302 LYCOPODIALES 



borne by the megasporophyll. So far as it goes, then, the evidence from 

 the fossils favours the conclusion that plants resembling Selaginella existed 

 in the primary rocks, and that even the more specialised heterophyllous 

 type of Selaginella dates at least from the Carboniferous period, while 

 it seems possible that a seed-like habit had already been established. 



The dendroid Lycopodiales are among the earliest known fossils, 

 dating from the Lower Devonian period to the Trias. They include the 

 families of the Lepidodendraceae, Bothrodendraceae, Sigillariaceae, and 



FIG. 150. 



Ground plan of a Tree-stump with Stigmaria-trunks. One-sixtieth the natural size. 

 (After Potonie.) 



Pleuromoiaceae. Underlying the differences of detail according to which 

 these families are distinguished, there is a general unity of morphological 

 plan : the essential features of it are as follows. The main axis was 

 upright, rising in some cases to a height of 100 feet. It was bulky 

 relatively to the numerous simple leaves which it bore : it branched 

 upwards in a dichotomous manner, in most cases profusely : in some of 

 the Sigillariaceae, however, and in Pleuromoia branching may be entirely 

 absent. The development of the branches of the dichotomy were in 

 various cases either equal or unequal, a fact which leads to differences of 

 habit, as is seen to be the case in Lycopodium or Selaginella. The axis 

 was fixed in the soil by a shallow and broadly spreading system of 

 Stigmarian trunks (Fig. 150). In Lepidodendron the main Stigmarian 

 trunks usually numbered four, which bifurcated repeatedly, thus forming a 



