486 



BOTANY OF THE LIVING PLANT 



But in the vast majority of Ferns the stele expands as the plant 

 grows stronger, and the leaves larger. In various ways it becomes 

 segregated into a number of vascular strands (meristeles), arranged in 

 a cylindrical network (Fig. 374, E. F). Each mesh in Dryopteris 

 corresponds to the insertion of a leaf -base, and is called a foliar mesh, 

 or gap. The vascular strands that run out into the leaf, called col- 

 lectively the leaf-trace, arise from the margin of it, and the cortex and 



Fig. 376. 



Transverse section of rhizome of Bracken, showing the outer and inner series of 

 meristeles, and the irregular bands of sclerenchyma between them. These are 

 embedded in soft ground-parenchyma, with a hard sclerotic rind. ( x 10.) 



pith are in direct communication through the foliar gaps. This is 

 readily understood if the vascular skeleton be dissected out, as it is 

 seen in Fig. 376 a. In transverse sections the meristeles would appear 

 as a ring of isolated tracts, and this gave rise to the misleading term 

 " polystelic " as applied to such stems. But the dissection shows 

 that they are all parts of one cylindrical network, which arises through 

 dilatation of the protostele of the sporeling (compare Figs. 374, 375)- 

 A similar disintegration of the leaf-trace may be followed in Dryop- 

 teris, and many of the larger Ferns. A simple strand appears to have 

 been the primitive type of trace, and it is seen generally in the leaves 

 of their sporelings. But a plurality of strands appears in the adult 



