VIII 



THE PTERIDOPH YTA—OPHIOGLOSSA CEJZ 



245 



corresponding pits in the walls of adjacent trachcids are 

 separated by a very delicate membrane. At intervals 

 medullary rays, one cell thick, extend from the pith to the 

 outer limit of the xylem. The cells are elongated radially, 

 and have uniformly thickened walls and granular contents. 



The phloem consists of large sieve -tubes and similar but 

 smaller parenchymatous cells. No bast fibres or sclerenchy- 

 matous cells are present. The whole cylinder is bounded by 

 a single layer of cells somewhat compressed radially, forming 



A 







-Tl 



cam.. 



Fio. 125.--A, Part of a cross-section of the stem bundle of £. I'if^iniann^n, x 200. — lettering as in 

 Fig. 123 ; B, a portion of the tracheary tissue, showing the peculiarly pitted walls, X400. 



the endodermis or bundle -sheath. Between the xylem and 

 phloem is a well-defined layer of cambium by whose growth 

 the thickness of the vascular cylinder is slowly but constantly 

 added to, and as a result there is a secondary growth of the 

 stem strictly comparable to that of the Dicotyledons. 



The outer layer of the cortex (the epidermis is quite 

 absent) develops cork, but not from a definite cork cambium.^ 

 These cork cells arise by repeated tangential divisions in cells 



' Holle (i), p. 249. 



