VIII 



THE PTERIDOPHYTA—OPHJOGLOSSACEAL 



249 



there was not the clear separation of the root -cap from the 

 body of the root that is so distinct in Equisetum^ for example. 



The central fibrovascular cylinder of the root is not provided 

 with a definite endodermis, and its limits are not clearly 

 defined. It varies in the number of xylem and phloem 

 masses, even in the same species. In B. Virginiajiuui the 

 larger roots show three or four xylcm masses (Fig. 128). B. 

 teniattini has a usually triarch bundle, while B. liinaria is 

 commonly diarch.^ The elements both of the xylem and 



Fig. 128. — Tetrarch vascular bundle of the root oi B. Virginianujit, X85. 



phloem are much like those in the stem and do not need 

 any special description. The roots increase considerably in 

 diameter as they grow older, but this enlargement does not 

 take place at the base, where the root is noticeably constricted. 

 The enlargement is due entirely to the cortical tissue, and is 

 mainly simply an enlargement of the cells. The diameter of 

 the central cylinder remains the same after it is once formed. 

 In the outer part of the root, as in the stem, there is a develop- 

 ment of cork. 



^ Holle (i), p. 245. 



