quickly and greatly, and die as soon as unbearably arid conditions set in.

In the same wash, where the soil was deep with an admixture of small stones and pebbles, the roots of other species were also examined. One of these was _Peganum harmala_, whose roots were examined at Ghardaia. The root-systems of the plant in the two regions were similar in being generalized. A main root was found extending downward over 61 cm. and it gave off three good-sized laterals, arising from 15 to 27 cm. beneath the surface. The uppermost lateral took a horizontal course. Figure 82 gives a fairly good idea of the general character of the root-system of the species.

There were many specimens of _Plantago albicans_ growing in the same habitat as _Haloxylon_ and _Peganum_. Its root-system was also studied with the following results: The tap-root is strongly developed. In one instance the slender main root was found to go straight down over 71 cm. Numerous laterals were borne between 8 and 20 cm. beneath the surface of the ground. The species has an interesting habit of propagating vegetatively by means of fleshy stolons. An examination of the stolons showed them to be of very unequal age, some having been lately formed, while others had been organized in previous years and were no longer living. Many specimens were examined to learn the probable service of the fleshiness of its stolons, and the conclusion was that this factor probably enables the species to pass over periods of excessive drought.

Running across country are the Bou Rhezal Mountains, of which the north and south faces were examined in one or two places, on each side somewhat in detail. The south face is precipitous and has little vegetation, but there are many plants on the opposite face, especially near the base, where the soil conditions are relatively favorable (fig. 84). Among the perennials were found a few specimens of _Rhus oxyacantha_ and _Zizyphus lotus_, which, although dwarfed, were the largest plants seen away from the oasis, being about 1.5 meters high. In another gulch were found the following species: _Acanthyllis tragacanthoides_, _Dæmia cordata_, _Ferula vesceritensis_, _Haloxylon scoparium_, and other plants unknown. Toward the upper portions of the gulches _Haloxylon_ was seen to be especially abundant, but in the bed of the washes and on their flood-plains _Peganum_ grows in large numbers (fig. 83). In the most favorable places on the northern slope in March the annuals were the most abundant observed in the vicinity of Biskra, although even there they failed to completely conceal the ground. Where the situation was less favorable as regards soil and water conditions (for example, on the side of the gulches, on the summit of ridges, and the like) there were almost no annuals and those present were relatively small.

In this brief account of some of the most striking features of the Biskra flora it should be noted that several habitats are omitted as not being pertinent to the points in view. Especially, nothing has been said regarding the flora of the salt flats along the Oued el Hamman or that of the dunes, since the influences here are largely edaphic, while the present interest lies mainly in the relation between plants and climate; and the flora of the oasis has been largely neglected for similar reasons.

Not only is there in certain regards a larger number of plants in the Biskra region than had been previously seen in southern Algeria, but there are certain types, mostly new, which point to more favorable conditions of plant life. These are such forms as have a water-storage habit, like _Asphodelus_, _Ferula_, _Plantago_, and _Phelypæa_, which, although not wholly absent farther south, appear to be much more numerous near Biskra. The presence of bulbous plants is well known as being one of the floral characteristics of the High Plateau, and it is also known that similar forms are not to be found where the arid conditions are the most severe, which probably accounts for the facts noted. It may be pointed out also that plants at Biskra exhibit exposure preference where soil conditions appear to be parallel. This condition is not so marked farther in the desert as at Ghardaia, for example, where, provided there is sufficient depth of soil, apparently any species may be found on any exposure. In other words, exposure preference implies a certain amount of water as well as sufficient soil.

So far as shown by observations of the root-systems carried out on similar species growing in the Biskra region and at Ghardaia, the essential root characters of plants growing in the two regions are the same. The single exception to this so far noted is that of the roots of _Euphorbia guyoniana_, which, contrary to the expectation, are apparently wholly fibrous at Biskra, although they are in part fleshy farther south, where the soil conditions are surely more arid. At Biskra, also, the only plant observed with typical generalized root-systems (_Peganum_) does not grow where the soil is shallowest, but where it is relatively deep. So far as seen, also, plants growing where the soil is shallow have either a generalized root-system or a root-system approaching this type, even if the branching is only relatively deep.

=GENERAL SUMMARY AND CONCLUSIONS.=

THE ENVIRONMENTAL CONDITIONS OF PLANTS IN ARID REGIONS.

The environmental conditions encountered by plants in the arid regions are widely different from those of moister regions. Precipitation is not only slight, but it shows an enormous range in variation from year to year; the rate of evaporation is high; the temperature of the air and soil varies widely both during the day and with the seasons; the light is of great intensity, and the soil is low in humus content and may contain an excess of salts. These, the most striking physical factors of deserts, are present in different combinations with resultant differences among deserts, and an arid region may be so large as to include such variation within its borders. Also, a desert may be so far from the ocean, or it may include such diversity of topography, as to show great variation in biological features as well as in its surface phenomena.

In the flora of any arid region, the mutual relations of the constituents of the flora and its general and detailed relation to the physical environment are quite different in the main from these features in the flora of the more humid regions. Thus, the leading relation is the relation to water, and on the response of the plants to this relation much of the phenomena associated with plant life in the desert directly depends. For example, in the extreme deserts it is probable that the elements of competition between the perennials, which is an important factor in the survival of a species in the moister regions, is wholly lacking. It should be noted, however, that in the less extreme deserts, as in the vicinity of the Desert Laboratory, competition exists between plants, although this is not at first apparent. In this case the competition is not for room, but for water, and is not manifest by palpable crowding, but by invisible competition of the roots. Thus the reactions are with the physical environment and are exhibited in a variety of ways, some of which concern the plants themselves in an intimate manner, being morphological and physiological, some being concerned with the flora as a whole. The environmental responses are often obscure and complex, but in other instances they are less obscure and apparently direct.

As is well known, a desert flora is in part perennial, lasting with but little outward change from season to season, and in part ephemeral, consisting of short-lived species which appear with the rains and which disappear with the return of the dry season. The ephemeral flora differs in no essential regard from annuals of the moister regions; also, the environment to which they are exposed closely resembles the environment of the annuals of such regions. But the perennial desert flora, on the other hand, offers very striking departures from the corresponding flora of the moister regions, just as the environment to which they are exposed for the most of the year is also different. It will be sufficient, for the purpose of bringing out the point of view, to notice a few of the leading characteristics of desert plants and of their physical environment and, in a few instances, to observe possible relations between the two.

The most obvious features of desert plants are associated, in whatever way, with the subaerial portions. Leaves are usually greatly reduced or wanting, during the dry seasons at least. Spines are frequently present and the exposed parts are often well covered with hairs. The stomata are sometimes deeply sunken, the cuticle often very heavy, and a waxy substance may cover leaves or stems. The chlorophyll-bearing cells are arranged with the long axis at right angles to the leaf or stem surface. All or most of these characters are associated with the low humidity of the air. In certain deserts plants are also to be found with greatly enlarged stems and branches which serve as water-storage organs. It should be noted, however, that plants which do not have a constant surface undergo many marked changes with a betterment of the water relations, particularly if this comes when the temperatures are favorable. For example, many cacti organize leaves which are unsuited in structure for periods of extreme drought, and which consequently fall away soon after the close of the rainy season. These leaves enormously increase the rate of transpiration at a time when this is not injurious.

It is not in the subaerial parts alone, however, that the plants of one desert are different from those of another, that plants are unlike in the same desert, or that plants of a desert are different from those of the more humid regions. The root-habits also exhibit not a little diversity and show marked reactions to the pressure of their environment. For instance, the desert shrubs of the region surrounding the Desert Laboratory at Tucson have well-marked root-systems, apparently constant under natural conditions, which may roughly be designated as the tap-root type, the superficial type, and the generalized type. Other conditions being equal, species with characteristic root-types have also characteristic distribution, or exhibit in other regards consistent reactions. Thus, the widely extending and superficial type of roots is confined, among independent plants, to such as have water-storage capacity. The relation of this root-type to the distribution will be mentioned later. Plants with a dominating tap-root are confined to areas where the soil is relatively or actually deep, while species having generalized roots have a local distribution which may be considered the maximum.

