LEPIDODENDRON 



iSi 



The secondary vascular tissues began to develop 

 rather later than the periderm. Very often the new 

 growth began on one side of the central cylinder, so 

 that for a time the secondary wood formed a crescent, 

 and not a complete ring (cf. Fig. 60) ; in other cases it 

 was fairly equal all round from the first. The cambial 



Fig. 61. — Lepidodendron selaginoides. Part of radial section, showing primary wood 

 on left, and secondary wood, with medullary rays, on right ; the narrow elements 

 between the two are the protoxylem. X about 25. From a photograph by Dr. 

 Bousfield. S. Coll. 24. 



divisions started in the conjunctive tissue between the 

 primary wood and phloem ; the very first beginning 

 of the new formation can be traced in some of the 

 specimens (see Fig. 60, on lower side of figure). The 

 secondary wood usually abuts directly on the small 

 external tracheides of the primary ring (see longi- 

 tudinal section, Fig. 61); sometimes a layer of 



