366 



MOSSES AND FERNS 



cha:\ 



tissue is formed, in the projecting apex of which a single large 

 cell occupies a central position." As in BotrycJihim the arche- 

 sporium is derived from a single hypodermal cell, which ap- 

 proaches more or less the tetrahedral form of the true Lepto- 

 sporangiates, 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-de- 

 veloped sporangium is in shape much like that of Botrychhim 

 Virginianuin, and has a very short massive stalk. Like Hcl- 

 ininthostachys and Angioptcris, it opens by a vertical cleft, and 

 like the latter there is a rudimentary annulus consisting of a 

 group of thick-walled cells (Fig. 207, r). 



The Gleicheniace^ 



These comprise about twenty-five species of tropical and 



sub - tropical Ferns, 

 which may be all placed 

 in two genera (Diels 

 ( I ) ) — Stromatopteris, 

 with a single species S. 

 monilifor m i s and 

 GleicJienia with about 

 25 species. The best 

 known is G. dichotoma, 

 an extremely common 

 Fern of the tropics of 

 the whole world. It has 

 very long leaves, which 

 fork repeatedly, and 



Fig. 20?,.—Glcichcnia pcctinata. ProthaIHa, X4; may bc prolifcrOUS from 



B, a large prothallium seen from below, show- , 4-1^111 



ing a dichotomy of the apex; C, the young ^^^^ grOWtll OI DUCiS Cle- 



sporophyte attached to the prothallium. VCloped ill the axils of 



the forked pinnae. 



The Gamctophytc 



The development of the prothallium has been studied by 

 Rauwenhoff ( i ), and shows some interesting points in which it 

 is intermediate between the Osmundace^e and the other Lep- 

 tosporangiatse. The spores of Gleichenia are usually tetra- 