The relation of the superficial type of root-system to the distribution of the species is not so apparent in this relation in plants as with the other types of roots, but is undoubtedly close and possibly definitive. The absorption roots of plants with water-balance mostly lie less than 10 cm. beneath the surface and are thus subject to extreme desiccation for the maximum time, or in other words they are exposed to favorable moisture conditions the minimum time. How long the optimum water-absorption time may be for such species is not known, but that it is longer than might be supposed (from the period certain species can survive without water) is highly probable from the facts concerning their distribution. In brief, the best development of the fleshy species in question occurs where the rainfall is periodic, occurring twice each year, and they are wanting or sparse where the rainfall is uncertain or occurs but once annually. Had these plants a deeply penetrating type of root-system the local as well as the general distribution would be very different from what it now is. Owing to the unfavorable character of the rainfall in southern Algeria, plants with a water-balance are wanting there, just as they are wanting in portions of southwestern United States where the amount or the character of the precipitation is likewise unfavorable.

Besides the characteristics of the root-systems of the desert perennials whose significance has been sketched, there are other features of importance. For example, the roots lying near the surface bear tufts of delicate roots, which are formed during periods of active growth and perish when such seasons cease. By the organization of deciduous rootlets the absorption area of the species is enormously increased, and quickly, without at the same time increasing the distance of water transport.

There need be mentioned here only one additional feature of the desert plants. It is now known, in brief, that the non-fleshy perennials of the desert, not halophytes, may possess a very dense cell-sap. This fact has been demonstrated in the subaerial parts of several species, and is assumed to hold for the roots also of the same species. As suggested, not all of the desert plants, however, are capable of developing dense juices. Thus, certain fleshy species, and such mesophytes of the desert as have been studied in this connection, do not have more highly concentrated cell-sap than the ordinary plants of the humid regions. Further, it appears that desert species which, under natural conditions and during the dry seasons, form extremely dense juices, lose this capacity when grown under humid conditions. So far as is now known, species capable of developing a cell-sap with high osmotic power have generalized roots, although this may be of no especial significance; but the relation of this capacity to survival in an arid substratum is apparent and vital.

ENVIRONMENTAL FEATURES OF THE FLORA OF ALGERIA.

The physical environment of the plants of southern Algeria is, in a few broad features, similar to that of the southwestern portion of the United States. These regions have about the same latitude, both are separated from a large sea by mountains, and the range in altitude is similar. There are other features, however, particularly as regards the amount and the distribution of the precipitation, in which the two widely separated regions are very unlike, and a correlated difference in the habit and composition of the floras of the two regions is apparent.

The Algerian climate as a whole is a mild, temperate one, but very diverse. The latitude and topography taken in connection with the presence of large seas to the north and west, and a large continent leading away to the south are its chief determinants. The climate, therefore, of the northern portion is coastal; that of the southern portion continental.

Probably the most important of the secondary factors which modify the climate of Algeria is its highly varied topography. An important mountain system, the Atlas, made up of many more or less detached groups and secondary systems, a plateau or steppe lying 3,000 feet more or less above the sea, and finally the northern edge of the Sahara, which has a very diverse topography of its own—such is the surface of Algeria.

Algeria is divided into three climatic provinces corresponding to the leading topographical differences: the Tell, including the littoral, or portion between the maritime Atlas and the Mediterranean; the High Plateau, or steppe, which lies between the Tellian Atlas and the Saharan Atlas; and the desert. These provinces have marked individual differences in rainfall, temperature, and other climatic features.

In the Tell and the High Plateau the winds from the sea deposit most of their moisture. Along the coast as much as 700 mm. of rain is recorded, while in other parts of the Tell it is about 570 mm. On the High Plateau the yearly precipitation sinks to 310 mm. In the desert south of the Saharan Atlas, however, where the altitude is lower and the temperature greater than in either of the other provinces, the yearly rainfall is 200 mm. and less. In some years, in fact, no precipitation whatever is reported in the desert.

The seasonal distribution of the rains in any arid or semi-arid region is of great importance as a factor in shaping the character of the vegetation. For example, in the semi-arid region of the southwestern part of the United States, in the Tucson region, there are two distinct rainy seasons—the rains of winter and those of summer—and here the plants with a water-balance are an important feature; but farther to the west, where there are no summer rains, there are no succulents. In Algeria, also, there is but one rainy season and it has already been noted that the absence of plants with water-storage facilities is one of the leading characteristics of its vegetation. The seasonal distribution of rains is as follows: In the Tell, in winter it is 42 per cent, in spring 27 per cent, in autumn 27 per cent, and in summer only 4 per cent. On the High Plateau the percentages are 30, 20, and 34 for winter, spring, and autumn, and 16 for summer. In the desert the percentages for winter, spring, and autumn are 37, 39, and 20 respectively, while in summer 4 per cent of the entire rainfall occurs. On the High Plateau, however, and in the Saharan Atlas the distribution of rain is much more equable, since 16 per cent falls on the High Plateau in summer and 13 per cent in the Saharan Atlas the same season. We have, therefore, the interesting result that both in the Tell and on the desert there is a long, dry summer season, but in the intervening country more or less rain falls at this time of year. It seems very probable that a careful study of the plants of these regions would show reflected in the vegetation this peculiar character of climate.[33]

The mean relative humidity changes in a marked manner as one passes from the Tell, across the High Plateau, and enters the desert. For example, at Fort National the mean relative humidity is 85 per cent. On the desert it varies from 54.6 per cent at Ouargla to 42.6 per cent at In Salah. At times in midsummer the humidity in the desert is too low to measure with instruments; it is often 7 or 9 per cent. On the other hand, the humidity in autumn is surprisingly high, owing in part to the lower temperature and in part to the northerly winds. However, no dew is reported and probably its occurrence is rare.

With so great difference between the Tell and the desert in relative humidity is associated marked variation in the rate of evaporation. For example, at Algiers the total annual evaporation is 1,654 mm., while at Ghardaia it is 5,309 mm., which is possibly the greatest amount of evaporation thus far reported. Thus, the difference in evaporation between the Tell and the desert is nearly as the ratio 4 to 1.

The evaporation-rainfall ratios for the Tell, High Plateau, and the desert are of great interest. The seasonal evaporation-rainfall ratio for the littoral is 2.5 to 1; that of the Tell is 3.5 to 1; that of the High Plateau is 9.4 to 1; and that of the desert is 46.5 to 1. If we represent the evaporation-rainfall ratio as unity, the ratio for the Tell becomes 1.4 and the ratio of the High Plateau becomes 9 to 3.7, while the desert ratio is 18.6.

The annual and daily variations of temperatures in the desert are naturally relatively great. At Algiers the annual variation is approximately 40.7° C., while at Ghardaia it is 47.9° C. As great an annual variation as 57° C. has been observed at Ghardaia. The daily variation of temperature is especially marked on the High Plateau and the desert, ranging 17° C. more or less on the High Plateau, and 20° C. more or less on the desert. These figures are occasionally overstepped: for example, at Batna in July, 1904, when the maximum daily range was 21.8° C.; and an observation made by the writer in the open desert between Ouargla and Touggourt, in November, 1910, showed a variation of temperature of 24.7° C. between 3 o’clock in the afternoon and 6 o’clock the following morning.

The absolute maximum temperatures in southern Algeria are fairly high. At El Golea, for example, they are 47°, 46.5°, 48°, and 49.2° C., while at In Salah, about 700 miles from Algiers, the maximum temperatures for four years have been found to be 50°, 49.2°, 50°, and 48° C. It is interesting to note that at Ouargla, which is much nearer the coast than In Salah, even higher temperatures have been recorded. The maxima for as many years are as follows: 50.2°, 51.0°, 52°, 49°, and 48.4° C. Usually in winter freezing temperatures are experienced at all stations in southern Algeria.

Very little has been done on the soil temperatures in the desert region, but at Ghardaia, in July, 1911, the temperature of the soil 15 cm. beneath the surface of the soil ranged, maximum from 36° to 37° C., minimum from 31° to 33° C., giving an absolute range of 6° C.

