Part 40

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.=

The asexual generation which follows from the further development of the fertilised oosphere, is, in the Mosses, only the sporogonium (according to one theory it is perhaps homologous with a spore-bearing leaf, situated upon a short stem, see p. 187); in Filicinæ, Equisetinæ, and Lycopodinæ, on the other hand, it is a highly developed plant differentiated into stem, leaf, and true root, and bearing the sporangia on its leaves. The ~MODIFICATION OF THE SHOOT~ is very slight in Filicinæ. The first leaves of the embryo are very simple in form (Fig. 205), but after a certain age all the leaves which arise are essentially alike. The fertile leaves do not differ from the barren ones, and are found associated with them, and their formation does not limit the growth in length of the stem. It is only in a few of the true Ferns, and in the Hydropterideæ, that the fertile leaves differ considerably from the barren ones. A division of labour in which certain leaves are set apart for nutrition, and others for reproduction, is found more pronouncedly in the Equisetinæ and Lycopodinæ, for in these groups, with a few exceptions, the fertile and barren leaves are very dissimilar; the former are collected in special ear-like cones, which terminate the further growth of the short stems on which they are borne. In connection with the cone, leaves are sometimes developed which form a transition from the barren to the fertile ones (the “annulus” in Equisetaceæ), and in these cases the first indication of a flower with perianth or floral-leaves is to be traced. Among the Cryptogams the highest division of labour is found in Selaginella and Isoëtes, which have the two kinds of sporangia borne on different leaves. The division of labour (modification) is, however, still more pronounced in the Phanerogams: the leaves which bear the microsporangia (“pollen-sacs”) have quite different forms from those which bear macrosporangia (the “nucellus” in the ovule), the former are termed stamens, the latter carpels; in certain instances, too, there is even a contrast between the “male plants” and the “female plants.” Moreover, a division of labour, in a much greater degree, takes place in the leaves which do not directly take part in reproduction, and it is thus possible in many plants to draw a sharp line not only between stamens and carpels, but also between four or five distinct kinds of leaves, which differ in form, structure, and corresponding functions, and which appear in regular sequence on the shoot: namely, between “scale-leaves” and “foliage-leaves,” both of which occur in the Cryptogams, and the “floral-leaves,” including the bracts and leaves of the “perianth,” which latter often differ from each other in form and colour, and are then separated into sepals and petals. The leaves--stamens and carpels--which bear the sporangia are termed sporophylls, and the shoot, or extremity of a shoot, whose leaves are modified into sporophylls, is terminated in its further growth by their production, and is known as a flower. The flower which is most completely furnished has calyx, corolla, stamens, and carpels arranged in this order. If the only sporophylls present are stamens, then it is said to be a male (staminate) flower, and if only carpels, then a female (pistillate) flower, and in both these cases the flowers are unisexual, or diclinous. If stamens and carpels are both present in the same flower, it is termed hermaphrodite. Diclinous plants in which the female flowers are situated on one plant, and the male flowers on another, are termed diœcious; and those in which the same plant bears the two kinds of flowers are termed monœcious. When the male, female, and hermaphrodite flowers are found in the same species, the plant is said to be polygamous.