VII PTERIDOPHYTA—FILICINEM—OPHIOGLOSSACE^ ^Z 



what intermediate between these and the elongated ones found 

 in most Ferns. The walls between the pits are very much 

 thickened, and the bottoms of corresponding pits in the walls of 

 adjacent tracheids 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 



Fig. 143.— Bo(r3K:W««i Virginianum, A, Longitudinal section of the stem apex of a 

 young plant, X260; B, cross-section of a similar specimen; L, the youngest leaf. 



a single layer of cells somewhat compressed radially, forming 

 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 (Holle, 

 (i), p. 249). These cork cells arise by repeated tangential 

 divisions in cells near the periphery, and have in consequence 

 the same regular arrangement seen in similar cells of the higher 

 plants. 