In addition to the rainfall, the evaporation, and the temperature, there is another important climatic factor which should be taken into account, but which can not be stated in accurate terms, _i.e._, air-currents. It is a matter of common experience that one rarely observes a calm day on the desert, but that usually the wind, which is often of considerable force, is found to be blowing. This is unquestionably an important factor in raising the total of evaporation and therefore in increasing the arid conditions of this region. The winds which are most effective in the direction mentioned are those which come from the desert and are known as the “sirocco;” these are most likely to blow in spring and summer, although they occur in autumn also, and to a very limited degree in winter. When the winds blow from the north cooler conditions occur, the relative humidity is lower, and therefore the evaporation rate is less. The sirocco, or desert wind, crosses the Mediterranean and is sometimes felt in southern Europe. It does not generally last more than three days at one time, but at Batna, in July, 1902, it was reported for eight consecutive days. When the sirocco blows the humidity is likely to be markedly affected; for example, at Batna during the eight-day sirocco alluded to, the relative humidity fell from 25.6 to 16 per cent on the first day.

SOME EFFECTS OF TEMPERATURE AND RAINFALL IN SOUTHERN ALGERIA.

While it is recognized that, generally speaking, climate shapes the character of the vegetation, its immediate effects can not well be measured, or, at least, have not been accurately measured, so that it seems necessary to confine one’s observations to supposed or probable effects, however unsatisfactory this may be. So far as suits the present purpose, climatic effects may conveniently be separated into those which are direct and those which are not direct, remembering at the same time that the division is purely arbitrary, since the climate is a complex of various factors and its effects on vegetation are also complex. Among the climatic factors whose effects are most striking are the air temperature and the rainfall, and only certain effects resulting from a variation of these will be commented on here.

In parts of the Sahara visited where the most rain is reported, especially Laghouat and Biskra, plants were observed to exhibit exposure preference. Here the south or southerly facing slopes may have a floral composition different from the opposite exposure. In each instance the soil conditions, and apparently the moisture conditions also, were alike. Exposure preference was not noticed in the southern portions of the colony. Another temperature relation was observed, namely, the renewal of growth in the autumn. This is probably direct effect, although the point is not certain. In Ghardaia it was seen that many of the perennials were taking on new growth and coming into flower, although no rain had fallen for 12 months. Analogous conditions, with a significant difference, occur each year in the Tucson region. Here with the change from a cooler to a warmer temperature, as from winter to spring, or from spring to summer, fairly independent of the rainfall, many perennials organize flowers or shoots. But, so far as is known, no species renews its vegetative activities with the coming of winter, or with a decreased temperature and also _independent_ of the rains, although there are characteristic winter and summer plants. Judging from analogy, therefore, it would appear that the stimulus to development on the part of the M’Zabite plants may be from the relatively better water relations made possible by a lower temperature without rain. In November at Ghardaia the evaporation rate is much below that of summer, that during the night being very small. Further, it was told me by good authority that the same species seen growing in autumn renew growth whenever rain chances to come, whatever might be the season. But it should be remembered that rain most commonly occurs in this region in winter, so that the plants may have a rhythm to which they usually conform, but from which they may depart, and that both stimuli (better water relations and lower temperature) are the annually recurring factors by which it may have been induced. Reference, of course, is made to perennials only, as no annuals were seen until the rains of spring made conditions favorable for their appearance.

The effects of a varying amount of precipitation are naturally the most marked of any climatic factor. It is especially striking as one goes south from the Mediterranean, crossing the Tell and the High Plateau and entering the desert proper. Whether the effects would be increasingly striking with deeper penetration of the desert is doubtful. As is well known, a leading characteristic of the vegetation of the littoral and of the Atlas Mountains is the presence of forests of whatever species. As the littoral is left behind the forests disappear until on the High Plateau there are only straggling trees along the dried water-courses. This steppe bears mainly shrubs, many of which are halophytes, with the perennial grass, _Stipa tenacissima_, and _Artemisia herba-alba_ away from salt spots. Such low forms are present in sufficient numbers as to give character to the landscape and to conceal the surface of the ground fairly well.

South of the Saharan Atlas a marked change occurs. Here, with a rainfall of 200 mm. and less, the trees are confined to the dayas, a narrow belt, the vicinity of oueds, and the oases, exclusively. The shrubs of the hamada also decrease in numbers as one goes south, and where the annual precipitation is least, as on the Gantara between Ghardaia and Ouargla, large barren areas extend. At no place on the hamada of the M’Zabite region are the shrubs present in sufficient numbers or size to conceal the surface of the ground or to give character to the landscape.

Aside from the effects following a lessened annual precipitation there is also to be taken into account the increasing uncertainty of rain, or its irregularity, which is also a marked characteristic of the Saharan climate. In the desert, also, storms are likely to be of the torrential type. I did not observe vegetation characteristics which appeared directly traceable to the irregularity in rainfall, but Hayward reports an interesting condition observed by him in the southern Sahara, near Kidal, where large tracts of _Mimosa_ had died from an unusually long period (five years) without rain.[34] It is not at all improbable that to the cause named much is directly traceable which is generally attributed to insufficient rainfall taken in the usual sense. It is probable that the vegetation of the desert—the amount as well as kind—is due to the capacity of desert forms to meet successfully the occasional, even rare, conditions, of whatever sort.

THE SOIL RELATION IN SOUTHERN ALGERIA.

A very important environmental factor, although one which can not at this time be adequately presented, is the soil relation. Nowhere is the edaphic factor more important than in the desert, where quantity and quality are always important and occasionally even determining factors. In this connection I do not refer particularly to dunes or to chotts, but to country soils, that is, the sort most commonly to be found, which in southern Algeria is a clay with sand present in greater or less amount. So far as the relation of plant to soil refers to the presence or the absence of the plant, the problem can be briefly stated thus: Given similar kinds of soil and an equal precipitation, areas where, within limits, there is greatest depth of soil will have the largest number of plants, and areas with light soil covering will have few or no plants. Also, having given sufficient soil, the kind of plants present, together with certain root-types, will depend on the soil depth. It should be understood that these generalizations are supposed to apply to southern Algeria and not the deserts in general, or at least not to semi-deserts where in certain regards a very different condition obtains. The plant distribution thus dependent on soil depth, and the root character also having relation thereto, will be presented below; the soil conditions, so far as they are known to me, can be given briefly in this place.

From a few excavations on the hamada near Laghouat, on an analogous area by Tilrempt, on the hamada at Ghardaia, between Ghardaia and Ouargla (Gantara), and by Biskra, in each instance with analogous topographical conditions, it was learned, in short, that on formations of this sort the soil is usually less than 50 cm. in depth, although where there are rocks or in drainage depressions the depth may be greater. In the oueds, where a different type of soil occurs, greater soil depth is naturally found; also on the reg, or alluvial plain, frequently at least the flood-plain of oueds, the soil conditions are peculiar and the soil is deeper than on the higher hamada. The special significance lies not so much in the differences in the soil _per se_, but in the differences in the water relation occasioned by variation in depth. Owing to want of data in regard to penetration of water and its retention on the Algerian soils, the soil-moisture relation can only be gathered from inference. As regards rains, it is probable that light showers, those so slight as not to penetrate over 1.0 cm., have little or no direct influence on the perennial vegetation, but greater penetration directly benefits such plants. The first conclusion is drawn from the observation that filamentous rootlets (seemingly like the deciduous rootlets of the perennials of the Tucson region) on _Haloxylon_ at Biskra were not found nearer the surface than 8 cm. Should the more superficial soil layer be moist for any considerable time, there would apparently be no reason why such temporary rootlets should not be formed nearer the surface. However, should there be sufficient moisture in the soil to permit absorption by roots, provided a slow rate was adequate to replace the transpiration loss, which rate was made lower by a more moist air, a slight rain would be of great significance, even if it did not penetrate to any appreciable depth. On the intensely arid desert such slight modifications of the water relations as the lowering of the temperature as winter approaches, causing decreased evaporation or rains, although actually small in amount, may be of large moment to plants. Such a condition was noted at Ghardaia,[35] where there had been a drought for over twelve months, but on the return of the cool season, with a lower evaporation rate, growth was resumed and several plants came into flower. This appeared to be not wholly the stimulus of lower temperature, since I was informed by good authority that the plants renewed their various activities whenever rains chanced to come, whatever might be the season.

