GENERAL RESULTS— LYCOPSIDA 635 



thickening of Gymnosperms and Dicotyledons is some- 

 what less exact in this class than in the Sphenopsida 

 or the Pteridosperms. In some forms (Lepidophloios 

 fuliginosus, for example,- p. 154) the zone of thickening 

 is extremely irregular, and although in other Lepido- 

 dendreae it approaches the- normal type more nearly, 

 it is doubtful whether in any case the same cambium 

 remained continuouslyactive throughout. The secondary 

 growth of Iso'etes presents some interesting analogies 

 with that of the Lepidodendreae. 



The enormous development of the secondary cortical 

 tissue, chiefly phelloderm, is a striking feature of the 

 tree Lycopods, and with this again the growth of the 

 cortex in Iso'etes alone presents any analogy, though, at 

 best, only a remote one. 



The difficult question of the morphology of 

 Stigmaria, the subterranean part of the Lepidodendroid 

 plant, already discussed (p. 259), suggests that the 

 differentiation between '' root and shoot " may have 

 been less sharp in this group than in other Vasculares. 

 Similar indications are to be found in the rhizophores 

 of Selaginella and the stem-like roots of some Lyco- 

 podiums at the present day. A suggestion has recently 

 been made by Professor F. E. Weiss that an analogy 

 for the Stigmarian axes may be found in the protocorm, 

 the rudimentary, somewhat thalloid, stem which is the 

 first product of germination in some species of Lyco- 

 podium, and appears to be persistent in the tuber of 

 Phylloglossum. 



It is a curious fact that among the very numerous 

 fructifications that have been investigated, no certain 



