XI 



THE HOMOSPOROUS LEPTOSPORANGIAT^ 



357 



the tetrahedral form of the true Leptosporangiatcs, but shows a 

 good deal of variation. As in these the wall of the sporangium 

 is only one-layered, and the tapetum ordinarily two, but 

 occasionally three-layered. The fully-developed sporangium is 

 in shape much like that of BotrycJiium Virginianum, and has 

 a very short massive stalk. Like HehnintJiostacJiys and 

 Angiopteris, it opens by a vertical cleft, and like the latter 

 there is a rudimentary annulus consisting of a group of thick- 

 walled cells (Fig. 184, r). 



The GleidieniacecE 



These comprise about twenty-five species of tropical and 

 sub-tropical Ferns, which may be all placed in two genera ^ — 



Fig. 185. — A, Pinnule of Glcichenia dichotoma (Willd.), showing the position of the sori (s), X4 ; B, 

 ventral ; C, dorsal view of the ripe sporangium, X 85 ; D, vascular bundles of the petiole and 

 stem of Gleichenia (sp.) (after Poirault). The dark masses represent the xylem masses; ph, 

 phloem ; en, endodermis. 



Platyzoma, with a single species P. luicrophyllum, and GleicJienia. 

 The best known is G. dicJwtoma, an extremely common Fern of 



^ Hooker and Baker ( 1 ) 