From the observations last given it appears that sufficient moisture persists in the soil to tide perennials over the long periods of drought, although not in sufficient amount to permit active growth during the dry seasons. This is not an uncommon occurrence with desert plants. For example, whenever in the Tucson region the arid seasons are uncommonly long, or there has been a relatively small rain, as 25 per cent less than usual, much of the vegetation may remain dormant. Under such conditions an evergreen like _Covillea tridentata_ drops all save the youngest and smallest leaves and maintains this nearly defoliate condition for long periods, with little other change. The plant behavior noted is always connected with insufficient moisture.

The soil in the oueds and probably also in the reg, as well as that of the dayas, is of considerable depth. The soil depth, or rather the depth before solid rock is encountered in the valley of the Oued M’Zab at Ghardaia, is from 20 to 30 meters, or even more; at the Daya Tilrempt the depth to water in one of the wells is about 95 meters. Although there is much gravel and sand in the fill of these depressions, it is likely that they afford the most favorable soil conditions in the desert for the development of a large root-system. Be that as it may, it is certain that large plants, such as _Tamarix_ and _Pistacia_, occur only in such places.

Where there is most soil on the hamada, as in certain pockets near the old hill town of Ghardaia, abandoned several centuries since, one finds also the most and largest plants of this, the hamada, formation. Moreover, at the time in November when growth was noticed in the oued plants, it was also taking place in these favoring situations, about 50 meters above the level of the valley of the M’Zab and much above any possible permanent water-supply such as is afforded by a water-table.

The various habitats, therefore, not including the oases, are naturally closely associated with soil differences. These are few and, in nearly each case, to name is sufficient definition. There are dunes (areg), hamada or stony desert, reg or alluvial desert, the daya, and the flood-plain of the oueds (reg?). Modification of the hamada, reg, and oued flood-plain occurs whenever white hard-pan (caliche) is present. The soil of the reg, often that of the oued flood-plain and that of the daya, is fine alluvial and is relatively or actually deep. The hamada has the poorest soil condition, being underlaid by rock, and often or always by hard-pan as well. Large stones and boulders are embedded in the soil or lie on the surface. A modification of the hamada occurs whenever sand is strewn over its surface, even if the sand is only a few centimeters in thickness. This acts as an effectual mulch, increasing the retentive capacity of the soil, and very strikingly changes the character of the vegetation. The final habitat to be mentioned is the salt spot, or chott, where gypsum constitutes an important salt.

As regards their relative importance the habitats in southern Algeria can probably be grouped in the following sequence, a relation which very possibly holds good for the Sahara taken as a whole: Hamada, dune, oued, flood-plain, reg, daya.[36]

The habitats are unlike as regards the relation to the rainfall and its effects. This is in part due to the differences in soils, or their depth, and in part to topographical differences. By the latter the low-lying areas receive a relatively large amount of water; and since their soil is relatively deep the water is retained longer than on the hamada, for instance, where the soil is shallow.

The habitat preferences of the plants of southern Algeria are marked, as would be expected from the striking differences in the habitats. On the dunes, for example, we find drinn (_Aristida pungens_) as possibly the most commonly occurring and the most widely distributed sand-plant. One finds on sandy areas also _Tamarix_ sp., _Euphorbia guyoniana_, _Ephedra_ sp., _Retama retam_, _Limoniastrum guyonianum_ (zaita), and other forms in smaller numbers. On the oued banks there are _Tamarix_, _Nerium oleander_ (Laghouat), and, near the oases, date palm and other introduced plants. On the flood-plains will be found a large number of species, among the most typical of which are _Peganum harmala_, _Retama retam_, _Ephedra_, _Genista_ sp., and _Haloxylon_ sp. The typical plants of the dayas are _Pistacia atlantica_ and _Zizyphus lotus_, the latter occurring on flood-plains and the reg as well. On the chotts we find mainly such halophytes as _Anabasis articulata_, _Halocnemon strobilaceum_, _Salsola_ sp., _Limoniastrum guyonianum_, and others. The flora of the reg, so far as my observations show the conditions obtaining, is essentially like that of the flood-plains, which would be expected from the relation of the two habitats. South of Biskra, however, where the reg is probably not of fluvial origin, one finds a forest of _Zizyphus lotus_, and much _Ephedra_ sp. among the most striking forms. Finally, the flora of the hamada, which has a peculiar stamp, can be briefly characterized.

On the hamada are to be found the fewest species and the smallest individuals. Probably most perennials of the hamada are under 30 cm. in height. Among the species characteristic of the hamada are _Artemisia_, _Teucrium_, _Deverra_, _Centaurea_, _Acanthyllis_, _Thymelæa_, _Echinops_, _Henophyton_, and _Haloxylon_. The last named is possibly the most widely distributed, occurring in other habitats as well.

ROOT-CHARACTERS AND SPECIES DISTRIBUTION IN SOUTHERN ALGERIA.

The general principles bearing upon the relation between the type of root-system and the distribution of the species, as observed in southern Algeria, can be briefly stated. Often the relation is close and apparent, but not always. For example, large perennials, such as _Tamarix_ and _Zizyphus_, have an obligate specialized root-system, with a long tap-root. These plants naturally occur where there is considerable depth of soil, and hence are not to be found on the hamada, for instance, where it is shallow. On the other hand, such species as have a generalized root-system, like _Acanthyllis_ and _Haloxylon_, are to be found on the hamada, but they occur also in other habitats where the soil is deep. The last type of root-system is flexible, accommodating the species to a wide range of soil conditions. In doing this the change in form is almost a change in type; for example, the roots of _Haloxylon_ on the hamada at Ghardaia develop both laterals and a main root, but in deeper soil, as at Biskra and Ghardaia also, the laterals are nearly suppressed and the tap-root is the striking feature. A marked exception to the rule that plants with a generalized type of root-system have also the widest local distribution lies in _Peganum harmala_, which, having roots of this character, is nevertheless restricted to habitats where the soil is deep. The species is a half-shrub, having a perennial subterranean portion and a short-lived subaerial portion, the life of which appears to depend on the character of the water-supply. As learned by Fitting, the species can develop in its leaves a very dense sap, enabling it to extract water from a very dry soil.[37] In spite of this fact it appears to act like an annual in certain regards, requiring at all times, particularly during the most arid season, a relatively good water-supply.

Thus, in brief, a study of the relation of the root-type of the Algerian plants to the plant’s distribution leads to the same general conclusion already obtained by similar but more extended study in the Arizona desert, namely, that the connection is often a very close one and often of definitive importance. Where the root-type is an obligate type the distribution of the species is much restricted, but where it undergoes modification with changed environment the distribution of the species is much less confined. It is of interest to note especially that as a rule it is the latter kind of root-system that is developed by such plants as occur where the soil conditions are most arid, that is, on the hamada or its equivalent, and not the former, from which it follows that the generalized type of root-system is really the xerophytic type _par excellence_, and not the type with the most deeply penetrating tap-root, as might be supposed.

THE BIOTIC FACTOR.

It will be well to summarize some of the main facts regarding another and important environmental feature of the Saharan plants, namely, the relation to herbivorous animals. In the western Sahara, wherever there is any forage, animals which subsist on it are to be found. Of the wild animals the gazelle is probably the most numerous and the most destructive. All travelers across the desert have noted the presence of this animal. In the northern Sahara Tristram remarked its abundance fifty or more years ago, and it may be frequently seen by the traveler at the present time. In addition to the native animals, the domestic animals, especially the sheep, goats, and camels, are very numerous, very destructive of plants, and range great distances for food. As a result, an area around every well or oasis, extending as far as 40 kilometers or even much more than this,[38] is repeatedly grazed over and has been utilized in this manner for centuries. As a result only the poisonous or the distasteful species, or the plants especially well armed, are left undisturbed to grow and reproduce, while the balance are more or less consumed, frequently so much so as to be quite unrecognizable. From the large number of camels, sheep, and goats which range the desert pastures it might be concluded that the leading types of plants to be found would be such as are not eaten by them, but this is not the case. On the other hand, possibly the most generally consumed form is _Haloxylon_, which grows on the oued flood-plain, the reg, and the hamada. Thus, so far as this type is concerned, there is probably little or no diminution in numbers because of the attacks of animals.

