A handbook of systematic botany

Sexual generation. The MICROSPORES produce an extremely rudimentary prothallium, formed of only a single cell, and having also a very much reduced bicellular antheridium with a small number of spermatozoid mother-cells in each cell (in Salvinia 4, in Marsilia and Pilularia 16). In Salvinia the microspores remain embedded in a hard mucilaginous mass (at first frothy) which fills up the cavity of the sporangium. The prothallium must therefore grow out through this slime and also through the wall of the sporangium (Fig. 214), and it thus terminates in a relatively long cell.

In Marsilia the microspores are set free from the microsporangium, and the prothallia, with the antheridia, remain in them until the spermatozoids are liberated. The latter are spirally-twisted threads.

The MACROSPORES, on germination, give rise to a very reduced prothallium, which in Salvinia bears 3 archegonia; but, if these are not fertilised, the prothallium may continue to grow and become a fairly large, green body with several archegonia (Fig. 215 A, B). In Marsilia the prothallium is still more reduced, it is enclosed in the macrospore, and only bears one archegonium. The archegonia are similar in structure to those of the Ferns, but are smaller, and sunk more deeply in the tissue of the prothallium.

The asexual generation is developed from the fertilised egg-cell. It is a dorsiventral, horizontal shoot. In Salvinia it bears at first a shield-like leaf, the scutiform leaf (Fig. 215 C, a), which is succeeded by the ordinary foliage-leaves. The young plants of Marsilia, likewise, have less perfect leaves in the very early stage.

The formation of the sporangium is the same as in the Leptosporangiate Ferns. (The 16 spore-mother-cells originate from one central, tetrahedric archesporium.)

The Hydropterideæ are divided into 2 orders, the chief differences between them being found in the asexual generation.

Order 1. Salviniaceæ. This order more nearly approaches the true Ferns, especially so on account of the form of the indusium. Only one species is found in Europe, Salvinia natans (Fig. 217). This is a small, floating, annual, aquatic plant, entirely destitute of roots. The dorsiventral, horizontal stem bears two kinds of leaves, which are arranged in whorls of three. Two of these which turn upwards are oval, entire, “aerial foliage-leaves” (Fig. 217 B, b²-b³); the third, the “water-leaf” (b¹) is submerged and divided into a number of hair-like segments, similar to the submerged leaves in many aquatic plants, for instance, Water-buttercup (see also Fig. 215 C). The whorls of leaves alternate with each other; there are thus 4 rows of dorsally-placed aerial leaves, and two rows of ventrally-placed submerged leaves. The sporangia are situated in sori, each sorus being borne on a small column (receptacle or placenta) and enveloped by a cupular, but entirely closed indusium (Fig. 218). The sori are situated on the submerged leaves (Fig. 217 B, s-s) and are unisexual, i.e. each sorus contains microsporangia only, or macrosporangia.

Order 2. Marsiliaceæ. The characteristic feature of this order, and one not possessed by other Fern-like plants, is that the sori (2–many) are enveloped in leaf-segments which close round them and form a “sporocarp,” just in the same manner as the carpels, in the Angiospermous Flowering-plants, close round the ovules and form ovaries. The sori contain both micro-and macrosporangia. When the spores are ripe, the sporocarp opens in order to disperse the spores (Fig. 220).

The two genera (with 57 species, Temperate, Tropics) are land-and marsh-plants, whose dorsiventral, creeping stem bears roots on the under surface, and the leaves in two rows on the upper side (Figs. 219, 221). The leaves of Marsilia are compound, and divided into four small leaflets springing from the apex of the petiole (Fig. 219), and resemble the leaves of Oxalis. In the bud the leaves are circinate (Fig. 219 b), and at night they exhibit the well-known sleep-movements. The sporocarps are borne on the petioles of the fertile leaves, near their bases (Fig. 219 f); they are oblong and resemble small beans, the outer cells being hard and sclerenchymatous, while the inner ones are divided into a number of loculi arranged in two rows. On germination, water is absorbed, the two sides separate slightly, as valves (Fig. 220 A), and a long vermiform mass of gelatinous, parenchymatous cells (Fig. 220), swollen by the water, emerges, bearing a large number of sori arranged pinnately. Each sorus (sr) is covered by a thin indusium. (The thin covering may be considered an indusium physiologically, though not morphologically).

Pilularia has linear leaves, without lamina. The sporocarps are spheroid (Fig. 221), brown and hard, and situated near the base of the leaves. They are 2–4 chambered and open by a corresponding number of valves.

Class 2. Equisetinæ (Horsetails.)

The characteristics of this class have been described on page 204.

It is divided into two sub-classes:—

1. The isosporous Equisetinæ. To this sub-class belong, with certainty, only the Equisetaceæ now existent, which are represented by only one genus, Equisetum.

2. The heterosporous Equisetinæ. Forms which are now extinct.

Sub-Class 1. Isosporous Equisetinæ.

Order. Equisetaceæ (Horsetails).

The sexual generation. The prothallium is green and leaf-like, as in the majority of Ferns, but irregularly branched and curled. It is often unisexual. The male prothallia bear antheridia only, and are smaller and less branched (Fig. 222 A) than the female; the latter may attain a diameter of ½ an inch, and bear archegonia only (Fig. 222 B). The antheridia and the archegonia resemble those of the Ferns, but the spermatozoids (Fig. 223) are larger and less twisted. On the last curve is situated a more or less elongated appendage of cytoplasm (Fig. 223 c).

The asexual generation. The embryo is similar to that of the Ferns. The fully developed Equisetum is a perennial herb, with widely creeping (in some species tuberous) rhizome, from which extend erect, aerial, most frequently annual shoots.

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.

Sub-Class 2. Heterosporous Equisetinæ.

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.

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.

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.

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.