374 



MOSSES AND FERNS 



CHAP. 



T. pyxidiferum (Bower (8)), the prothallium remains fila- 

 mentous, and forms a densely branching structure very much 

 like the protonema of some Mosses, but coarser in texture. 

 Other species, however, e. g., T. alatum, produced flattened 

 thalloid prothallia from branches of the filamentous forms, and 

 Hymenophyllum always has a flat hepatic-like prothallium, 

 which in its earHer stages, according to Sadebeck ((6), p. 

 i6i), always develops a two-sided apical cell, and differs in no 

 wise from that of other Ferns. These prothallia, however, 

 remain single-layered throughout, although they reach an ex- 

 traordinarily large size, and branch much more freely than 

 those of most other Ferns (Fig. 215). The rhizoids are 

 always very short and dark-coloured, and generally occur in 



s 



Fig. 2 IS- — Hymenophyllum (sp). A, Large prothallium of the natural size; B, part of 

 the margin of one of the growing branches, showing two similar initial cells, Xi8o; 

 C, a filamentous male prothallium derived from a bud, X6o. 



groups upon the margin only. The branching of the prothallia 

 is either monopodial or dichotomous, and the latter method 

 may be repeated a number of times. They may live for an in- 

 definite time apparently. The writer has kept prothallia of 

 both Trichomanes and Hymenophyllum for nearly two years, 

 at the end of which time they showed no diminution of vigour. 

 They form ordinary adventitious shoots, but there are also 

 special gemmae developed in many of them, often in great num- 

 bers. In an undetermined species of Hymenophyllum col- 

 lected In the Hawaiian Islands (Fig. 216) these gemmae oc- 

 curred very abundantly upon prothallia that had ceased to form 

 sexual organs. A marginal cell grows out and curves upward, 



