366 



MOSSES AND FERNS 



CHAP. 



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

 cell occupies a central position." As in Botrychium 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 Botrychium 

 Virginianum, and has a very short massive stalk. Like Hel- 

 minthostachys 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. 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 ,5\ 

 monilifor m i s and 

 Gleichenia 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 

 may be proliferous from 

 the growth of buds de- 

 veloped in the axils of 

 the forked pinnae. 



FIG. 208. Gleichenia pectinata. Prothallia, X4; 

 B, a large prothallium seen from below, show- 

 ing a dichotomy of the apex; C, the young 

 sporophyte attached to the prothallium. 



The Gametophyte 



The development of the prothallium has been studied by 

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

 is intermediate between the Osmundacese and the other Lep- 

 tosporangiatse. The spores of Gleichenia are usually tetra- 