A similar conclusion would doubtless be drawn after study of other forms, but there lies at least one notable exception, namely, the influence of animal grazing on the distribution of the betoum (_Pistacia atlantica_). The betoum, which is the largest arboreal species in the Sahara, is confined to the region of the Dayas; that is, to the country immediately south of Laghouat. The tree is unarmed and is eagerly sought after by all herbivorous animals for its foliage and tender twigs. Owing to the presence of such animals, wild and domesticated, the young tree would have no chance to survive were it not that, growing in association with it, is the jujube (_Zizyphus lotus_), which is armed and is not eaten by any animals. The jujube affords safe protection for the seedling betoum, and in its capacity as nurse prevents predatory attacks by animals during the critical period. The survival (and probably the distribution as well) of the betoum is mainly conditioned on the presence of its protector.

When I first visited southern Algeria it seemed improbable that any portion of it, or at least any portion that I should be likely to see, would exhibit the possibilities of plant growth as unaffected by herbivorous animals. But finally there were found two classes of plant formations in which animals either had not intruded at all, or not to a harmful degree. One of these is the wide flood-plain of the Oued M’Zab, or one of its tributaries, lying about 50 kilometers east of Ghardaia. Despite the fact that the flocks are very numerous in the vicinity, and that the flood-plain is on the regular caravan route between Ghardaia and Ouargla, there are few signs of grazing. The entire plain is so well covered by shrubs that the vegetation gives the tone to the landscape—a rare thing in the desert. Here one finds _Retama retam_, _Genista saharæ_, and _Ephedra_ sp., as well as other species in fair abundance. Thus the plants are not only numerous, but are of a good size. It should be observed that on either side of the flood-plain, on the hamada, scarcely any vegetation may be found.

The second formation referred to is that of protected areas, especially at Ghardaia, which have been little disturbed at any time, and portions of them not at all disturbed for centuries. These are the cemeteries. Such areas are situated not only in the floor of the M’Zab Valley, where the plant conditions are relatively favorable, but also on the hamada, where they are relatively very unfavorable. Both in the valley and on the hamada, as well as on the valley wall between the two wherever there chances to be soil, the plants are relatively numerous and of fair size. This fact has been detailed under the section on Ghardaia and need not be more than mentioned here.

From these two general observations (exceptions to the usual conditions), that on the flood-plain of the Oued M’Zab and the cemeteries at Ghardaia, it is concluded that the grazing of animals has had a very marked influence in modifying the flora of southern Algeria. So far as could be told from the limited opportunity to observe, the modification has gone along on at least two lines, which are, of course, closely related. The size of the plants eaten is much under normal for the particular locality, and at the same time the capacity of the plant for reproduction has been greatly lessened. That such species as are not touched by animals have not spread more rapidly, or even have not become the dominant forms, is an interesting problem, and one that would have to be worked out for each species. It can be suggested, however, that the restricted distribution probably lies in the fact that the struggle of desert plants is mainly with an adverse physical environment rather than with one another, and that such conditions would not be affected by grazing animals. For example, large shrubs do not occur on the hamada, hence _Tamarix_, _Zizyphus_, and _Rhus_, as well as _Peganum harmala_, are limited to situations where the soil is fairly deep and the water relatively favorable. _Euphorbia guyoniana_ also occurs only in sandy soil, and the number of plants limited in their distribution to soil characters is necessarily a large one.

COMPARISON OF SOME GENERAL FEATURES OF THE VEGETATION OF SOUTHERN ALGERIA AND OF SOUTHERN ARIZONA.

When we compare the most striking characteristics of the vegetation of the Algerian Sahara with that of the Tucson region where the Desert Laboratory is situated, we find some interesting differences, which may be summed up in the terms “desert” and “semi-desert,” as applied to the two widely separated regions. What is meant by these terms will be apparent from the following short characterization:

Passing into the Sahara from the Saharan Atlas, over the route which I followed, one encounters a great variety of topography, of which the most extensive may for the moment be classed as plains. The plains are divided into three well-marked regions, that of the dayas, the Chebka, and the Gantara (hamada). The topography is further diversified by oueds and their flood-plains and by low, flat-topped mountains. On the northern portion of the plains one encounters a sparse population of low perennials, and as Ghardaia is approached the plains vegetation becomes continuously poorer until at Ghardaia there appears to be none. On the hamada between Ghardaia and Ouargla areas are to be crossed, several kilometers in width, where perennials are wholly lacking. The decreasing plant population of the plains, until it entirely disappears, is entirely due to the increase in aridity as one goes from the mountains to and across the Ghardaia-Ouargla country. The low mountains are almost entirely barren. The flood-plains of the oueds, however, support a surprisingly luxuriant population of perennials.

Should we contrast the topography and vegetation of the Algerian Sahara with that of southern Arizona we would find little that is similar and much that is different. The wide-stretching plains (bajada) of southern Arizona are well covered with perennials of good size. The water-courses are fringed with trees, and often an open forest is to be found on the flood-plains. The low mountains have a fairly dense plant population, partly of trees, and the lower mountain slopes are often covered with a mixed flora of shrubs and trees. It may be said that there is probably no large area in southern Arizona, where the soil conditions are favorable for plants, where the water conditions are too meager to support a perennial flora of some sort. The greater aridity of the northern portion of the Sahara is evident, therefore, from the great contrast in its flora.

In crossing the plains of southern Algeria one is likely to call a region barren when close inspection will show that this is not the condition. In fact, it was found that areas on the plain, 16 by 16 meters in extent, carried as many as 330 or more perennials, although a casual glance did not reveal the presence of any conspicuous vegetation.[39] Both of these conditions are the immediate result of the small rainfall. The reason for the large number of plants in certain areas, as above noted, lies in their small size, since it would probably be difficult to find an equal number on this area were the plants as large, for example, as in southern Arizona. The fact that the perennials are inconspicuous is in part because they are small and in part because the leaves are either absent or greatly reduced.

When viewed somewhat more closely, one finds other features in which the flora of southern Arizona and of southern Algeria are unlike. Travelers, botanical as well as non-botanical, have described the armed condition of the Saharan plants until the impression is general that such plants as persist from season to season are usually well provided with spines. What may be the proportion of armed to unarmed plants in the northern Sahara I do not know, but to a person familiar with the plants of southern Arizona, where spinose forms are very numerous, the Algerian plants do not appear especially well protected. As this appears to be a general condition, it is scarcely an accident that the spines of the American species of the genus _Zizyphus_, for example, are much better developed than are those of the Algerian representative of the same genus. From the circumstance that grazing by wild as well as by domestic animals is very destructive in Algeria, apparently more so than in Arizona, where the results of grazing are scarcely to be noted, the general facts regarding spininess in plants, as given above, suggest the really small influence such animals play in shaping such a character in desert plants.

Finally, it need only be remarked that plants with a water-balance are wanting in southern Algeria, and that they constitute one of the striking features of the flora of the southwestern United States.

[Plate 2:

Illustration: Fig. 4. Shoot-habit of Acanthyllis tragacanthoides. Laghouat.

Illustration: Fig. 5. Shoot-habit of Zollikoferia spinosa. Laghouat.

Illustration: Fig. 6. Acanthyllis tragacanthoides on sandy plain. Laghouat.

Illustration: Fig. 7. Zollikoferia spinosa in habitat, plain (hamada). Laghouat.]

[Plate 3:

Illustration: Fig. 8. Detail of north slope of Nomad Mountains where Zollikoferia spinosa is the dominant species. Laghouat.

