XV] LEPIDODENDRON 123 



and much smaller cells of the meristematic zone. This is seen 

 in fig. 155, which shows a small piece of fig. 168, A, on a larger 

 scale. In plants with a normal cambium the segments cut off 

 from the initial layer fit on to the elements of the xylem or 

 phloem to which they are to form additions, but in Lepido- 

 dendron it seems to be a general rule to find each of the most 

 external lignified elements abutting on a group of two or three 

 much smaller cells. It is difficult to believe that the meristem 

 shown in fig. 155, m, could produce secondary xylem elements 

 equal in size to those already formed: in all probability had 

 growth continued there would have been a marked difference 

 between the size of the secondary tracheids, as in fig. 152, A, x^, 

 where there was no doubt some cause which interfered with 

 normal cambial activity. This disparity in size between the 

 secondary xylem elements and the adjacent parenchymatous 

 tissue of the meristematic zone is by no means exceptional and 

 may be described as the general rule. It is at least certain 

 that in Lepidodendron vasculare, as in other species, the secondary 

 xylem was succeeded by a broad band of parenchymatous 

 tissue, from which new tracheae and medullary-ray elements 

 were produced, and not by a narrow cambium such as occurs 

 in recent plants. 



V. Lepidodendron stems as represented by casts and 

 impressions of partially decorticated specimens. 



The differentiation of the outer cortex of a Lepidodendron 

 into comparatively thin-walled and more resistant tissue has 

 been the cause of unequal decay and the consequent formation 

 of shrinkage cavities. In addition to the unequal resisting 

 power of contiguous tissues, another important factor in deter- 

 mining the nature of casts and impressions is the existence of 

 the cylinder of delicate cells in the outer cortex of stems and 

 branches. As already pointed out, this meristematic cylinder 

 or phellogen constitutes a natural line of separation, as in the 

 case of the cambium layer between the wood and the external 

 tissues in a fresh Sycamore twig. The result of the separation 

 of an outer shell of bark from the rest of the stem and the 



