Chapter IV. Of these the Sundews (_Drosera_), Butterworts

(_Pinguicula_), and Bladderworts (_Utricularia_) supply very interesting examples within our own flora, which anyone may study on a holiday spent on the moors or mountains. The Sundews are familiar to all plant lovers--little plants of the bogland, usually growing among Sphagnum, and well distinguished by their leaves decked with spreading red hairs, each of which is tipped with a little drop of sparkling sticky fluid. It is these hairs or tentacles and their movements which place the Sundews among the most interesting of all plants. It is important to note that they are not hairs in the ordinary sense, which are organs of very simple structure arising from the epidermis or skin of the leaf. The tentacles of _Drosera_ have a complicated structure resembling that of leaves, and the tip is occupied by a gland which produces the sticky secretion already mentioned. These glands are exceedingly sensitive, and, moreover, sensitive in a selective way. They are unaffected by the drops of rain which frequently fall on them, but the touch of any solid body, especially of organic material, immediately affects them; most of all nitrogenous substances of any kind. Darwin found that a morsel of human hair weighing only 1/78,740 of a grain was sufficient to set the machinery of _Drosera_ in motion, and that immersion of a leaf in a solution of phosphate of ammonium so weak that each tentacle could absorb only 1/20,000,000 of a grain acted as a strong stimulus. In nature the stimulus is usually given by some unwary insect--a midge or other small flying creature--which, attracted by the bright colour or by the odour of the leaf, ventures too close, and becomes entangled among the sticky hairs. Then a most interesting series of events takes place. Almost at once the tentacles--first the ones actually touched, and then the adjoining ones--bend towards the point of disturbance, closing down one by one on the unfortunate victim till the leaf resembles a closed fist. At the same time the production of secretion increases, so as further to entangle the victim. When it is firmly secured, the secretion changes in character. Digestive ferments, closely resembling those by which animals digest their food, are poured out. These dissolve the animal’s body, all except the horny parts; the digested materials are then absorbed into the plant, which, as experiments show, benefits considerably by the addition to its diet of this animal food. When digestion is completed, the tentacles open again and prepare for a fresh victim. While the details of this remarkable process have been worked out only by careful and minute research in the laboratory, the main movements may be watched by anyone on any British moorland; or, bringing home a few plants in the damp moss in which they grow, we may amuse ourselves by experiments in feeding them.

In comparison with the Sundews, the other insectivorous plants which are included in the British flora are of less interest. The Butterworts (_Pinguicula_), of which four species are known in these islands, have a rosette of smooth, broad, yellowish leaves covered with glands which exercise the same functions as those of _Drosera_. To the touch of raindrops, sand-grains, or other inorganic substances they are indifferent; but a tiny insect alighting on the sticky leaf at once provokes an outpouring of secretion, while the leaf rolls inward from the edges till the victim is securely caught; it is then digested as in the Sundew.

The Bladderworts (_Utricularia_), of which several species may be found floating in boggy pools, are rootless, limp plants with finely divided leaves, among which are numerous little bladders (in reality strangely modified leaflets), and upright stems bearing pretty yellow Snapdragon-like flowers. The bladders do not help the plant to float, and appear to have for their sole function the securing of animal food. In the Common Bladderwort (_U. vulgaris_) they are about 1/10 inch long. At the upper end is a little hinged door, which is kept closed as by a spring against a thickened rim or door-frame. Outside the door are a few stiff hairs, a convenient perching-place for small aquatic creatures such as the minute Crustaceans known as Water Fleas. Should one of these try to explore the bladder, the door opens easily, but closes at once behind the rash wanderer, imprisoning it. The Bladderworts do not _digest_ the victims which they secure in this manner, but when the bodies are decomposed by means of bacteria, the products of decomposition are absorbed. How fatal this mousetrap arrangement is to Water Fleas can be determined by dissecting the bladders of the plant.

