Cones. The terminal portions of the branches of Selaginella usually bear smaller leaves of uniform size which function as sporophylls, but in this genus the fertile shoots do not generally form such distinct cones as in many species of Lycopodium. In S. grandis (figs. [130], 3; [131], G) the long and narrow strobili consist of a slender axis bearing imbricate sporophylls in four rows: each sporophyll subtends a sporangium situated between the ligule and the axis of the shoot. The sporangium may be developed from the axis of the cone or, as in Lycopodium, from the cells of the sporophyll[126]. In some species the lower sporophylls bear only megasporangia, each normally containing four megaspores, the microsporangia being confined to the upper part of the cone. This distribution of the two kinds of sporangia is, however, by no means constant[127]: in some cases, e.g. S. rupestris, cones may bear megasporangia only, and in the cone of S. grandis, of which a small piece is represented in [fig. 131], G, all the sporangia were found to contain microspores.
The occurrence of two kinds of spores in Selaginella constitutes a feature of special importance from the point of view of the relationship between the Phanerogams, in which heterospory is a constant character, and the heterosporous Pteridophytes. One of the most striking distinctions between the Phanerogams and the rest of the vegetable kingdom lies in the production of seeds. Recent work has, however, shown that seed-production can no longer be regarded as a distinguishing feature of the Gymnosperms and Angiosperms. Palaeozoic plants which combined filicinean and cycadean features resembled the existing Phanerogams in the possession of highly specialised seeds. This discovery adds point to the comparison of the true seed with structures concerned with reproduction in seedless plants, which in the course of evolution gave rise to the more efficient arrangement for the nursing, protection, and ultimate dispersal of the embryo. In the megaspore of Selaginella we have, as Hofmeister was the first to recognise in 1851, a structure homologous with the embryo-sac of the Phanerogam. The embryo-sac consists of a large cell produced in a mass of parenchymatous tissue known as the nucellus which is almost completely enclosed by one or more integuments. Fertilisation of the egg-cell within the embryo-sac takes place as a rule while the female reproductive organ is still attached to the parent-plant and separation does not occur until the ovule has become the seed.
In a few cases, notably in certain plants characteristic of Mangrove swamps, continuity between the seed and its parent is retained until after germination. The megasporangium of Selaginella dehisces[128] along a line marked out by the occurrence of smaller cells over the crest of the wall. It has been customary to describe the megaspores as being fertilised after ejection from the sporangia. This earlier separation from the parent and the absence of any protective covering external to the spore-wall constitute two distinguishing features between seeds and megaspores. In Selaginella apus, a Californian species, Miss Lyon has shown that fertilisation of the egg-cell usually takes place while the megaspore is still in the strobilus. On examining withered decayed strobili of this species which had been partially covered with the soil for some months after fertilisation of the megaspores, several young plants were found with cotyledons and roots projecting through the crevices of the megasporangia[129]. From this, adds Miss Lyon, “it seems safe to assume that an embryo may have two periods of growth separated by one of quiescence quite comparable to those of seed plants with marked xerophilous features.”
In another Western American species S. rupestris described by the same writer the cotyledons of young plants were found protruding from the imbricate sporophylls of a withered cone ([fig. 131], D). This species is interesting also from the occasional occurrence of one instead of four megasporangia in a sporangium; a condition which affords another connecting link between the heterosporous Pteridophytes, on the one hand, and the seed-bearing Phanerogams in which the occurrence of a single embryo-sac (megaspore) in each ovule is the rule. The cones of Selaginella rupestris retain connexion with the plant through the winter and fertilisation occurs in the following spring. After the embryo has been formed the megasporangium “becomes sunken in a shallow pit formed by the cushion-like outgrowth of the sporophyll around the pedicel.” It is suggested that this outgrowth may be comparable with the integument which grows up from the sporophyll in the fossil genus Lepidocarpon[130] and almost completely encloses the sporangium. In the drawings given by Miss Lyon no features are recognisable which afford a parallel to the integument of Lepidocarpon. I have, however, endeavoured to show, by a brief reference to this author’s interesting account of the two Californian species, that the physiological and morphological resemblances between the megasporangia of Selaginella and the integumented ovules of the seed-bearing plants are sufficiently close to enable us to recognise possible lines of advance towards the development of the true seed.
Professor Campbell[131] records an additional example of a Selaginella—probably S. Bigelovii—from the dry region of Southern California in which the spores become completely dried up after the embryo has attained some size, remaining in that state until the more favourable conditions succeeding the dry season induce renewed activity.
Isoetaceae.
The genus Isoetes is peculiar among Pteridophytes both in habit and in anatomical features. In its short and relatively thick tuberous stem, terminating in a crowded rosette of subulate leaves like those of Juncus and bearing numerous adventitious roots, Isoetes presents an appearance similar to that of many monocotyledonous plants. The habit of the genus is well represented by such species as Isoetes lacustris and I. echinospora[132] ([fig. 132]) both of which grow in freshwater lakes in Britain and in other north European countries. The latter species bears leaves reaching a length of 18 cm. The resemblance in habit between this isolated member of the Pteridophytes and certain Flowering plants, although in itself of no morphological significance, is consistent with the view expressed by Campbell that Isoetes may be directly related to the Monocotyledons[133].
Fig. 132. Isoetes echinospora (After Motelay and Vendryès).
- Stem of I. lacustris.
- Base of sporophyll: l, ligule; spg, sporangium partially covered by velum.