36o MOSSES AND FERNS chap. 



The Hymenophyllacece 



The Hymenophyllaceae have been the subject of much 

 discussion on account of the assumption made by all the 

 earlier writers that they were the most primitive of the 

 Pteridophytes. This was based very largely upon the apparent 

 resemblance between the delicate sporophyte of many of them 

 and the leafy gametophore of the Mosses. More recent study 

 of their development, especially the gametophyte, has led to a 

 modification of this view, although it is still held by many 

 botanists. It seems more probable that the peculiarities of 

 both gametophyte and sporophyte are due to the peculiar 

 environment of these plants, which grow only in very moist 

 places, indeed are almost aquatic at times. They are for the 

 most part extremely delicate Ferns, of small size, and with few 

 exceptions are exclusively tropical. Many are epiphytes, and 

 these have the roots very poorly developed or even entirely 

 wanting. The leaves are, with few exceptions, reduced to a 

 single layer of cells, except the veins, which gives them a 

 striking resemblance in texture to the leaves of some of the 

 larger Mosses, e.g. species of Mnium. Hooker ^ reduces them 

 all to three genera, which, however, are often further divided. 

 Of these Loxsoina is represented by but one species, L. Ctin- 

 ninghamii, a form which seems to be intermediate in general 

 characters between the Cyatheaceae and the other Hymeno- 

 phyllaceae, but its life history and anatomy are not known. 

 Of the other genera Hooker gives seventy-one species to 

 HyinenopJiylhivi and seventy-eight to Tridiomanesr 



TJie GaiJietophyle 



The gametophyte is known more or less completely in 

 several species of both Trichoinanes and Hymenophyllum. The 

 large spores germinate promptly, but their subsequent develop- 

 ment is very slow. They contain chlorophyll, and often begin 

 to germinate within the sporangium, where they may often be 

 found divided into three equal cells by walls radiating from 

 the centre (Fig. i86). All of the cells begin to grow out 



^ Hooker and Baker (i). 

 ^ The number of species known now considerably exceeds this. 



