PTERIDOPHYTA 



277 



Gametophyte. The gametophyte does not differ essentially from that of the 

 Polypodiacese, and, so far as it is known, the embryo also is very similar. 



Sporophyte. The sporophy te has a prostrate, or short, upright rhizome, 

 from which are sent up the leaves. The latter in Lygodium have an unlimited 

 apical growth, and the leaf -stalk twines so that these are known as climbing 

 Ferns. The tissues are much like those of the Gleicheniaceae. The sporophylls 

 are usually modified, either the whole leaf being strongly contracted, e.g. 

 Schizcea pusilla, or special fertile leaf-segments are formed, as in Lygodium 

 and Aneimia. In the latter genus, the lower pair of pinnae are fertile, and often 

 long-stalked. Some species of Aneimia are also characterized by the peculiar 

 form of the stomata. The wall which cuts out the mother-cell is circular, so 

 that the stoma lies in the middle of an epidermal cell. 



B 



FIG. 244. A, Hymenophyllum recurvum (X 3). B, Trichomanes parvulum (x3). 

 C, T. cyrtotheca (X3). D, 1, indusium more enlarged; 2, section of indusium, 

 showing the sorus, s. 



Sporangium. The sporangia (Fig. 243, C) are large, and possess a terminal 

 annulus, which in Lygodium and Aneimia forms a conspicuous cap of thickened 

 cells. The sporangia are covered by an indusium formed by the leaf-margin, or 

 there may be a special indusium for each sporangium (Lygodium). 



Family 5. Hymenophyllaceae 



The Hymenophyllacese are especially characteristic of the moist 

 mountain forests of the Tropics, where their exquisite filmy fronds 

 sometimes quite cover the trunks of trees with their graceful dra- 

 pery. A few species extend beyond the Tropics, but only two 

 species, and these extremely rare, occur within the United States. 

 There are two genera, Hymenophyllum and Trichomanes, each com- 



