Part 20

The vegetative aerial STEMS are divided into a number of internodes by the whorls of leaves (Fig. 224). The internodes are hollow, the cavities being separated from each other by the transverse partitions of the solid nodes. The lower portion of the internode, which is encased by the leaves, has much thinner and softer cell-walls, so that the stem is easily separated into segments just above the nodes. Each internode has a large number of ridges and furrows, and bears at its apex a whorl of leaves whose number and position correspond to the ridges of the internode. As in the case of other verticillate plants, the whorls are placed alternately, one above the other; the same arrangement is also found in the ridges on two successive internodes. In addition to the large air-cavity in the centre of each internode (the central cavity), a whorl of tubular air-passages is found in the cortex of the stems, opposite the furrows (vallecular canals). There is also a similar air-passage (carinal canals) in each of the vascular bundles, which are placed in a ring, one opposite each ridge, and therefore alternating with the vallecular canals. The vascular bundles are _collateral_ as in the majority of Flowering-plants, but poorly developed. The xylem of each bundle consists of two groups of annular or spiral vessels, close to the outer border of the carinal canal, and two groups of scalariform tracheides, each placed on a radius passing through a group of spiral vessels. The phloëm is placed between these four groups, each of which has only a few vessels. The stiffness of the stems is mainly due to the large amount of silica in the cell-walls of the epidermis, and to the sclerenchymatous cells of the ridges.

All LEAVES are situated in _whorls_. The VEGETATIVE are simple, undivided, 1-nerved, and are united into toothed sheaths (Fig. 224 _a_, _b_). The branching of the stems in some species (_E. arvense_) is very abundant. The branches break through the base of the leaf-sheaths (Fig. 224 _b_), and generally _alternate with the teeth_ (leaves).

The FERTILE LEAVES (_sporophylls_) are different from the barren ones. They are _free, shield-like_, each one having a short stalk bearing usually an hexagonal plate (Fig. 224 _d_), and closely compressed into an ear or cone (Fig. 224 _a_, _c_). The _Equisetums_ thus present an advance in development distinctly beyond that of the Ferns, which is further emphasized by the circumstance that a transition from the sheath-leaves to the fertile-leaves is found in the involucre or annulus, a “collar” of specially modified leaves situated at the base of the cone (Fig. 224 _a_ and _c_). The cone may be considered as a very rudimentary flower, and the annulus may be regarded as a very early stage in the formation of a flower (perianth). See page 235.

The SPORANGIA are situated on the underside of the sporophylls, one at each angle; they are sac-like, and open inwardly by a longitudinal cleft (Fig. 224 _d_). An annulus is wanting; but in the wall of the sporangium, as in the pollen-sacs of the Flowering-plants, a layer of cells, with annular or spiral thickenings, is developed, which assists in the dehiscence of the sporangium.

The SPORES are green; the walls composed of four distinct layers, of which the outer is gradually separated, except at one point, and becomes split into four long bands (_elaters_) (Fig. 225). The elaters are extremely hygroscopic, coiling round the spore when moistened, and expanding as soon as dry, presenting a most lively object under the microscope when breathed upon and allowed to dry. The second layer, when germination commences, becomes detached from the inner wall, which is formed of the exospore and endospore.

The order has become much reduced, and at the present time includes only one genus, _Equisetum_, with about twenty-five species, which are distributed over the entire globe, particularly in damp situations. In SOME SPECIES the barren shoots are green and very much branched, but the fertile ones are unbranched, pale brown, and possess no chlorophyll (_E. arvense_, Field-Horsetail, Fig. 224, and _E. maximum_). IN OTHERS the fertile and barren shoots are alike green, and either both unbranched (_E. hiemale_), or branched (_E. palustre_, _E. limosum_, etc). The fertile shoots of _E. silvaticum_, up to maturity, resemble those without chylorophyll of _E. arvense_, but after that period they produce green branches, and thus resemble the barren ones.

EXTINCT ISOSPOROUS EQUISETINÆ. In addition to several true species of fossilized _Equisetums_, the order of the CALAMITES, which no doubt is closely allied to the Equisetinæ, is also found in the fossil state. These were gigantic forms, attaining about twenty times the size of those of the present day, and stems of nearly 10–12 metres in height are known. They reached the culminating point of their development in the Carboniferous period, and died out towards the close of the Palæozoic. The stems had hollow internodes and alternating grooves, similar to their relatives of the present day. The leaves must either have been absent or very perishable, since they have not been identified with certainty. If the determinations of certain remains of cones which of late have been discovered are correct, they were heterosporous and had two kinds of sporangia as in the following sub-class. A cambium formation and an increase in thickness has been found in the stems.

