246 STUDIES IN FOSSIL BOTANY 



words : " The meristemic activity of the cambial layer 

 may have manifested itself irregularly rather than 

 periodically." l At the exterior of the wood, some 

 remains of the cambium can sometimes be traced, 

 passing over externally into a zone of delicate tissue, 

 more or less radially arranged, and consisting, as 

 shown by longitudinal sections, of elongated elements. 

 This v/as no doubt the phloem. On its outer margin 

 is an imperfectly preserved lacunar zone, such as often 

 occurs in a corresponding position in the stems of 

 Lepidodendron. The middle cortex (Fig. 98, c) is never 

 perfectly preserved, but where parts of it remain, it 

 appears to have the same curious trabecular structure, 

 as if made up of interwoven filaments, which we so 

 often find in the same region in Lepidodendron. In 

 some specimens the preservation of the outer cortex 

 is very perfect (see Figs. 98 and 100). The primary 

 structure of this region can only be observed in fairly 

 young examples (Fig. 98, c 2 ). Most of its thickness 

 is made up of a very uniform large-celled parenchyma. 

 On the external surface is a hypodermal zone of smaller 

 cells, marked off from the rest of the cortex by a 

 narrow band of dark tissue, probably sclerotic (Figs. 

 98, c z , and 100). 



Periderm -formation set in early, and the older 

 specimens were provided with an immensely thick 

 bark, fragments of which are among the " common 

 objects" of the coal -balls. The development of 

 periderm (Figs. 98 and 100, pd) began in a deep- 

 seated layer of the outer cortex, and appears to have 

 gone on for some time, chiefly in the centripetal 



1 " Monograph of Stigmaria ficoides," p. 17. 



