A greatly added interest in Equisetum arises from comparison with allied fossils; for not only were these numerous and of early occurrence, but they attained dendroid proportions, while some were heterosporous. They are grouped as the Calamariaceae, which will be specially treated elsewhere (see PALAEOBOTANY).
The Equisetales, thus comprising the Equisetaceae and the Calamariaceae, form a natural and closely related class, of which the nearest affinity was with the Sphenophyllales, but with some degree of relation also with the Psilotales; all of these being sporangiophoric Pteridophytes.
V. Lycopodiales.—This class comprises a considerable num ber of species now living widely distributed upon the earth, and known as club-mosses; though they are in fact vascular plants, and quite distinct from the true mosses. They are all relatively small, and indeed insignificant as features in the present flora, compared with the fossil types which, though they may have in cluded a number of relatively small species, comprised also some of the largest plants of the forests of the coal period. It may be that among the lycopods no actual diminution in size took place as time went on, but rather that the types which were always small survived, while the giant members of the group became extinct.
The features which all club mosses have in common are that the leaves are relatively small and simple in form (microphyllous), while the sporangia are seated singly, one in the axil of each leaf of the fertile region, or spreading outwards on its base. This marks off the Lycopodiales clearly from the sporangiophoric Pteridophytes on the one hand, and from the Filicales on the other. As in the former, however, the axis is dominant in the adult shoot, and forks equally or unequally. According to the position assumed by the stem the habit of living club mosses is upright or pendent ; but frequently, as in the stag's horn moss common on Scottish hills, with its creeping stem rooted in the soil, and bearing upright fruiting branches (fig. 8, B).
The class is divided systematically according to the presence or absence of a ligule, which is a small scale-like body borne on the upper surface of each leaf, near its base. Those in which no ligule is present, the ELIGULATAE, inciude Lycopodium and Phyl loglossum, together with certain early fossils designated Lycopo dites; those which possess a ligule, the LIGULATAE, include Selag inella and Isoetes, and with these are associated the fossil Lepi dodendraceae and Sigillariaceae. The distinction is accentuated by the fact that the former are all homosporous, the latter het erosporous.
A. Lycopodiales Eligulatae. Lycopodium comprises about i oo species of small plants of varied habit, creeping, shrubby or epiphytic. The construction of the shoot is uniformly microphyl
lous, the bifurcating stem dominating the conformation of the whole plant. The leaves are simple and small, and in some species uniform throughout the plant. In others the sterile leaves are larger than the membranous sporophylls, the latter being asso ciated in definite strobili, or cones (fig. 8, B). The former prob ably represent the more primitive type. Each of the isolated sporangia is seated at the base of its sporophyll; it is large, and kidney-shaped, with a short massive stalk, and it dehisces like an oyster-shell, in a plane parallel to that of the leaf. The arrange ment of the leaves is sometimes in regular whorls, but frequently it is according to some more or less interrupted spiral ,scheme. The plant is fixed at its base by roots which spring endogenously from the stem, and show bifurcation.
The stem of Lycopodium is seen in transverse section to be surrounded by a bulky and often indurated cortex, and traversed by a stele continued to the apical cone itself, while from each leaf a minute vascular strand passes to its periphery. In the young stem there is a solid woody core, surrounded by phloem and ill defined sheaths. But in fully-grown stems the core may be in vaded by tracts of phloem, moulding it into a cruciform or stellate transverse section, or even separating it into distinct radiating plates, or permeating it to form a sort of woody The sporangia are massive from the first, and vary slightly in spread along the leaf-surface, a point of interest for comparison with the ligulate types. Each produces after the usual tetrad division a large number of homo sporous spores, which germinate slowly. The prothalli produced from them, and the embryology that follows, vary more than is usual in a single genus. The prothallus sometimes grows at soil-level and is green (L. cernuum) ; but often it is un derground and wholly sapro phytic (L. clavatum, fig. 8, D, A). Whatever its form, the sex-organs are massive and deeply sunk, and both are present on the same prothallus. The antheridia produce numerous bi ciliate spermatozoids; the arche gonia vary in length of neck, and sometimes have numerous canal-cells. But the egg is deeply seated, and produces an embryo with a suspensor. This thrusts the embryo deep into the prothallus, where it often develops juvenile characters of impor tance for comparison. The most striking of these is the swollen "protocorm," a tuberous growth seen in Hornea, and present also in Phylloglossurn. Bursting out from the prothallus the embryo develops shoot and root, the former in the underground types finding its way upwards to the soil-level (fig. 8, A).