Their USES are very limited. A few species, such as _E. hiemale_ are used for polishing on account of the hard siliceous cell-walls of the epidermis, found in all species of _Equisetum_.

Sub-Class 2. =Heterosporous Equisetinæ.=

The two orders which come under this head are united by the characteristics, that the verticillate leaves are not united into sheaths (Fig. 226), and that between each whorl of fertile leaves there is also a whorl of barren ones. The fertile whorls in ANNULARIÆ are situated about midway between the barren ones (Fig. 227), but in ASTEROPHYLLITEÆ they occur immediately above a barren whorl (Fig. 228) and contain only half as many members as the latter. The lower whorls bear macrosporangia with one macrospore, the upper, microsporangia with many microspores.

The ANNULARIÆ were distichous (Fig. 226), and presumably floating plants. The ASTEROPHYLLITEÆ had verticellate branches. These also died out after the Carboniferous period, at the close of the Palæozoic.

Class 3. =Lycopodinæ= (=Club-Mosses=).

The characteristics of this class have been given on page 205. It consists of two sub-classes, one embracing isosporous, the other heterosporous forms.

Sub-Class 1. =Lycopodieæ= (ISOSPOROUS Lycopodinæ).

One kind of spore. Prothallium large, partly green. Leaves without ligule.

Order 1. =Lycopodiaceæ.= The PROTHALLIUM is only known in a few species at present, but in these it is more or less tubercular, and bears both antheridia and archegonia.

In _L. annotinum_ the prothallium is a relatively large mass of cells, without chlorophyll, and subterranean, in which the antheridia and archegonia are embedded (Fig. 229). In the widely distributed tropical species, _L. cernuum_, and in _L. inundatum_, it is a small tubercular body which has a subterranean portion, with either little or no chlorophyll; and an aerial green portion. The prothallia of _L. phlegmaria_ and others live saprophytically in the crevices of the bark of trees; they are partly filamentous, branched, and possess no chlorophyll.

The =asexual generation=. PERENNIAL PLANTS. The stem branches monopodially (often apparently dichotomously), and is thickly covered by small, simple, triangular or scale-like leaves. The leaves are spirally arranged in some species (Figs. 229, 230), and in others, whose stem is compressed with unequal sides, opposite (Fig. 231). The roots of _Lycopodium_ are dichotomously branched.

The SPORANGIA in _Lycopodium_ are situated singly at the base of the leaves, almost in their axils; they are reniform, unilocular and open like a mussel-shell by two valves (Fig. 230 _h_). The sporangia are developed from a group of surface cells. The archesporium is formed from one hypodermal cell (or perhaps a cell-row).

The fertile leaves are collected upon definite regions of the stem. They are either similar to the barren ones, and then the fertile portions of the stem pass gradually, without any break, into the barren portion (_L. selago_); or they differ from the barren leaves, and are then collected into special apical cones (Fig. 230 _a_). The SPORES are tetrahedral or bilateral (Fig. 232).

About 100 species, chiefly tropical.

Five species of _Lycopodium_ are found in Great Britain. _L. clavatum_ and _L. selago_ are common in mountainous districts. _L. annotinum_ is common in the Highlands of Scotland. The other genus of the order is _Phylloglossum_, with one species, _P. drummondi_ (Australia, Tasmania, and New Zealand), a small plant only a few centimetres high, with two tubers, and about eleven linear leaves at the base of the stem which is terminated by a cone of sporophylls.--FOSSIL Lycopodiaceæ in the Carboniferous period.

OFFICINAL: “Lycopodium,” the spores of _L. clavatum_.

Family 2. =Psilotaceæ=. The sporangia are placed on the apex of short, two-leaved stems, as 2–3, seldom four, small capsules. Small herbs, with angular stems; leaves small, simple, and one nerved. Only four species.--_Psilotum_ (Madagascar, Moluccas, Sandwich Islands, etc.) is destitute of roots, their place being supplied by special underground stems which bear a few modified leaves, very much reduced, especially when buried deeply in the soil. Three species.--_Tmesipteris_ (Australia), one species.

Sub-Class 2. =Selaginelleæ= (HETEROSPOROUS Lycopodinæ).

Micro-and macrospores. The prothallia are very much reduced, especially the male; the female does not leave the spore. The leaves are ligulate.

