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-like 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 leaf-scars. 1 



The leaves of the fossil 

 Lycopodiales were 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 leaf-scars, whenever the cast 

 of a stem-surface is well preserved (Fig. 152). 



The vegetative region appears to have been, as a rule, purely 

 vegetative : the sporangia are restricted to well-defined cones or strobili, 



1 Lehrbnch der Pflanzenpalaeontologie, p. 212, Fig. 215. 



FIG. 151. 



Pleuromoia Sternbergii. Swollen base of stem with root- 

 scars, and showing part of the aerial stem, with the epidermis 

 and leaf-scars on the right, and on the left the sub-epidermal 

 sculpture. (After Bischof, from Engler and Prantl.) Two-thirds 

 natural size. 



