Page:EB1911 - Volume 22.djvu/625

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609
PTERIDOPHYTA


sclerenchymatous. The central cylinder of the root often shows a striking resemblance to that of the stem. The Lycopodiaceae are homosporous. The spores are formed in sporangia of considerable size, situated on the upper surface and near the base of the sporophylls. The latter may differ from the foliage leaves and be arranged in definite cones, or the two may be similar and occupy alternate zones of a shoot with continued growth; sometimes rudiments of sporangia are found at the bases of the leaves (fig. 4). In the development of the sporangium the sporogenous tissue is derived from a number of superficial cells by divisions parallel to the surface. The tapetum is derived from the layer of cells surrounding the sporogenous group. Short trabeculae of sterile tissue have been found to project into the cavity of the sporangium of some species. The spores, when liberated by the dehiscence of the sporangium, give rise to the prothallus, which is now, owing mainly to the investigations of Treub and Bruchmann, known in a number of tropical and temperate species. In habit and mode of life of the prothallus these present striking differences, which may be correlated with the situations inhabited by the sporophyte, and are perhaps to be regarded as adaptations which have enabled the species to survive. Thus in L. cernuum and others the prothallus is green and grows on the surface of the soil (fig. 1, b); in the species living on the moors it is subterranean and saprophytic, though sometimes capable of developing chlorophyll when exposed to light (fig. 1, d); while in L. Phlegmaria and other epiphytic forms the prothallus consists of fine branches growing saprophytically in rotting wood (fig. 1, c). A comparison of these various types would appear to indicate that the primitive form of prothallus in the genus was radially symmetrical and contained chlorophyll. The prothalli of L. cernuum come nearest to this; in them the meristem forms a zone slightly below the summit, which may bear a number of green lobes. The different forms of the prothallus found in L. Selago give an idea of how the more extremely modified types could be derived from such a prothallus as that of L. cernuum. All the saprophytic prothalli contain an endophytic fungus in definite layers of their tissue. The antheridia and archegonia are produced above the meristematic zone, and are more or less sunk in the tissues of the prothallus. The most important difference in the sexual organs concerns the length of the archegonial neck; this is shortest and has only a single canal cell in L. cernuum, while in L. complanatum it is longer than in any other Vascular Cryptogam, and contains a number of canal cells. The spermatozoids are biciliate. The embryo in L. cernuum and other forms with superficial green prothalli is attached to the prothallus by a small foot, and develops at first as a tuberous body (the protocorm) bearing rhizoids; this forms a number of simple leaves, and upon it the apex of the shoot arises later. In the saprophytic forms the protocorm is absent, and in some of them the foot is of large size (fig. 4, B). When new individuals of species which possess a protocorm arise vegetatively from the leaves or roots of young plants, the protocorm appears in the young sporophyte. This fact leads to the consideration of Phylloglossum, which resembles the embryo of Lycopodium cernuum in so many respects that it has been spoken of as a permanently embryonic form of Lycopod: it is in some respects the simplest existing Pteridophyte. Its prothallus resembles that of L. cernuum, but wants the crown of assimilating lobes. The plant is reproduced by tubers, which resemble the protocorm in bearing first a number of protophylls and later the upright shoot with its single terminal strobilus. The sporangia agree with those of Lycopodium in structure and position.

EB1911 Pteridophyta - Selaginella.jpg
(From Strasburger's Lehrbuch der Botanik.)
Fig. 5.—Selaginella.
A, S. helvetica (nat. size).
B, S. denticulata, young plant attached to the megaspore (enlarged).