=The sexual generation.= In the ~MICROSPORES~ are formed: (1) a very small “vegetative” cell, representing the vegetative part of the prothallium (_f_ in Fig. 233 _A, B_), and (2) a cell many times larger and which divides into a number (4–8) of primordial cells, each of which divides into four spermatozoid-mother-cells, though all of these may not develope spermatozoids. On germination, when the spore-wall is ruptured, the spermatozoids and spermatozoid-mother-cells are ejected into the water.

The ~SPERMATOZOIDS~ in _Selaginella_ are elongated and club-shaped, with two cilia (Fig. 234); but in _Isoëtes lacustris_ they are spirally-twisted threads which differ from all other spermatozoids by having a bunch of cilia _at each end_; the other species of _Isoëtes_ have cilia only at the anterior end.

The MACROSPORES. Shortly after the macrospores have been set free, or in _Selaginella_, while still enclosed in the sporangium of the mother-plant, they germinate and soon become filled with the cellular tissue of the prothallium, and even in _Selaginella_ the archegonium begins to be formed before the rupture of the spore-cell-wall has commenced (Fig. 235 _A_).

The ARCHEGONIA are constructed on the same plan as those of the other Archegoniatæ, but are quite embedded in the prothallium (Figs. 235 _æ_, 236).

=The asexual generation= varies very much in the different orders.

Order 1. =Isoëtaceæ (Quill-worts).= The only known genus, _Isoëtes_ (Quill-wort), has an extremely short, tuberous, _unbranched_ stem with very short internodes (Fig. 237). The STEM is remarkable as being the only one among the Vascular Cryptogams which increases in thickness (see page 202). The meristematic cells are situated round the axial cylinder, and form, especially, parenchymatous tissue in two or three directions, giving rise to 2–3 grooves in which the dichotomously-branched ROOTS are produced. The LEAVES are arranged spirally in a close rosette. They are awl-shaped and have at the base a semi-amplexicaul sheath, with a groove (_fovea_), in which a sporangium is situated (Fig. 238). The ligule is a foliar outgrowth from the upper edge of the groove.--The MACROSPORANGIA (each with a number of macrospores), are situated on the outer leaves, the MICROSPORANGIA (Fig. 238), on the inner ones. Between each cycle of fertile leaves there are a number of imperfect or barren ones as in the case of the female plant of _Cycas_. The spores are liberated by the decay of the sporangium. The two kinds of sporangia develope at the commencement in the same way. The archesporium is, at first, a hypodermal layer of cells which grow out in the direction perpendicular to the surface of the leaf, and divide by a number of walls parallel to this direction, forming a sporogenous mass of cells. Some of the cell-rows of this sporogenous mass lose their rich protoplasmic contents, and are arrested in their growth; thus incomplete divisional walls of sterile cells, “_trabeculæ_” arise in the sporangium, dividing it into a number of compartments one above the other (Fig. 238 _t_). (The trabeculæ, according to Goebel, play the same part as the nutritive cells of the sporangium of _Riella_; the tapetal cells, as in the Ferns, are in a great measure dissolved at a later period.) The sporogenous cell-rows, in the microsporangia, give rise to a large number of spore-mother-cells, but in the macrosporangia only one spore-mother-cell, with tapetum, is developed from each fertile archesporial cell.

The two native species, and several others, are aquatic plants, the remaining species are land plants, or are amphibious. About 50 species. In temperate and tropical regions.--FOSSIL species in the Tertiary period.

Order 2. =Selaginellaceæ.= This order contains only one genus, _Selaginella_. The STEM, in the majority of species, is dorsiventral, long and slender, and apparently branches dichotomously, but in reality _monopodially_, with well developed lateral shoots. The LEAVES are small, round, or ovate, in the majority of species arranged in whorls of two leaves each; these whorls, however, are not decussate, but are considerably inclined towards each other, an arrangement by which four rows of leaves are produced, each whorl having one large and one small leaf. The two leaves in each whorl are of unequal size, the smaller one being placed on the upper surface and the larger on the lower surface of the stem (Fig. 240). Some species have spirally-arranged leaves, more resembling the arrangement in the _Lycopodiums_.