Thus far, then, as regards some of those peculiar members of our flora which make their living by the unusual method of stealing their neighbour’s goods, or which eke out their existence by the capture of animal food. Let us now take another line of exploration and consider the conditions which prevail on the loftiest portions of our islands, and how these affect the vegetation. Mountain-tops are always attractive and interesting places--the keen rarefied air, the freedom and openness of the summits, fill us with exhilaration. Our own mountains are not lofty; nowhere in the British Islands is a height of a mile attained. But we have only to ascend to a couple of thousand feet to note a great change in the vegetation. The plants of the lower grounds to a great extent die out (though some accompany us to our highest summit), and the vegetation takes on a low compact form, which becomes more emphasized as we ascend farther, till in sheltered nooks alone do we find any plants more than a few inches in height. Furthermore, we notice an incoming of new plants unknown at lower levels, which search will show us to be confined to the mountains, each of them having a more or less definite limit below which (also above which, though our mountains are not high enough to render this point well marked) it is not found.

Among the plant formations and associations of the lower grounds which we considered in Chapter II. it was noted that the controlling factors were mainly connected with the nature of the soil and the amount of the water-supply. Here on the mountains another factor, the climatic, comes in emphatically, and takes charge. The temperature of the atmosphere falls one degree centigrade for about every 200 feet of elevation, so that a sharp frost on the lowlands may easily mean zero Fahrenheit on a 4,000-foot hill. The rarefaction of the atmosphere, too, tends to produce a much greater range of temperature, both diurnal and seasonal. Again, the velocity of the wind is much higher on the summits than on the plains, where friction is greatly increased by trees and other obstacles. These high winds have a very great cooling effect, as we may notice on our own bodies even in summer. In fact, as regards climatic change, an ascent of a thousand feet is comparable to a journey of several hundred miles northward. Anyone who has, on a winter tramp, been caught in a snowstorm on a 3,000-foot hill is forcibly reminded of what he has read of winter conditions in the Arctic regions. In ascending Ben Nevis we travel, in a sense, to the Arctic Circle. But the analogy is false, for conditions, especially in summer, are very different in the two places. The plants of our mountains have all the advantages of the high summer elevation of the sun, very different from the weak, sloping sunlight of the

Arctic. On our loftier hills, indeed, the heat is on occasions oppressive.

Again, the mountain climate, with its heavy rainfall and long cold period, tends to the formation of peat; and the acids thus engendered in the soil, as well as the low temperature prevailing during most of the year, render difficult the absorption of water by the roots of plants. The conditions under which alpine plants, then, live may be summed up as follows: a long cold winter, a short summer; great exposure; scarcity of food-supply. The modifications which plants have undergone to meet these conditions are very marked, and render alpine plants a source of constant interest to the traveller and of delight to the gardener. The effect of low temperature (also of peaty soil) in rendering difficult the absorption of food materials, and causing extensive root production and limited stem and leaf growth, is immediately observable. In Fig. 33 is seen an alpine Stonecrop (_Sedum primaloides_) as growing on the Chinese Alps at some 12,000 feet. The root is out of all proportion to the aerial parts. The same plant in the garden forms a little bush with branching stems half a foot long, and flowers borne on leafy axillary shoots a couple of inches long, while the roots are short and tufted. The most characteristic form which alpine plants assume may be called the cushion type. This is produced by excessive branching of the stems of small-leaved plants, accompanied by but little longitudinal growth; and it is excellently shown in many well-known plants such as the Mossy Saxifrages, the Kabschia Saxifrages, the Cushion Pink (_Silene acaulis_), and a number of others. The same type of

plant growth is characteristic of semi-desert regions, where the points of similarity of environment to those of the mountain-tops are evident. This cushion form has many advantages for the alpine plant. It keeps it warm in winter and cool and damp in summer; it allows it to produce a great amount of blossom without the necessity for extensive growth; it resists the utmost efforts of furious gusts of wind almost as well as would a half-buried stone; on the most storm-swept cliffs its fresh green blobs “welcome every changing hour, and weather every sky.” Fig. 32 shows a boss of this kind, composed of the Cushion Pink (_Silene acaulis_), with an admixture of Filmy Fern (_Hymenophyllum unilaterale_) and a Moss (_Mnium hornum_). The shrubs of the alpine zone are mostly small and creeping, weaving themselves among the vegetation, and with low grasses and sedges forming a mat which is equally resistant to all inimical conditions. Their leaves are small, to avoid damage by wind or by excessive transpiration. In some genera--for instance, _Veronica_--the diminution of leaf surface accompanying more elevated habitat is very striking. In the New Zealand lowlands broad-leaved forms (Fig. 34, _left_) are met with, which give way, as one ascends to 8,000 feet, to such forms as _V. Hectori_ (Fig. 34, _right_), in which the leaves are reduced to mere scales, and the plant much resembles some of the Cypresses or other Conifers with marked xerophile characters.