Illustration: Fig. 9. Vegetation of plain (hamada) at Tilrempt. The conspicuous shrub is Haloxylon articulatum.

Illustration: Fig. 10. The Daya of Tilrempt from the plain, showing the character of the depression.]

[Plate 4:

Illustration: Fig. 11. Near view of the Daya of Tilrempt. The fortified stage station, bordj, and a nomad camp are to be seen. The flattened, level, lower surface of the trees is the effect of grazing, mainly by camels.

Illustration: Fig. 12. Jujube (Zizyphus) shrubs and betoum (Pistacia) at the Daya of Tilrempt.]

[Plate 5:

Illustration: Fig. 13. A young specimen of betoum in the midst of a protecting jujube, at the Daya of Tilrempt.

Illustration: Fig. 14. Leaf and shoot habit of the jujube (Zizyphus lotus). Daya of Tilrempt.

Illustration: Fig. 15. Leaves of the betoum (Pistacia atlantica), from the Daya of Tilrempt.]

[Plate 6:

Illustration: Fig. 16. South wall of the valley of the Oued M’Zab at Ghardaia.

Illustration: Fig. 17. Detail of an eroded bank of the Oued M’Zab at Ghardaia. The overhanging stratum is hardpan similar to the “caliche” of southwestern United States.]

[Plate 7:

Illustration: Fig. 18. Shoot-habit of Haloxylon articulatum, from the plain near the Daya of Tilrempt.

Illustration: Fig. 19. Agriculture at Ghardaia. The fields are divided into plots about 3 by 4 feet in size, for the more economical use of water. Young barley is shown growing (November).]

[Plate 8:

Illustration: Fig. 20. Vegetation in upper part of a “draw” on plain north of the M’Zab Valley Ghardaia.

Illustration: Fig. 21. Situation of square No. 2. on plain north of valley at Ghardaia.]

[Plate 9:

Illustration: Fig. 22. Capparis spinosa at base of valley wall at Ghardaia.

Illustration: Fig. 23. Leaves of Capparis spinosa, from Ghardaia.]

[Plate 10:

Illustration: Fig. 24. Leaf-habit of Dæmia cordata. Ghardaia.

Illustration: Fig. 25. Shoot-habit of Salvia ægyptica. Ghardaia.

Illustration: Fig. 26. Upper surface of rocks, showing small incrusting lichens, on a low mountain about 5km. north of the M’Zab Valley, Ghardaia.]

[Plate 11:

Illustration: Fig. 27. Habitat of Peganum harmala, at Melika, Ghardaia.

Illustration: Fig. 28. Shoot and leaf habit of Peganum harmala, from Ghardaia.]

[Plate 12:

Illustration: Fig. 29. Habit of Henophyton deserti at Ghardaia when growing in a protected situation (cemetery).

Illustration: Fig. 30. Leaf-habit of Henophyton deserti. Ghardaia.]

[Plate 13:

Illustration: Fig. 31. View in a cemetery at Ghardaia, to show the relatively abundant vegetation.

Illustration: Fig. 32. View in an M’Zabite cemetery, Ghardaia. Haloxylon articulatum is the leading species shown.]

[Plate 14:

Illustration: Fig. 33. Habit of Deverra scoparia, from plain about 3km. north of M’Zab Valley, Ghardaia.

Illustration: Fig. 34. Root-habit of Gymnocarpos fruticosum, from plain about 3km. north of M’Zab Valley, Ghardaia.]

[Plate 15:

Illustration: Fig. 35. Root-habit of a mature specimen of Peganum harmala, from the floor of the M’Zab Valley near Ghardaia. The soil at the place is comparatively deep. (See text for further explanation.)

Illustration: Fig. 36. Root-systems of Helianthemum sessiliflorum (right), Haloxylon articulatum, and Nolletia chrysocomoides (left), from the flood-plain of the Oued M’Zab, Ghardaia.]

[Plate 16:

Illustration: Fig. 37. Shoot and root habit of Citrullus colocynthis. Oued M’Zab, Ghardaia.

Illustration: Fig. 38. Euphorbia guyoniana, in the valley of the Oued M’Zab at Ghardaia.]

[Plate 17:

Illustration: Fig. 39. Habit of Euphorbia guyoniana. Ghardaia.

Illustration: Fig. 40. To the left, shoot of Centaurea sp., showing effects of grazing; to the right, shoots of Teucrium pseudo-chamæpitys. From Ghardaia.]

[Plate 18:

Illustration: Fig. 41. Habit of Salsola sp. (below), and Echinopsilon muricatus, from the M’Zab Valley, Ghardaia.

Illustration: Fig. 42. Ghardaia to Ouargla. View overlooking the hamada about 28km. from Ghardaia. The relatively abundant vegetation is associated with a light cover of sand over the area shown. The leading species are Aristida, Deverra, and Haloxylon.]

[Plate 19:

Illustration: Fig. 43. Retama retam, in dunes about 58km. from Ghardaia.

Illustration: Fig. 44. Dates at the bordj Zolfana, about 58km. from Ghardaia. One of two wells encountered between Ghardaia and Ouargla.

Illustration: Fig. 45. Ghardaia to Ouargla. View overlooking flood-plain of the Oued M’Zab, or a tributary of this oued. The adjacent upland is apparently without plant life.]

[Plate 20:

Illustration: Fig. 46. Vegetation on edge of the Oued M’Zab, about 63km. east of Ghardaia, showing habitat of Rhantherium adpressum in foreground.

Illustration: Fig. 47. Sandy flood-plain of the Oued M’Zab, about 63 km. from Ghardaia. Retama, Genista and Ephedra are the leading plants in this place.

Illustration: Fig. 48. Habit of Ephedra alata in habitat shown in figure 47. This specimen was 1.5m. high.]

[Plate 21:

Illustration: Fig. 49. View of habitat of Ephedra alenda, 138km. from Ghardaia.

Illustration: Fig. 50. Detail of suckering habit of Ephedra alenda, from habitat shown in figure 49.]

[Plate 22:

Illustration: Fig. 51. Vegetation, mainly Ephedra and Retama, of the western edge of the Chott Mellala.

Illustration: Fig. 52. Approach to western edge of the Chott Mellala, showing characteristic rounded hills, or mamelons.]

[Plate 23:

Illustration: Fig. 53. View between the Ouargla plain and the Chott Mellala, showing characteristic appearance of eroded hills.

Illustration: Fig. 54. Looking toward the Ouargla plain (reg).

Illustration: Fig. 55. Shallow well about 25km. north of Ouargla.]

[Plate 24:

Illustration: Fig. 56. Ouargla to Touggourt. Leaf-habit of Limoniastrum guyonianum. The surface of the leaves is covered with an incrustation of salts.

Illustration: Fig. 57. Habit of Limoniastrum guyonianum. About 25km. north of Ouargla.]

[Plate 25:

Illustration: Fig. 58. Vegetation of the reg. desert, about 25km. north of Ouargla. Ephedra and Retama are the leading species of the area a spreading dune.

Illustration: Fig. 59. Shoot-habit of Halocnemon strobilaceum, about 28km. north of Ouargla.]

[Plate 26:

Illustration: Fig. 60. Habit of Halocnemon strobilaceum, on the edge of a chott, about 28km. north of Ouargla.

Illustration: Fig. 61. Shoot and leaf habit of Anabasis articulata, about 32km. north of Ouargla.]

[Plate 27:

Illustration: Fig. 62. Detail of the shoot-habit of Salsola tetragona?, about 25km. north of Ouargla.

Illustration: Fig. 63. Habit of Nolletia chrysocomoides near the edge of a chott, about 80km. north of Ouargla.]

[Plate 28:

Illustration: Fig. 64. Tamarix sp. as a sand-binder near Bled-el-Amar, south of Touggourt.

Illustration: Fig. 65. Biskra. Habitat of Euphorbia guyoniana, looking toward the Djebel Maouya, with the Chaine de Sfa in the background.]

[Plate 29:

Illustration: Fig. 66. Characteristic vegetation on the north slope of the Djebel Bou Rhezal, Biskra. Haloxylon scoparium is a prominent species.

