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 .S". 

 moniliformis 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. ProthalHa, 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- 

 tosporangiatae. The spores of Gleichenia are usually tetra- 