Other plants, again, escape climatic rigours by burrowing underground and throwing up short aerial stems in summer; the spindly plants of the lowland, with diffuse stems, and also the light-rooted annuals,

are conspicuous by their absence. The brief summer and long winter are unsuitable to the economy of annual plants; and the alpine perennials are so constructed that with the passing away of the cold, flowering and fruiting may be accomplished quickly, before winter descends again. The abundance and vividness of the flowers of alpines is almost proverbial. Several explanations have been put forward to account for these features, and probably there is some truth in each of them. It has been held that the brilliancy of the sunlight is accountable; the shortness of the period available for seed-production, and the consequent need of prompt pollination by insects, have been suggested, as leading to urgent advertisement by means of brilliant coloration; while the fact that the pollinating insects are largely Butterflies, the most æsthetic of flower visitors, has also been put forward as accounting for it. Be that as it may, the glowing patches of colour produced by many quite minute alpine plants are among the most delightful things in nature. Our own flora contains but few of the more striking of these jewels; but where will one find a more delightful sight than a well-flowered patch of Spring Gentian (_G. verna_) or Mountain Avens (_Dryas octopetala_) or Purple Saxifrage (_S. oppositifolia_)?

As we mount higher and higher on the hills, plants become fewer and more stunted, but hardy forms persist even long after the level of perpetual snow is reached. In the Alps, _Ranunculus glacialis_ occurs up to an elevation of about 14,000 feet. In West Tibet, strange stunted species of _Saussurea_, a genus of _Compositæ_ allied to the Thistles, exist at elevations of 17,000 to 19,000 feet. Some of the Cryptogams go higher still. Lichens grow on the summit of Kilimanjaro (over 19,600 feet); and Schimper suggests[11] that this may by no means represent the absolute limit of vegetation. The prevalence of snow and ice does not of itself inhibit the lower forms of life. Since “red snow” was shown, nearly a century ago, to be due to colonies of a minute Alga, many microscopic organisms of like habitat have been discovered, and these algal colonists of snow and ice are now known to extend far over the frozen deserts of the highest hills, and to penetrate into the remotest regions of the Arctic and Antarctic.

As we get up to the level of perpetual snow on the higher mountains, or go northward within the Arctic Circle, the conditions under which plant life exists become very severe. It has been pointed out that in spite of a superficial similarity, wide disparity exists between the sets of conditions prevailing in the two kinds of habitat just mentioned. In the Arctic the winter is continuously dark and the summer continuously light; and in summer the sun is never far above the horizon, so that the temperature remains low, though it rises amply far enough above freezing-point to allow of plant life. On high mountains, on the other hand, there is the same succession of day and night which prevails on the plains below, the height of the sun above the horizon being a question of latitude. On mountain-ranges situated within the Temperate Zone, such as the European Alps, and much more on those nearer the Equator, the day temperature in summer is very high wherever the sun strikes, and while plants may have to withstand at night a temperature comparable to that borne by the Arctic flora, they must endure by day the most intense insolation.

