MEDULLOSEAE 435 



A zone of internal periderm was formed around" the 

 central part of the stem, enclosing the group of steles, 

 and separating them from the outer cortex and leaf- 

 bases (Fig. 164, pd). The periderm was probably for 

 the most part of a suberous nature ; it is extremely 

 well preserved, and was evidently formed wholly or 

 chiefly on the external side of the phellogen. 



In one specimen, apparently forming part of an old 

 stem, the whole of the outer cortex, together with the 

 leaf-bases, had been exfoliated, leaving the periderm 

 exposed. It may be doubted, however, whether this 

 exfoliation was a normal process. 



A point of some anatomical interest is the occasional 

 presence, in the cortex, of accessory vascular strands, of 

 concentric structure, and probably of wholly secondary 

 origin (Fig. 164, an). These formations recall the 

 similar cortical bundles occurring in the genus Cycas 

 and some other members of the Cycadaceae at the 

 present day. 



The petioles of Medullosa anglica were of large size, 

 having a diameter of 4 cm. or more, at the point where 

 they became free from the stem. Each leaf-stalk received 

 a large number of vascular bundles, as many as seventy 

 or eighty in all, derived from the repeated ramification 

 of several of the principal leaf-traces. The supply of 

 bundles did not all enter the leaf-base from the stem at 

 the same level ; they passed in successively, in groups 

 corresponding to the various principal leaf-traces from 

 which they were derived. Thus their number, in the 

 leaf-base, increased from below upwards. At a certain 

 height, however, the leaf-base was marked off from the 

 interior of the stem by a band of internal sclerenchyma, 



29 



