SPHENOPHYLLEAE 



9i 



hexarch, though the triangular sectional form is retained. 

 In the young specimens but little remains of phloem 

 have been found ; the inner layers of the cortex are of 

 delicate tissue, but towards the outside is a fibrous 

 zone (Fig. 37). The external outline of the cortex, 

 as seen in transverse section, is characteristic, for there 



Fig. 37. — SJrhenopkyllum plurifoliatum. Transverse section through whorl of leaves, 

 m. x, primary wood ; jr 2 , secondary wood ; g; phloem (?) ; h, periderm ; k, outer 

 cortex. X about 20. After Williamson, Phil. Trans, Will. Coll. 874. 



are three well-marked furrows, lying opposite the three 

 flat sides of the triangular stele (see Fig. 36, B). 



It is only in the most minute specimens that the 

 primary structure just described is found unaltered. 1 

 At a very early stage the formation of secondary wood 

 and bast, by means of a normal cambium, began 

 (Fig. 37). The secondary wood possesses some very 



1 See Fig. 36, B and C, where only one layer of secondary wood is 

 shown. Cf. Fig. 41, B. 