Neither in the Arctic nor on the high hills does plant life cease merely on account of low temperature. Species belonging to many families venture even beyond the limit of perpetual snow. The coldest known area on the earth’s surface lies in Siberia, actually within the limits of forest growth, and trees and herbs of many species survive winter temperatures which may fall below -60° C. (76 degrees of frost Fahrenheit). They freeze into solid lumps of ice without injury, and indeed the thawing process in spring is more dangerous to them than their congealment in autumn. Many of the high alpine plants are frozen solid every night only to be roasted alive by day; it seems amazing that any living organisms can endure under such circumstances. Yet it is not only species confined to areas where such extremes exist, and specially adapted thereto, which can resist them successfully. In Central Europe the Common Chickweed and Common Daisy are often frozen solid, so that leaves and stems snap between the fingers like sealing-wax, yet with a rise of temperature they continue growth quite unperturbed, just as they do in areas where frost is unknown. The main difficulty induced by cold would appear to be the withdrawal of available water; if that goes on for too long, life ceases. Of course the suspension of activities which accompanies freezing cannot continue indefinitely, and in the cold regions of the Earth plants are found only where for a sufficient portion of the year the maximum temperature rises above freezing-point enough to allow of ordinary vital functions being resumed. A curious point in this power of resistance in plants to extremes of temperature is that they display no obvious protective adaptations. “Our present powers of investigation,” Schimper concludes,[12] “do not enable us to recognize in plants any protective means against cold. The capacity of withstanding intense cold is a specific property of the protoplasm of certain plants, and is quite unassisted by protective means that are external.”

It is a far cry from the high Alps to the seashore, but it will be of interest to examine next the lower limit of the range of the Seed Plants. While the upper limit varies much in different latitudes, according to the distribution of temperature, the lower is controlled by sea-level, which (for our purpose at least) is uniform over the whole globe. The level of the fresh waters, whose margin marks the limit of the bulk of the Seed Plants, is, on the other hand, various, lakes being situated at different heights above (and occasionally below) sea-level, while rivers slope across the lands down to the ocean. While the sea margin forms a very real barrier to the spread of Seed Plants, the lakes and rivers, on the other hand, yield many inhabitants, and we must examine the relations existing between the aquatic and the terrestrial species.

As has been stated on a former page, the evidence points to life having originated in the water, at a period extremely remote. The most lowly as well as the most minute of all organisms are the bacteria, some of which are in size beyond the limit of the most powerful microscope to detect, their presence being known only by their chemical actions. The most primitive groups of bacteria, known as prototrophic, are able to live without light, deriving their nourishment by the breaking up of inorganic chemical compounds. It is difficult to conceive of any living organism more primitive than these, and quite possibly they recall that dim borderland where merely chemical structure and action mysteriously advanced into the cell structure and purposive chemical changes which we call life. From that lowly stage the evolution of plant life has been marked especially by three great forward bounds, of inestimable importance. The first of these was the “invention” of chlorophyll, which allowed plants to use for their life-processes the vast supply of energy furnished by the Sun. Sunlight then became essential to life, and the Algæ, the probable ancestors of all the higher plants, were developed, presumably through the peculiar _Cyanophyceæ_, or “Blue-green Algæ,” in which the chlorophyll is in a somewhat undifferentiated condition. Much later than this stage, yet far back in the history of evolution, occurred the second of the great forward steps. This was the desertion of the water for the land, which opened up for the plant world vast new fields and a great variety of new conditions. The final stage was reached by the abandonment of the aquatic mode of pollination by means of swimming spermatozoids, as still found in the Maidenhair Tree (_Ginkgo_), Cycads, Ferns, and groups lower in the scale, and the adoption instead of pollination through the medium of the air, “which” (to quote Mrs. Arber’s happy phrase) “has won for them the freedom of the land.” The Seed Plants, then, achieved their wonderful abundance and variety owing to the highly stimulating conditions offered by a terrestrial existence; we must assign to all the existing types a long terrestrial ancestry. How, then, about the water plants whose leaves and flowers so decorate our lakes? There seems no doubt[13] that they are species which have left the land to resume the aquatic habits of their remote ancestors. With few exceptions they retain the aerial mode of pollination which is the pride of the specialized land plants. The pressure of competition has probably driven them into the water, where they descend as far as the lessening light-supply will allow. Some--presumably the earliest to take to an aquatic life--have all their relations to keep them company, the remote ancestor which adopted an aquatic habit being now represented by many species, or even by many genera. In other cases a terrestrial genus or order has few or only a single aquatic representative. It may be assumed that in such a case the aquatic habit has been recently acquired. The great majority of water plants send their flowers up above the surface to be pollinated by wind or (more rarely) by insects. It may be noted that few of the more highly evolved groups of Seed Plants are represented in the aquatic flora; wind-pollinated flowers of a rather primitive type of structure are the rule in our lakes and rivers; which points to an early assumption of the aquatic habit, and suggests that the land is more favourable than the water for the evolution of higher types.

