LEPIDODENDRON 133 



upper side of the cushion in the younger specimens, 

 but that the leaf-bases bent downwards in the older 

 stems, bringing the scar to a lower level (cf. Fig. 63, p. 

 159). Lepidophloios is of interest, though differing so 

 slightly from Lepidodendron, because some of the 

 specimens with structure certainly belonged to it, as 

 did also some of the forms to be described further on, 

 in which the probable position of the fructifications on 

 the branches has been determined. So far as is known, 

 however, there is no constant difference in internal 

 structure between Lepidophloios and Lepidodendron, and 

 in dealing with the anatomy, the latter name will 

 usually be employed, treating Lepidophloios, for our 

 present purpose, as a subgenus. 



2. Stem. — We have now a very considerable mass 

 of material for the study of the internal structure of 

 Lepidodendreae, and of this material the greater part 

 has been derived from the British Carboniferous rocks. 

 Our knowledge of the anatomy of the group is chiefly 

 due to the researches of Williamson, who worked out 

 and described the anatomy of no less than nine distinct 

 forms of the group, though some of these are better 

 known than others. The Continental strata have so 

 far yielded comparatively few specimens of Lepidoden- 

 dron, with structure preserved, so our description will 

 be chiefly based on the British fossils. 



The great anatomical feature, which is common to 

 all known stems of Lepidodendron, is the presence of 

 a single stele, with centripetally developed primary 

 wood. The occurrence of a pith is inconstant, not 

 only among different species, but even in different 