Selaginellaceae.—The single genus of this order (Selaginella) contains between three and four hundred species. There is considerable diversity among them as regards external form, the majority having dorsiventral aerial shoots with dimorphic leaves (fig. 5, A), while in others the shoots are radially symmetrical and the leaves alike. The stem contains one, two or several steles; in one species the stele is tubular. The phloëm completely surrounds the xylem, which usually develops from two protroxylem groups. In the aerial stem of the British species (S. spinosa) the radial stele has a number of protoxylem groups arranged round the periphery, much as in Lepidodendron. The cells of the endodermis are developed as trabeculae, which traverse the continuous air-space surrounding each stele. The simple, uni-nerved leaves have a ligule near the base; the base of the ligule is somewhat sharply marked off from the other tissues of the leaf. In some species a depression of the leaf-surface encloses the ligule, regarding the function of which little is known. The roots, the stele of which is monarch, may arise directly from the stem, or are borne on rhizophores, which spring from the shoot at the point of branching, and root on reaching the soil. In structure they resemble the roots, but their morphological nature is uncertain. The sporophylls are arranged radially in the cones, which are terminal on the branches. A single sporangium is borne on the axis just above the insertion of each sporophyll. Selaginella is heterosporous, the megasporangia being often found towards the base of the cone. The development of the micro- and megasporangia is the same up to the stage of isolation of the spore mother-cells. The sporogenous tissue, which is referable to several archesporial cells, is surrounded by a tapetum, mostly derived from the sporogenous group. In the microsporangium all the mother-cells undergo the tetrad division, giving rise to the numerous microspores. In the megasporangium, on the other hand, the four megaspores, which arise from a single mother-cell, are nourished at the expense of the other sporogenous cells and of the tapetum. On germination the micro spores give rise to a reduced prothallus, consisting of the small cell first cut off and a wall of cells enclosing two to four central ones; from these latter the biciliate spermatozoids originate. The megaspore becomes filled with the female prothallus, the formation of cell-walls commencing at the pointed end of the spore, where from the first the nuclei are more numerous, and later extending to the base. The surface of the prothallus, which is exposed when the thick wall of the spore is ruptured, may produce a few rhizoids; upon it the archegonia, consisting of a short neck and the central series of ovum, ventral canal cell and canal cell, arise (fig. 1, e). After fertilization the embryo forms a short suspensor; the apex of the stem, with a leaf on each side of it, is first distinguishable; at the base of this is the foot; while the root arises on the farther side of the latter. Thus the position of the root in Selaginella is different from what obtains in the other Vascular Cryptogams. A point of interest in this heterosporous genus is that the formation of the prothallus may commence before the megaspore is liberated from the sporangium.

Lepidodendraceae.—This order includes only extinct forms, the best known of which are the plants placed in the genera Lepidodendron and Sigillaria. These plants, a fuller description of which must be sought in the article Palaeobotany: Palaeozoic, underwent secondary increase in thickness and attained the size of large trees; the aerial stem was more or less branched dichotomously. The leaves, which were of simple form and provided with a ligule, were, as the leaf-scars on the stem show, variously arranged. In Sigillaria the latter form vertical rows, while in Lepidodendron the arrangement is a complicated spiral. The stem had a single stele, the primary xylem of which was polyarch and centripetally developed. The upright stems were attached to the soil by a number of dichotomously branched members (Stigmaria), which, whatever their morphological nature may be, appear to have performed the function of roots: they bore numerous cylindrical appendages, which penetrated the soil on all sides. The cones, which in some instances at least were heterosporous, presented a general resemblance to those of Lycopodium and Selaginella, a single sporangium being situated on the upper surface of each sporophyll. The cavities of the large sporangia were sometimes traversed by trabeculae of sterile tissue resembling those found in Isoëtes. In some of the heterosporous forms (Lepidocarpon, Miadesmia) the sporangia were sometimes surrounded by an integument; and since only a single megaspore attained maturity, the structure of the megasporangium suggests a comparison with an ovule.

Isoëtaceae.—The single genus (Isoëtes) contains about fifty, mostly aquatic, species, though a few are amphibious or terrestrial. The plants present considerable uniformity in general habit, consisting of a short, unbranched stem, bearing the closely-crowded awl-shaped leaves, which in the larger species attain the length of a foot. Each leaf bears a ligule resembling that of Selaginella in structure and position. The stem is monostelic, the centre of the stele being occupied by a mass of short tracheides; but little can be said as to the primary structure of the central cylinder, which appears to be reduced. A meristematic zone forms a short distance outside the xylem, from which secondary tissue is developed both internally and externally; that to the inside contains both xylem and phloëm elements. By the unequal development of the secondary cortex the stem becomes two- or three-lobed; the roots, which branch dichotomously, spring from the furrows between the lobes. The leaves have a single main bundle, and in the mesophyll are four longitudinal series of large intercellular spaces separated by transverse diaphragms. The sporangia, which are situated singly on the adaxial surface of the leaves, between their insertion on the stem and the ligule, arise from a considerable number of epidermal cells. The cells composing the young sporangium are at first similar, but ultimately become differentiated into sterile trabeculae, which may stretch from the inner to the outer wall, and the mother-cells of the spores. The latter are more numerous in the microsporangium than in the megasporangium. The tapetal layer is partly formed from the sporangial wall and partly as a layer covering the trabeculae. The spores, which are set free by the rotting of the sporangial wall, germinate much as in the case of Selaginella, though the similarity may be a case of independent resemblance. Important points of difference are found in the multiciliate spermatozoids, and in the embryo, which has no suspensor.