While the fresh waters of the globe have thus acquired from the land an abundant population of higher plants, the presence of salt, in water as on land, has had a deterrent effect. The sea was at first fresh. The primitive ocean derived by condensation from a cooling atmosphere in the early days of the world’s history contained no excess of salts. Whether life arose while this condition still persisted it is not possible to say; but as the sea grew salter owing to the rivers bringing into it incessantly salts derived from the land, the Seaweeds alone of the great groups of plants adapted themselves to saline conditions, and the ocean is now their unchallenged kingdom. The divisions which are represented by the Mosses, Liverworts, Club-mosses, Horsetails, and Ferns, have not, and so far as is known never had, a single representative in the sea. Only one or two Fungi--often symbiotically combined with Algæ to form Lichens--and a very few Flowering Plants, have attempted marine colonization, after long ages spent on land; and they have met with indifferent success. As we pass from fresh to brackish water, the population decreases rapidly, till in the seas surrounding our islands only one Seed Plant--the Grass-wrack, _Zostera marina_--has adopted a habitat which is thoroughly marine, and very few are found in other parts of the world. A study of the meeting-ground of the land and sea plants, such as we may make on rambles along the coast, supplies us with some interesting material. On sandy shores, the wave-trampled beach, shifting under the influence of winds and currents, offers a stretch of “no-man’s-land”--a desert strip untenanted alike by terrestrial or marine plants. The former do not descend below spring-tide mark, if they go so far; the latter cannot obtain foothold on the unstable substratum. The peculiar characters of the terrestrial beach plants has been referred to on a previous page (p. 36). On rocky shores the “desert” strip is much narrowed, and a certain overlap may often be found, for the Lichens--essentially a terrestrial group--descend from the plant-covered slopes into the spray-swept zone below, and on to mix with the Seaweeds which occupy the belt under high-water mark, some of them, species of _Verrucaria_ and _Arthropyrenia_, continuing downward till the low-water mark of spring tides is reached. On steep rocky shores the dividing-line between the Flowering Plants and the Seaweeds is quite narrow, and varies in elevation with the exposure. On cliffy coasts open to the Atlantic waves the uppermost Seaweeds, such as _Pelvetia_, which only asks to be wetted periodically by spray, occur far above high-water mark, the lowest Seed Plants perching on the rocks much higher still--sometimes not venturing to within 100 feet of the water-level. Under such extreme conditions none of the higher land plants venture down towards the unfriendly sea. To see the overlap of the terrestrial and maritime vegetation well developed we seek conditions entirely different, where amid shallow inlets and salt-marshes land and sea merge imperceptibly. Here the absence of higher plants from the areas below high water, as compared with their abundance above water-level, is a conspicuous feature. This is a noteworthy point, because if we assume that the presence of salt is the main factor which has prevented the land plants from spreading downwards, we are faced with the fact that the soil of the salt-marsh, where many such plants occur, may by evaporation of water become much more highly charged with salt than the sea itself. Yet the salt-marsh flora includes representatives of many Natural Orders, including some of the most highly specialized families--_Ranunculaceæ_ (_R. sceleratus_), _Cruciferæ_ (_Cochlearia_ spp.), _Caryophyllaceæ_ (_Alsine_), _Umbelliferæ_ (_Apium graveolens_, _Œnanthe Lachenalii_), _Compositæ_ (_Aster Tripolium_, _Artemisia maritima_), _Primulaceæ_ (_Glaux maritima_), _Plumbagineæ_ (_Statice_, _Limonium_). It seems clear that it is the assumption of the marine habit which is the stumbling-block, not the presence of salt. The Grass-wrack or _Zostera_, our only marine Seed Plant, comes of one of the oldest stocks of aquatic plants, and its nearest relatives have long been toying with the idea of a maritime habitat. The Order to which it belongs, the _Naiadaceæ_ or Pondweed family, from their worldwide range, their number, their variety, and their uniformly aquatic habit, may be set down as among the earliest Seed Plant colonists of lakes and rivers; some of them favour brackish water, while others besides the Grass-wrack have taken to marine life. Without going beyond the limits of our native _Naiadaceæ_ we can study the various stages, and form a picture of how the Grass-wrack migrated to the sea. First we have the numerous Pondweeds which grow in our lakes and rivers--plants with leaves broad and floating, or narrow and submerged, and inconspicuous flowers which rise above the water and are pollinated by the wind. Next we find several narrow-leaved Pondweeds which grow in brackish pools; and with them are some allies, the Tassel Pondweed (_Ruppia_) and Horned Pondweed (_Zannichellia_), with more reduced flowers and often a more nearly marine habitat, as they sometimes mix with Seaweeds on the open shores of estuaries; in these plants we find the stages of a most interesting return to the archaic method of water-pollination, so long discarded by the great mass of the Seed Plants. In the flower of _Ruppia_, which consists merely of two stamens and four carpels without corolla or calyx, the pollen is liberated under water, and, being light, rises to the surface; older flowers have already, by growth of the flower-stalk, reached the surface, and they become pollinated by the floating grains. In _Zannichellia_ the process is in general similar, save that the flowers are either male or female, the former consisting of nothing but a single stamen. The Naiads (_Naias_) form an allied genus, and are slender annual herbs, growing completely submerged in fresh or brackish water. One of them (_N. flexilis_) occurs in lakes at rare intervals along the western edge of the British Isles; and another, _N. marina_, is found living in only one spot in Britain--Hickling Broad in Norfolk; their fossil seeds embedded in old lake deposits show that in former times both were more widely spread than now in Western Europe, and that other species of the genus also occurred. In the Naiads complete reversion to water-pollination is found. When the very simple male flowers shed their pollen, the grains, which are heavy owing to the presence of starch, fall through the water on to the female flowers which are borne below them, or are carried by currents to other flowers. Lastly we come to the Grass-wracks, a small group of submersed marine plants. While some of them, like our little native _Z. nana_, haunt muddy sands between tides, our more familiar species, the common _Z. marina_, is thoroughly marine, growing tall and vigorous among the large Seaweeds down to far below low-water mark (to over 30 feet in the Baltic). The plant has, nevertheless, not yet developed submersed pollination, the pollen-grains rising to the surface, where they are caught by the stigmas of floating female flowers. It follows that the individuals rooted in the deeper water, though growing vigorously, do not mature seed, for the production of which the species has to rely on plants which, at least at low water, are rooted sufficiently near the surface to allow the flowers to rise above it. Could the species achieve submersed pollination, it appears quite capable of colonizing throughout the Laminarian zone, wherever there is a soft substratum for its creeping stems.

