iSo STUDIES IN FOSSIL BOTANY 



was really of the nature of cork is doubtful — the more 

 so, as the bases of the leaves outside it certainly 

 persisted for a very long time. 



The periderm (as we may call the whole of the 

 secondary cortical tissue) consisted of elongated, rather 

 thick-walled cells, and must have contributed very 

 materially to the mechanical strength of the stem. It 

 may have had other functions as well, and was certainly 

 much more than a mere bark. 1 The late M. Hove- 

 lacque, to whom we owe the most detailed study of the 

 anatomy of this species, 2 was mistaken in supposing 

 that the periderm was formed entirely from within ; on 

 its inner margin the tissue is, as a rule, thick-walled, and 

 quite unlike a meristem, while a delicate zone of cells 

 is constantly to be found in its outer part (see Fig. 

 59). It appears probable, however, that additions 

 may sometimes have been made to the periderm from 

 the interior also, new layers of primary cortical cells 

 taking up the divisions. 



In old specimens, the periderm attained a thickness 

 of as much as 2 inches. The course of the leaf-trace 

 bundles through it is marked by radial strands of tissue 

 more delicate than the rest. The periderm also shows 

 concentric markings, due to the alternation of zones 

 of wider and narrower elements. What has been said 

 of the periderm of Lepidodendron selaginoides may 

 be taken as holding good for the genus as a whole 

 (cf. Figs. 58 and 59). 



1 In L. Wunschianwu it contained strands of cells probably with a 

 secretory function. 



2 Hovelacque, " Recherches sur le Lepidodendron selaginoides" 3&m. 

 Soc. Linnienne de Normandie, 1892. This fine memoir is magnificently 

 illustrated. 



