Fig. 130. Selaginella grandis. (1–3, nat. size.)

In an erect species, such as S. grandis Moore[114] ([fig. 130] and [fig. 131], G) from Borneo, the main shoots, which may attain a height of 2–3 feet, bear small and inconspicuous leaves of one kind, but the lateral and repeatedly forked shoots are heterophyllous. The passage from the homophyllous to the heterophyllous arrangement is shown in the transition from the erect to the dorsiventral habit of the lateral shoots ([fig. 130], 2). The monopodially or dichotomously branched shoots produce long naked axes at the forks; these grow downwards to the ground where they develop numerous dichotomously forked branches. For certain reasons these naked aerial axes were named rhizophores and have always been styled shoots, the term root being restricted to repeatedly forked branches which the rhizophores produce in the soil. It has, however, been shown by Professor Harvey-Gibson[115] that there is no sufficient reason for drawing any morphological distinction between rhizophores and roots, the term root being applicable to both.

Our knowledge of the anatomy of Selaginella, thanks chiefly to the researches of Harvey-Gibson[116], is much more complete than in the case of Lycopodium. The stems, which may be either trailing or erect, are usually dorsiventral, and it is noteworthy that different shoots of the same plant or even the same axis in different regions may exhibit considerable variation in the structure and arrangement of the vascular tissue. In the well-known species, Selaginella Martensii, the stem, which is partly trailing, partly ascending, possesses a single ribbon-shaped stele composed of scalariform tracheids with two marginal protoxylems formed by the fusion of the leaf-traces of the dorsal and ventral leaves respectively. As in Lycopodium the metaxylem tracheae are as a rule scalariform, but reticulate xylem elements are by no means unknown. The tracheal band, surrounded by parenchymatous elements, is enclosed by phloem with external protophloem elements. The characteristic features of the stele are shown in the diagrammatic drawing of a section of another species—S. Willdenowii—represented in [fig. 131], A.

Fig. 131.

  1. Selaginella Willdenowii. Transverse section of stem: a, outer cortex; p, phloem; t, trabeculae.
  2. S. spinosa, stem: px, protoxylem.
  3. S. laevigata var. Lyallii, section of stele: t, ridge of xylem cylinder; e, endodermis.
  4. S. rupestris, seedlings with cotyledons (c) protruding beyond the sporophylls (b).
  5. Transverse section of Selaginella leaf-base: l, ligule; lt, leaf-trace.
  6. Portion of G. enlarged.
  7. S. grandis. Longitudinal section of strobilus: bb, sporophyll-trace; l, ligule.

A pericycle composed of one or two layers of chlorophyll-containing cells encircles the whole stele which is suspended in a lacuna by trabeculae ([fig. 131], A, B, t) connecting the pericycle with the inner edge of the broad cortex. The trabeculae consist in part of endodermal cells characterised by cuticular bands. The cortex is usually differentiated into three fairly distinct regions. Mechanical tissue of thick-walled fibres constitutes the outer region (a); the middle cortex consists of thinner-walled parenchyma, the elements of which become smaller and rather more compactly arranged in the inner zone. The middle cortex is frequently characterised by the presence of spaces and by the hyphal or trabecular structure of the tissue, a feature which, as Bower[117] pointed out, is common to many recent and fossil members of the Lycopodiales. In some cases, e.g. S. erythropus, from tropical America, the cortex of the creeping stem consists entirely of thick-walled cells. Selaginella grandis ([fig. 130]) has “a short decumbent stem rooted at close intervals[118],” from which thick erect aerial shoots rise to a height of one foot or more. In the apical region these erect axes give off repeatedly forked foliage shoots on which the spiral phyllotaxis of the homophyllous axis is gradually replaced by four rows of two kinds of leaves ([fig. 130], 2). The anatomy of this species agrees with that of S. Martensii. The trailing or semi-erect and homophyllous shoots of Selaginella spinosa[119] present a distinct type of vascular anatomy. The upper part of the ascending stem has an axial strand of xylem with seven peripheral groups of spiral protoxylem tracheae ([fig. 131], B); in the trailing portion of the shoot the protoxylem elements occur as one central group in the solid rod of metaxylem through which the leaf-traces pass on their way to the axial protoxylem. This type is important as affording an exception, in the endarch structure of the xylem, to the usual exarch plan of the stelar tissues. This species is the only one in which any indication of the production of secondary xylem elements has so far been recorded. Bruchmann[120] has shown that, in the small tuberous swelling which occurs at the base of the young shoot (hypocotyl), a meristematic zone is formed round the axial vascular strand and by its activity a few secondary tracheids are added to the primary xylem. With this exception Selaginella appears to have lost the power of secondary thickening, the possession of which constitutes so striking a feature of the Palaeozoic Lycopods. Another type is represented by S. inaequalifolia, an Indian species, the shoots of which may have either a single stele or as many as five, each in its separate lacuna. The homophyllous S. laevigata var. Lyallii Spr., a Madagascan species, affords a further illustration of the variation in plan of the vascular tissues within the genus. There is a considerable difference in structure between the erect and creeping shoots; in the former there may be as many as 12–13 steles, which gradually coalesce before the vertical axis joins the creeping rhizome to form one central and four peripheral steles. In the rhizome there is usually a distinct axial stele without protoxylem, surrounded by an ill-defined lacuna and enclosed by a cylindrical stele (solenostele)[121] usually two tracheae in width with four protoxylem strands on its outer edge. The continuity of the tubular stele is broken and, in transverse section, it assumes the form of a horse-shoe close to the base of an erect shoot to which a crescentic vascular strand is given off. Harvey-Gibson[122] has figured a section of the rhizome of this type in which the axial vascular strand is represented by a slight ridge of tracheae ([fig. 131], C, t) projecting towards the centre of the axis of the tubular stele. The cylindrical stele consists of xylem with external and internal phloem (p): cuticularised endodermal cells occur at e and e.

Reference has already been made to the descending naked branches given off from the points of ramification of the foliage shoots of Selaginella. It has been shown by Harvey-Gibson[123] that these branches, originally designated rhizophores by Nägeli and Leitgeb, as well as the dichotomously branched roots which they produce below the level of the ground, possess a single vascular strand of monarch type. It is interesting to find that in some species the aerial portion of the rhizophore has a xylem strand with a central protoxylem, an instance of endarch structure like that in certain portions of the shoot-system of S. spinosa. The root-anatomy of Selaginella and the dichotomous habit of branching afford points of agreement with the subterranean organs of Lepidodendron and Sigillaria.

Leaves. The leaves of Selaginella[124] usually consist of a reticulum of loosely arranged cells, but in some cases part of the mesophyll assumes the palisade form. The single vascular bundle consists of a few small annular or spiral tracheae and at the apex of the lamina the protoxylem elements are accompanied by several short reticulated pitted elements. Both foliage leaves and sporophylls are characterised by the possession of a ligule, a structure which may present the appearance of a somewhat rectangular plate ([fig. 130], 4, l, and [fig. 131], E–G, l) or assume a fan-shaped form with a lobed or papillate margin. The base, composed of large cells, is sunk in the tissue of the leaf close to its insertion on the stem ([fig. 131], E, l) and enclosed by a well-marked parenchymatous sheath. The sheath is separated from the vascular bundle of the leaf by one or more layers of cells, and in some species these become transformed into short tracheids. The ligule is regarded by Harvey-Gibson[125] as a specialised ramentum which serves the temporary function of keeping moist the growing-point and young leaves.