Illustration: Fig. 67. North base of Ed Delouatt hills, west of Biskra, showing the low-facing dunes. To the right is a glimpse of an oued which pierces the hills in the middle distance.]

[Plate 30:

Illustration: Fig. 68. Flood-plain of the Oued Hamman es Salahin, Biskra. Halophytes of various species occupy the foreground.

Illustration: Fig. 69. Habitat of Phelypæa violacea shown in figure 70.]

[Plate 31:

Illustration: Fig. 70. Young shoots of Phelypæa violacea, at north base of Ed Delouatt hills, Biskra.

Illustration: Fig. 71. Habit of young specimens of Phelypæa. Except for about 15cm., plants were buried by sand. Biskra.

Illustration: Fig. 72. Asphodelus sp. at north base of Ed Delouatt hills, Biskra. Photographed in March.]

[Plate 32:

Illustration: Fig. 73. Detail of square No. 1, on low hills north of Biskra.

Illustration: Fig. 74. Vegetation of north slope of the low hills which are north of Biskra. Ferula vesceritensis is the perennial appearing in the figure.]

[Plate 33:

Illustration: Fig. 75. Detail of square No. 2, neat area shown in fig. 74. Ferula and Haloxylon are the leading species.

Illustration: Fig. 76. Root-habit of Haloxylon scoparium, from a wash near Biskra.

Illustration: Fig. 77. Young shoots and mature roots of Ferula vesceritensis. North of Biskra.]

[Plate 34:

Illustration: Fig. 78. Root-habit of Fagonia growing on edge of a wash. Biskra.

Illustration: Fig. 79. Large lateral root of Haloxylon, with numerous deciduous rootlets, no longer functional.

Illustration: Fig. 80. Shoot-habit, taken from above, of Fagonia from the flood-plain of a small oued near Biskra.]

[Plate 35:

Illustration: Fig. 81. Spring annuals, March 17th, on north slope of the Djebel Bou Rhezal, Biskra.

Illustration: Fig. 82. Root and shoot habit of Peganum harmala, Biskra. The main root is especially well developed although the species has a generalized root-system. (See text.)]

[Plate 36:

Illustration: Fig. 83. Annuals growing with Peganum, near Biskra.

Illustration: Fig. 84. General view of the north face of the Djebel Bou Rhezal, Biskra. Apparently barren, plants are rather numerous in the rock crevices and small washes of the mountains.]

FOOTNOTES:

[Footnote 1: Kearney and Means, Agricultural explorations in Algeria, Bul. No. 80, Bur. Plant Ind., U. S. Dept. Agric., 1905.]

[Footnote 2: A. Engler, Die Vegetation der Erde IX. Die Pflanzenwelt Afrikas. 1 Bd., 1910, page 902.]

[Footnote 3: The seasonal distribution of rain (by percentages) is somewhat different from that given by Engler, which is given in the accompanying table.

+---------+-----------+-------+---------------+ | Season. | Littoral. | Tell. | High plateau. | +---------+-----------+-------+---------------+ | Winter | 41 | 36 | 45 | | Spring | 27 | 32 | 46 | | Summer | 4 | 7 | 11 | +---------+-----------+-------+---------------+

]

[Footnote 4: The most recent records available, 1897-1908, do not give a satisfactory account of the precipitation on the desert. For instance, meteorological records covering eight years for El Golea do not report on the rainfall. Records of five years at In Salah take the rainfall into account on one year only, and on that year no precipitation occurred.]

[Footnote 5: Unless otherwise stated, the climatological statistics given in this paper were taken or compiled from Observations Météorologiques du Riseau Africain, 1907-1908. The evaporation data are based on readings of the Piche evaporimeter. The amount of evaporation given in the text can be reduced to the evaporation from a free-water surface by multiplying by 0.737 (Meteorological Notes, J. I. Craig. Cairo Scientific Journal, vol. vi, May 1912).]

[Footnote 6: At Touggourt, however, no rain was reported during the summer season of 1908.]

[Footnote 7: Kearney and Means, _loc. cit._]

[Footnote 8: The Root Habits of Desert Plants. W. A. Cannon. Carnegie Institution of Washington Publication No. 131. 1911. This paper gives a descriptive classification of the main root-types in the deserts of the southwest, in which such a root as found in _Acanthyllis_ is called _generalized_, in distinction to roots like those of most of the cacti, or _Zizyphus_, the former having a system wholly superficial and the latter a system wholly deeply placed, as _specialized_. The specialized root-systems appear to be so fixed in character as to be not easily changed, while the generalized type is flexible. It will be self-evident that the type of root-systems may be of great importance in determining the local distribution of a species.]

[Footnote 9: Compare the root-system of _H. scoparium_ at Biskra, p. 64.]

[Footnote 10: The most striking change in the general character of the vegetation which the traveler notices as he goes from the less arid to the more arid portion of southern Algeria is its decrease in amount. This occurs through dwarfing effects of whatever cause and through a decrease in the number of individuals. Within certain limits the results observed are to be attributed mainly to the first of these, since there is often a surprisingly large number of perennials on any given area. But in other and more intensely arid regions (as portions of the Arabian-Egyptian desert, and indeed a limited area on the hamada between Ghardaia and Ouargla) plants are wholly wanting. Whether such is generally the case on the reg or the hamada farther south in southern Algeria is not known.]

[Footnote 11: Tristram remarks that it “seems that the larger wild animals have been rapidly decreasing in numbers and are in process of speedy extinction. Dr. Shaw, 150 years since, enumerates in his travels . . . five species of ruminants, which from his descriptions must be the bubale, the aoudad or wild sheep, the addax, and the gazelle, as well as the stag. . . . As the population has not increased, but rather retrograded, we can only surmise that the substitution of the flint and steel gun for the matchlock of the Bedouin . . . has been fatal in its results to all larger game.” It may be remarked that the French impose such restrictions on the Arab as regards the character of the guns he may use (only the army and certain officials of the government employing modern arms) that for the region visited Tristram’s description holds fairly well for to-day.]

[Footnote 12: Statistique Générale de l’Algérie, 1908.]

[Footnote 13: Un voyage botanique au Sahara, Bull. Soc. Bot. Belg., 1898.]

[Footnote 14: Massart, _loc. cit._]

[Footnote 15: Joan’s Guide de l’Algérie et de la Tunisie.]

[Footnote 16: Massart, _loc. cit._, p. 314, suggests that the Sahara may be gradually becoming more arid and says that as a result the betoum is becoming more and more rare and may become extinct. He says “L’extinction du Pistacia atlantica présente le caractère, tout à fait exceptionnel, d’ètre uniquement l’effet du climat.” It will appear from what is said in this study regarding the effects of grazing in general, as well as the especial effects on the betoum, that if the betoum is becoming extinct the sole cause, or perhaps the main cause, is not the adverse climate.]

[Footnote 17: Dr. Charles Amat, Le M’Zab et les M’Zabites, p. 70, gives a somewhat higher temperature for the rocks of the southern Chebka, placing it at 90° to 100° C., or even higher.]

[Footnote 18: Les Oasis du Souf et du M’Zab, La Géographie, 1902.]

[Footnote 19: Foureau, d’Alger au Congo par le Tchad, 1902, mentions having met with indications of early settlement of the Sahara by people now forgotten, and whose tombs, inscriptions, and other remains, were well known by his Touareg servants, although not at all understood by them. So far as I have learned, however, it is not supposed that the region of the M’Zab was inhabited before the coming of the Beni M’Zabs.]

[Footnote 20: There are seven cities of the Beni M’Zab, of which five lie in the M’Zab Valley, close to one another. These are El Ateuf, Ben Noura, Melika, Beni Isguen, and Ghardaia. In the pre-French times these cities were bound together in a confederacy with Ghardaia as the capital. The M’Zabites are at present, and probably always have been, a peaceful trading folk. They are heterodox Moslems. In an early time they aroused the antagonism of their more warlike as well as more orthodox Arab neighbors of the Tell, who drove them away from the coast region, and again from Ouargla and other places settled by them, until safety was at last secured in the eleventh century in the “inhospitable Chebka.” Palm gardens were established which for centuries have been irrigated laboriously by very primitive methods, and the inhabitants have accumulated wealth in flocks and by barter. The relatively large population (there were 92,761 inhabitants in 1908), the really great number of domestic animals, and the great length of time which the region has been occupied, are all factors of importance in bringing about a modification in whatever way of the primitive flora.]