The land plants of the salt-marsh, as well as the aquatic species, furnish interesting examples of overlap with the sea flora, but a brief reference must suffice. The Glasswort (_Salicornia_), for instance, has furnished itself with a very complete equipment for the difficult conditions of salt-marsh life (see pp. 17, 18), and grows far out on the mud-flats in green colonies, often below the upper limit of the Bladder-wrack or _Fucus_, the common brown Seaweed of our shores. The Glasswort has discarded leaves, its stems have become thick and succulent, and its flowers, reduced to the minutest and simplest dimensions, are almost buried in the fleshy branches. Thus armed, it braves the salt-desert of the mud-flats, and repeated submersion by the tides leaves it uninjured. Under the peculiar conditions of its life, it relies neither on insects nor wind nor water for pollination, the flowers being self-pollinated. A more surprising commingling is that which is illustrated by A. D. Cotton in his report on the Seaweeds of the Clare Island district (_Proc. Royal Irish Academy_, vol. xxxi., 1912), where, on peaty soil a little above mean high-tide level, the Sea Pink is shown forming a sward with a peculiar dwarf form of _Fucus_ (_F. vesiculosus_, var. _muscoides_) and a few other salt-marsh Seed Plants, such as _Schlerochloa_ (_Glyceria_), _Glaux_, _Salicornia_. The Sea Pink is highly evolved florally, and differs widely from the Saltwort in its abundant production of leaves and showy flowers, the absence of any conspicuous xerophile characters, and the fact that it is not confined to the coasts, being often a member of the alpine flora of our higher hills. In its association with _Fucus_ it may be claimed that it is the latter which is “out of water,” as it never produces fruit, increasing solely by means of vegetative growth. At the same time, so closely does it press its partner in the struggle for room, that the Sea Pink fails to form its usual robust clumps, its stem being mostly unbranched and its stature dwarfed.

