. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. GENERAL MORPHOLOGY 303 widely spreading system : from these the rootlets radiated in all directions, developing to a length of a foot or more, and showing dichotomous branching. The underground system was thus proportional to that above ground. In the Sigillariaceae similar trunks are found, but it seems doubtful whether they show the same constancy of initial type as in Lepidodendron. In Pleuromoia the base of the upright stock swells into a tuberous body, which is very Stigmaria-Wke in the fact that
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. GENERAL MORPHOLOGY 303 widely spreading system : from these the rootlets radiated in all directions, developing to a length of a foot or more, and showing dichotomous branching. The underground system was thus proportional to that above ground. In the Sigillariaceae similar trunks are found, but it seems doubtful whether they show the same constancy of initial type as in Lepidodendron. In Pleuromoia the base of the upright stock swells into a tuberous body, which is very Stigmaria-Wke in the fact that it is covered by root-scars, while it extends into four blunt processes corre- sponding in position and character to Stigmarian trunks, though much shorter (Fig. 151). It would seem probable that in this relatively late Triassic fossil (which is unfortunately known only in the form of casts, not structurally), a simple representative of the Lepido- dendroid basal region is cor- rectly recognised. In all of the dendroid forms the Stig- marian trunk appears to have been present, as a basis for the roots : but the latter were not restricted to that position: Potonie shows how the scars of their insertion may be sometimes found on the leaf- bearing axes also, associated with some degree of regularity with the The leaves of the fossil Lycopodiales wrere sometimes of considerable size, but un- branched and of simple form. They expanded at the base into the well-known cushions, which in many forms occupy the whole external surface of the axis : this corresponds to what is seen among the living Lycopods. On the upper surface of the leaf, near its base, the ligule is seated : it appears to have been a constant feature in the dendroid Lycopodiales, and the occurrence of it links them rather with Selaginella than with Lycopodium. It was often seated in a deep pit—as it is in some living Selaginellas—and this pit persists as a marked feature in the neighbourhood of the le
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