Lycopodiales. (Club-Moss allies.)—The lycopods are typically small-leaved plants. There are three living genera : Selaginella, Isoetes and Lycopodium, interesting in view of their relation to the Palaeozoic members of the group : the first two are ligulate and the second has secondary thickening in its rootstock in these respects comparing with the majority of the fossils. The living forms must be regarded as the remnant of a once important group; they are all herbs, whereas the fossils were almost all trees.
(a) Ligulateae. Lepidodendron (fig. 7) which may be taken as the type of the group is of extended geological range ; while Protolepidodendron, a very doubtfully related plant, is of Middle Devonian age Lepidodendron itself ranges from the Upper Devo nian to the Permian. The trunk reached a height of over 114 ft. while above that the crown of forking branches rose for a further 20 ft. The lance-shaped leaves are usually from one to four inches in length but sometimes reached the length of a yard ; they were shed when the part of the plant bearing them reached a certain age leaving a scar on the persistent leaf-base (fig. 8 and Pl. I. fig. 7). The ligule, a small secretory scale, is situated in a pit on the leaf-base just above the point of attachment of the leaf. The leaf-bases are diamond-shaped in outline and are very regu larly arranged on the surface of the stem. The leaves have a single median vein and the ventilating tissue of the leaf was connected with that of the stem by two strips of a similar termed parichnos which have left the two small marks one on each side of the vascular strand on the leaf-scar (fig. 8). There were deciduous side branches arranged in two vertical rows on each side of the trunk or main branches: when these became detached they left a characteristic scar. In other species there were nu merous vertical rows of these de ciduous branches and while it is possible that these branches may have borne the cones in yet other species the cones are known to have been placed on the ends of the ordinary foliage twigs (fig. 7). The stems and branches were traversed by a cylindrical column of primary xylem, in some species solid, but in others with a parenchymatous core. The protoxylems are situated on the outer surface of this column and from them the small vascular strands to the leaves passed out through the external tissues. In most species secondary wood was formed round the primary. External to the wood was a tissue corresponding to the phloem surrounded by the inner and outer cortex, the former a lacunar, ventilating tissue, while the latter had a cambial zone near its outer limit which gave rise to corky tissue. A thin layer of the cortex outside the corky tissue afforded a foundation for the attachment of the persistent leaf bases and became fissured later by the expansion due to the sec ondary growth within. The cones were in general construction like those of Selaginella only much larger and as in the living genus each sporophyll bore a ligule on its upper surface close to the abaxial face of the sporangium. In Lepidostrobus, cones which belong to various species of Lepidodendron, each sporophyll con sists of a stalk bearing on its upper surface a radially elongated sporangium• with the ligule placed in a pit just beyond. The stalk terminated in an upturned blade. Some cones are heterosporous the upper sporophylls bearing micro-sporangia, while those at the base of the cone bear mega-sporangia (fig. 9).
Each megasporangium contained 8—i6 megaspores. Other spores are known which bear only microsporangia or only mega sporangia. It is also possible that such species bore the two kinds of cones on separate trees. Megaspores have been found with the prothallus and archegonia preserved ; their reproduction must have been similar to that of Selaginella and it is probable that the spermatozoids which fertilised the ova in the archegonia were liberated from microspores which became entangled in the spinous processes found on the outsides of the megaspores. The chances
of megaspores and microspores becoming associated in this way were largely augmented by the enormous number of microspores produced in each microsporangium. Some coals of Carboniferous age consist very largely of compressed Lycopod spores. In Lepido carpon (fig. 1o, B), another type of cone, only one megaspore is present in the mature sporangium and an integument, formed from an outgrowth from the sides of the sporophyll-stalk invested the sporangium except for a narrow opening along the top. In Miadesmia (fig. Io, A), the fructification of a herbaceous Lycopod of Carboniferous age, the integument is prolonged in the form of a sheath beyond the top of the sporangium. A fairly close comparison may be made between these two fossils and true seeds ; in both fossils however, the sporophyll broke away from the parent plant with the sporangium before fertilisation took place or possibly even before microspores were deposited on it, unlike a true seed in which an embryo is usually developed before the seed is liberated from the parent plant. Among other Carbonif erous lycopods Sigillaria occupies an important position. Sigil laria had a large trunk with persistent leaf bases placed either like those in Lepidodendron or else in vertical rows. It is probable that many Sigillariae were unbranched except for small, lateral, cone-bearing twigs and the tall trunks with the tufts of leaves at the tops must have presented a peculiar appearance. Sigillaria was heterosporous and the distribution of the two kinds of spores in the cones was probably the same as in Lepidodendron. In Mazocarpon, a sigillarian cone, the sporophylls were curiously constructed; the wall of the sporangium had a curved plate pro jecting from its distal end and there is no integument. A large mass of sterile tissue is present inside, and attached to, the base of the sporangium and megaspores up to the number of eight were arranged in a single layer between this sterile tissue and the wall of the sporangium. Vascular tissue is present in the centre of the sterile tissue and it connects up with the vascular strand in the sporophyll. The megaspores are concave on the side next to the sterile tissue and archegonia are present in the prothallus in each spore opposite to the place at which the spore broke open. The megaspores when dispersed broke away part of the overlying sporangium wall. Lepidodendron and Sigillaria are sometimes found attached to their root bearing parts which receive the name Stigmaria. The base of each Lepidodendron or Sigillaria consisted of four main downwardly directed branches which spread out and forked equally and repeatedly (fig. 7). These branching systems are often found penetrating the barren sandstone or shale under neath the seams of coal and offer certain evidence that the coal has formed in the actual place of growth of the trees from whose !mains it has been formed. The surfaces of these underground ranches are covered with roots which penetrated the surrounding )il. The morphology of these root-bearing structures is not yet illy elucidated ; internally they are constructed more like stems 'Ian roots and the true roots which are attached to them are only ery slightly endogenous. The whole branching, root-bearing tstem may be closely compared with the root stock of the living sates in which there is a peculiar type of secondary thickening nd which bears roots very similar in structure to those of Stig taria.