The ~FERTILE LEAVES~ most frequently differ from the barren ones, and are collected into spike-like cones (a kind of flower; Fig. 239). Micro-and macrosporangia are found in the same cone (Fig. 239). Each sporangium arises from a group of superficial cells of the stem, directly over the leaf on which it will be situated later on. Each sporangium has a hypodermal, unicellular archesporium, and contains a layer of tapetal cells; these are dissolved later, when the spores are ripe, and not before as in the Ferns. In the very early stages of their development, the micro-and macrosporangia are precisely similar, and the differences between them arise later on. In the microsporangium all the spore-mother-cells divide, and each forms four tetrahedrically-arranged microspores (Fig. 204); but in the macrosporangium only four macrospores are formed, by the division of a _single mother-cell_, while the remaining spore-mother-cells are aborted. It is rarely that the macrosporangia contain 2 or 8 macrospores.

For the ~GERMINATION OF THE SPORES~, see pages 228, 229. The prothallium arises in the macrospore (_f-f_, in Fig. 235 _A_), probably by division of the meniscus-shaped protoplasmic mass, which is marked off at the apex of the spore; primordial cells are thus formed which later on are surrounded by a cell-wall. In six to seven weeks after sowing, the spore-wall is ruptured by the growing prothallium, which already has developed archegonia (Fig. 235 _œ-œ_). The prothallium so formed does not occupy the entire cavity of the spore, but four to five weeks after sowing, the large-celled parenchyma is developed in the lower portion of the spore by free cell-formation; this has been termed by Pfeffer, “endosperm,” since it is similar to the endosperm of Flowering-plants. Goebel, however, has termed it “secondary prothallium,” as the homology with the endosperm of the Angiosperms is very doubtful.

The ~FERTILISED OOSPHERE~ divides into an upper (hypobasal) and a lower (epibasal) cell; from the latter alone the embryo is developed with its root, stem, foot, and two _cotyledons_, and the former gives rise to an organ which appears in this instance for the first time, but which occurs in all Flowering-plants, viz. the _suspensor_. This forces the embryo down into the “endosperm,” which is entirely or partially absorbed by the embryo. In the case of the Flowering-plants the embryo is developed with its longitudinal axis in the elongation of the suspensor, but in _Selaginella_ the embryo is situated _transversely_ to it.

_Selaginella_ (300–400 species), is essentially tropical, only one species living in the North (_S. spinulosa_), but others grow in Central and South Europe.

Order 3. =Lepidodendraceæ= are extinct, tree-like Lycopods, which are found especially in the Lower and Middle Carboniferous. Vegetatively they are most nearly related to _Lycopodium_, but the stem attained much larger proportions (about eleven metres in height and one metre in thickness), and had a cambium by which it increased in thickness. It was regularly dichotomous, and closely studded with spirally-placed leaves, which left behind them peculiar rhombic scars. The large cones resemble Pine-cones, and bore sporangia much larger than any which are now produced (the male ones as much as 2 cm.’s in length). The macrosporangia were situated at the base, and the microsporangia at the apex.

Order 4. =Sigillariaceæ.= These are, presumably, another group of extinct tree-like Lycopods (especially in the Middle Carboniferous). The name has been derived from the seal-like scars, which the fallen leaves have left behind in longitudinal rows on the grooved stem. The rhizomes of these plants were formerly termed _Stigmaria_, and placed in a separate genus.

Order 5. =Sphenophyllaceæ= form an entirely extinct group. They do not definitely belong to any of the three large classes of Vascular Cryptogams, but it is perhaps best to place them in juxtaposition to these. They were herbaceous plants with verticillate, wedge-shaped leaves, with nerves branching dichotomously into equally strong branches. Micro-and macrosporangia were formed in the same cone; and were situated in the axils of the leaves, as in the Lycopods.

The Transition from the Cryptogams to the Phanerogams.

All the plants considered in the preceding chapters are included in the term CRYPTOGAMS; all in the following chapters under the head of PHANEROGAMS (see page 3). Hofmeister’s pioneer works (1851, _Vergleichende Untersuchungen der höheren Kryptogamen_, etc.) and the numerous researches published later by other investigators, have closed the gap which was formerly thought to exist between these plants; so that we now, in the series: Bryophyta--Pteridophyta--Gymnospermæ--Angiospermæ see the expression of a single line of development in accordance with a definite plan. The forms through which this gradual development has taken place have in course of time, however, to a great extent died out, and only single links of the chain connecting the lowest to the highest still remain.

THE ALTERNATION OF GENERATIONS, which we found indicated in certain Thallophytes, can be proved with the greatest clearness in all the higher Cryptogams, from the Mosses upwards; it is also found in the Phanerogams, but not in such a pronounced degree, because one of the generations is so far reduced that it has almost given up its independence. For the sake of greater clearness, we will begin with the comparison of the sporophyte, asexual (second) generation.

=The asexual (2nd) generation of the Cormophytes.=