Viewing generally the migration of the Seed Plants from land to water, we see that the fresh waters of the world, untenanted by other large plants, have been fully colonized, generally a long time ago, and by plants of rather early types. But as regards the sea, the luxuriant Algal vegetation which is in possession of our shores has no reason to tremble for its supremacy. Beautifully adapted for their life, whether in sheltered bays or on stormy rocks, the Seaweeds show no sign of relinquishing the domain that has been theirs since the earliest rocks which still display traces of organic life were laid down in Cambrian seas.

INDEX

Agriculture, 135

Alien plants, 143

Alpine plants, 190

Animal-eating plants, 77

Animals and plants, 75 and seed-dispersal, 69 dependence on plants, 155

Arctic deserts, 19 plants, 196

Bog plants, 41

British flora, 167 Isles, vegetation, 25, 30

Chlorophyll, 180

Cultivated plants, 145

Deserts, 16

Fertilization, 82

Flowers, 126 display, 85 structure, 81

Fruit, 131

Fruits, explosive, 55

Glacial Period, 165

Grassland, 25

Insectivorous plants, 78, 186

Insects and flowers, 81

Leaves, 119

Life, origin of, 15

Man and vegetation, 135

Marine plants, 201

Migration, 48

Mountain plants, 189

Mud-flats, 17

Mycetozoa, 156

Myxomycetes, 156

Ocean depths, 19, 76

Origin of life, 199

Parasites, 183

Peat flora, 41

Planets, question of life on, 11

Plant associations, 30 economy, 98, 141 formations, 32 migration, 48

Plants, cultivated, 145 earliest, 154

Pollination, 82

Roots, 105

Salt-marshes, 23, 40

Saprophytes, 181

Seed-dispersal, 49

Seeds, 50

Semi-deserts, 22

Shingle beaches, 39

Soil, 99

Stems, 109

Symbiosis, 79

Types of vegetation, 31

Vegetation, closed, 24

Vegetative reproduction, 53

Water, dispersal by, 61 flora, 43, 199

Wind, dispersal by, 62

Woodland, 25

Xerophytes, 36

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FOOTNOTES:

[1] SVANTE ARRHENIUS: “The Destinies of the Stars.” Translated by J. E. Fries. Putnam, 1918.

[2] F. SODDY: “Matter and Energy,” 1912, p. 194.

[3] A. G. Tansley: “Types of British Vegetation,” 1911, p. 63.

[4] H. B. GUPPY: “Plants, Seeds, and Currents in the West Indies and Azores,” 1917, p. 425.

[5] W. B. BARROWS: “Seed-planting by Birds.” Report of the Secretary of Agriculture, U.S.A., 1890, p. 281.

[6] See A. H. CHURCH: “The Plankton-phase and the Plankton-rate,” _Journal of Botany_, June, 1919, supplement.

[7] G. H. CARPENTER: “Insects: Their Structure and Life,” p. 300.

[8] W. B. BOTTOMLEY in “The Exploitation of Plants,” edited by F. W. Oliver, 1917, p. 12.

[9] To be accurate, certain groups of Bacteria, the lowest forms of organized life, must be excluded. They appear capable of building up their bodies directly out of inorganic substances.

[10] F. J. HANBURY and E. S. MARSHALL: “Flora of Kent,” 1899, p. xxxv.

[11] A. F. W. SCHIMPER: “Plant Geography” (English translation, 1903), p. 719.

[12] A. F. W. SCHIMPER: “Plant Geography” (English translation, 1903), p. 41.

[13] See AGNES ARBER: “Aquatic Angiosperms: the Significance of their Systematic Distribution,” _Journal of Botany_, 1919, p. 83.