[Footnote 21: In 1908, according to the Statistique générale de l’Algérie, there were cultivated in the territory of Ghardaia 572,114 fruit trees, among which were: almond, 5,850; fig, 101,722; date palm, 209,898; other sorts of fruits, 211,761. There were also 17,268 hectares of grain under cultivation.]

[Footnote 22: We were informed by our Arab attendant that the kabar, particularly the fruit, made such animals as ate it insane. The spicy flavor of the plant might otherwise be distasteful to animals.]

[Footnote 23: Fitting (Die Wasserversorgung und die osmotischen Druckverhältnisse der Wüstenpflanzen, Zeitschr. f. Bot., 4, 209-275, 1911) states that water-storage tissue is wanting, that in addition to being large, the leaves are much divided, without trichomes, and provided with thin cuticle. The stomata are fairly large, rather numerous, and not sunk. The leaves transpire rapidly and wilt soon after being removed from the stem. The osmotic pressure of the cell-sap of the leaves was found to equal 35.3 to 64 atmospheres, from which it is assumed that this plant, like others growing under desertic conditions, has a root cell-sap of great osmotic pressure, which permits it to extract water from a fairly dry soil or at a rather rapid rate. But neither at Biskra nor elsewhere, so far as I know, does _Peganum_ grow where the conditions are extreme, as might be concluded from the habit of the plant as given above.]

[Footnote 24: It has already been shown that the number of days on which rain may be expected to fall each year is greater at Laghouat than at Ouargla, and probably at Ghardaia also. According to reports, the rainy days at Laghouat vary from 20 to 84 (seven years’ observation), with an average of 49 each year. The average number of days on which rain falls at Ouargla is 14.2. The amount of rain at Laghouat is 200 mm., at Ouargla 90.2 mm. It would appear, therefore, that the average rain at Laghouat is less in amount than the average rain at Ouargla; or, in other words, it points to the torrential as being the type of the desert storm. Since, other things being equal, the greater storms would penetrate the ground the most deeply, we may here have an explanation of the emphasis at Ghardaia on the tap-root as against the generalized root as the type of the root-system.]

[Footnote 25: The Root Habits of Desert Plants, _l. c._]

[Footnote 26: Compare the root-system of the species at Biskra, p. 64.]

[Footnote 27: In the vicinity of Tucson (see The Root Habits of Desert Plants) is to be found a slender-stemmed _Opuntia_ whose roots are fleshy and are placed within 2 to 4 cm. of the surface of the ground. It has been observed that if the roots are examined in the midst of a dry season, as in June, they are gorged with water, but if the soil is removed for a few hours they become shriveled. A similar habit was seen in another species of the same genus. Two other genera of the cacti from the Tucson region have the water-storage organs wholly or partly protected by the soil. In _Cereus greggii_ the subterranean portion forms an organ 15 to 30 cm. in diameter, and in the other form the fleshy subaerial stem is partly drawn under the surface of the soil, so that only the flat upper surface is visible.]

[Footnote 28: In the case of annuals the differences in development of the shoot between plants well watered and those with only a meager supply are very striking. In one instance in the Tucson region specimens of _Parietaria debilis_ growing in extreme conditions, one moist and the other arid, varied in length between 39 cm. and 8 mm., or a difference with a ratio of 49 to 1. (Root Habits of Desert Plants, _loc. cit._)]

[Footnote 29: The vegetation in the vicinity of Biskra is so well known that a sketch will suffice as a basis of comparison with the flora and conditions of plant life farther south.]

[Footnote 30: Liste des plantes observées aux environs de Biskra et dans l’Aurés, Trabut _et al._, Alger, 1892.]

[Footnote 31: Briefly the case is as follows (see The root systems of desert plants, _loc. cit._): _Opuntia arbuscula_ growing near Tucson develops fleshy roots, but what is probably the same species growing about 100 miles distant has fibrous roots. Also, seedling opuntias have fleshy roots. _Opuntia vivipara_, which occurs naturally in the bottom of an arroyo (oued), may or may not have fleshy roots. By preliminary series of experiments it was learned that all opuntias tested which had an abundant water-supply developed fleshy roots, and it is assumed from this that the differences in this character as observed in nature had also such a physiological basis.]

[Footnote 32: Eine botanische Exkursion nach Algier and Tunis, Bericht der Senckenbergischen Naturforschenden Gesellschaft in Frankfort a. M., p. 76, 1910.]

[Footnote 33: Plants with subterranean water-storage organs—bulbous plants—are said to be a feature of the High Plateau.]

[Footnote 34: Through Timbuctu and across the Great Sahara, 1912, p. 266.]

[Footnote 35: Soil samples were taken from the plain about 3 kilometers north of Ghardaia from an area where the vegetation is relatively good (see p. 40). The surface of the soil at the place is slightly depressed. Soil samples from the depressed area and samples from portions of the plain adjacent to it were settled under water, with the result that the proportion of fine soil was found to be less in the depressed area. Samples of the soil from the lower area were placed in air-tight cans and the moisture content determined subsequently. The soil was found to contain 0.8 per cent water. Through the kindness of the Bureau of Soils, U. S. Department of Agriculture, the critical moisture-content of the same soil was determined, which was 5 per cent. The critical moisture-content of mesa soil, taken from the creosote-bush slope at the foot of Tumamoc Hill, Desert Laboratory, as determined by the Bureau of Soils, is 10.5 per cent, which forms an interesting comparison of nearly similar situations.]

[Footnote 36: The mountains have been disregarded, since in southern Algeria they are nearly barren. The only exception to this that I saw was that of the crustaceous lichens in small numbers at Ghardaia. In the central Sahara, however, where the mountains are of great elevation, the mountain climate brings about favorable conditions for plant life.]

[Footnote 37: Die Wasserversorgung und die osmotischen Druckverhältnisse der Wüstenpflanzen. Zeitschr. f. Botanik, 4, 1911.]

[Footnote 38: Hayward, _loc. cit._, p. 320, says that at In Salah camels are driven 200 kilometers before finding suitable grazing-grounds.]

[Footnote 39: It should be understood that such densely populated areas are separated by wide stretches where are few or no plants.]

Transcriber's note:

pg 15 Added period after: vegetation of the country

pg 17 Changed: _Quercus bollota_ to: _ballota_

pg 24 Changed: _Rhus oxycantha_ to: _oxyacantha_

pg 25 Changed: _Artistida pungens_ to: _Aristida_

pg 32 Changed: Astericus pygmæus to: Asteriscus

pg 35 Changed: _Petama spherocarpa_ to: _Retama sphærocarpa_

footnote 21 (pg 38) Changed: Statique to: Statistique

pg 43 Changed: were in in pre-M’Zabite to: were in pre-M’Zabite

pg 43 Changed: iteslf to: itself

pg 50 Changed: _Zillikoferia_ to: _Zollikoferia_

pg 53 Changed: Fig. 441. to: 44.

pg 54 Changed: _Astericus graveolens_ to: _Asteriscus_

pg 55 Changed: _Tragonum nudatum_ to: _Traganum_

pg 56 Changed: _Phargmites_ to: _Phragmites_

pg 57 Changed: Maricandia cinerea to: Moricandia

pg 57 Changed: Haloxylon articulata to: articulatum

pg 57 Changed: Sorzonera nudulata to: Scorzonera undulata

pg 59 Changed: P. albans to: albicans

pg 63 Changed: _Ferula visciritensis_ to: _vesceritensis_

pg 65 Changed: _Ferula vesciritensis_ to: _vesceritensis_

pg 73 Changed: edaphacic to: edaphic

Other spelling and formatting inconsistencies have been left unchanged