or Vi Filicales

The prothalli of both genera have been discovered since They are colourless, nourishing themselves saprophytically through fungal agency. In fact these primitive plants conform in their life-cycle to what is seen in many primitive ferns and Lycopods. Moreover, the embryology has been traced in Tmesip teris; there is neither suspensor, nor root, nor cotyledon. The em bryo with its apex directed, like that of the Bryophytes towards the neck of the archegonium, soon bursts its way out, proceeding to branch distally and form a leafless rhizomic system from which aerial shoots arise later. Such details of the gametophyte link the Psilotales, and indirectly the Psilophytales, with other Pterido phytes as regards alternation on the whole ; but the rootless and leafless embryo presents a state that finds its nearest comparison with the Bryophyta.

III. Sphenophyllales.

The two genera Sphenophyllum and Cheirostrobus are the representatives of this class, but in more or less loose association with them are such types as Pseudo bor nia and Hyenia; these suggest that the articulate type of land plants, which they all illustrate, in common with the horsetails, was well represented in the primary rocks. The existence of the Sphenophyllales extended from the Upper Devonian to Triassic time. (For details of Sphenophyllales see PALAEOBOTANY.) IV. Equisetales.—Those Pteridophyta which have their ap pendages disposed in successive whorls, with long internodes in tervening between them, have been designated the Articulatae. In the distant past this type was strongly represented by the Sphe nophyllales, and by the Calamariaceae and others. But it is fa miliar to us to-day as it is seen in the horsetails, included in the cosmopolitan genus Equisetum, the only species of Equisetaceae, and the only living representatives of the Equisetales. These are semi-aquatic plants ; they vary in height from a few inches to 3oft. or more, and are rhizomatous, with erect shoots arising from richly branched, subterranean stems, which are themselves rooted in the soil. The habit of the plants depends upon the method of branching of the shoot rather than upon the foliage, for the leaves are minute (microphyllous), and closely appressed to the stem that bears them. Each whorl of them forms a sheath closely investing the base of the next higher internode, while the teeth projecting upwards from it are all that represent the individual leaves. Their position alternates in successive nodes (fig. 7). The internodes are fluted, the ridges being continuous downwards from the next higher leaves; consequently those of successive in ternodes alternate. The number of the leaves in a whorl may vary according to the size of the stem, from three to 20 or 3o. This is the scheme of the shoot in all species of Equisetum, and the shoot is constantly of radial construction. In some species the branching is sparse (E. limosum) ; in others it is profuse (E. syl vaticum and maximum), and the branches may themselves branch again repeatedly. Their number and the degree of secondary branching defines the habit, and justifies for the more bushy types the familiar name of horsetails. The branches arise in the axils of the leaf-sheaths, but they alternate with the leaves them selves. Many of those initiated remain dormant. A root is

found at the base of each bud, but it also is frequently dormant, especially in aerial shoots. The structure of the underground rhizome is on the same plan as that of the aerial branches that it bears. Thus the whole plant consists of a succession of shoots, each with a dominant axis, whorls of subordinate leaves, and accessory roots.

In the transverse section of an aerial internode the sinuous out line shows the projecting ridges and depressed furrows of its fluted form, though this is less evident in the rhizomes (fig. 7, D). The centre of the section is occupied by a large air-cavity, surrounded by the remains of the pith. A circle of isolated vascular strands, corresponding in number to the leaves in the next whorl above, is in most species enclosed by a sinuous endo dermis, which thus delimits the stele (E. arvense, etc.). Outside this lies the cortex, marked by an equal number of large air-cavi ties, which alternate with the vascular strands, and correspond to the furrows of the fluting. The vascular strands themselves are opposite the projecting ridges, and are extended upwards into the several leaf-teeth. Between the ridges are regions of green chlorophyll-parenchyma, while the whole is invested by a well marked epidermis, with curiously elaborate stomata: these are ranged along the furrows, and so are opposite the photosynthetic tissue. The anatomy is clearly that of a semi-aquatic plant with its reduced vascular tissue and large air-spaces.

The sporangia of Equisetum are borne upon lateral append ages of the axis, sporangiophores, which are disposed in whorls, and are associated in definite strobili or cones, borne distally (fig. 7, A). Each sporangiophore consists of a stalk that ex pands into a peltate disc, from the inner surface of which some six to nine large sporangia hang parallel with the stalk (fig. 7, B, C). A single vascular strand passing through the stalk supplies a branchlet to each sporangium. These arise as massive growths from the first, and each produces a large output of homosporous spores (eu-sporangiate). The spores themselves are large; in ripening the outermost layer of the wall of each splits along spiral lines, giving rise to four elaters that straighten out when dry, and close round the spore in damp air. They are effective in forcing open the sporangium, which dehisces by a longitudinal slit : and those of different spores hooking together, the spores are grouped in germination, a matter of importance since the sexes are usually borne on separate prothalli. The spores germinate at once, producing each a green prothallus, which may be filamentous, flattened, or irregularly lobed. The smaller bear sunken antheridia, each with numerous spermatocytes, from which spermatozoids are liberated on access of water. The larger bear archegonia. The conditions of syngamy and formation of the embryo are essen tially as in ferns ; but here the embryo has its apex directed to the archegonial neck (exoscopic), and it grows out directly into the apex of the young plant, successive whorls of leaves arising lat erally upon it ; a root derived from the basal region fixes the young plant in the soil. Notwithstanding the differences of de tail, the life-cycle of a horsetail is comparable as a whole with that of a fern.