THE PTERIDOPHYTA : FILICALES, THE FERNS 519 



is filamentous. In Hymenophylhim a short-lived apical cell is formed, from 

 which the first broadening of the prothallus originates, but this soon gives 

 place to an apical meristem and marginal growth. Branching is brought 

 about by the dichotomy of the apical meristem, followed by unequal growth 

 of the two limbs, so that the weaker of the two looks like a side shoot. 

 Rhizoids are produced in marginal groups. 



Gemmae are a peculiar feature of the prothallus in this family. In 

 Hymenophylhim they develop from the margin in the form of little flat plates 

 of cells, provided with an apical cell, very like the young stage of the pro- 

 thallus itself. They are detached and form a means of vegetative propagation 

 of the prothallus. 



Antheridia and archegonia are borne in closely set groups on the under 



ANTHERIDIA .^-r<CnT7r5t3CCr>^ ARCHEGONIA 



Fig. 515. — Hymenophylhim dilatotum. Lobe of the 

 prothallus with antheridia and archegonia. {After 

 Goebel.) 



sides of lobes of the thallus, which are really specialized branches formed by 

 the dichotomy of the prothallus apex. Such fertile lobes may be produced 

 at intervals over a considerable period of time. The filaments of the 

 Trichomanes prothallus develop the archegonia in groups on very short 

 branches which are called archegoniophores and which probably correspond 

 to the fertile lobes in Hymenophylhim. The gametangia do not differ essentially 

 from the usual Fern type, nor does there appear to be anything peculiar in 

 the embryogeny, so far as it is known. 



Leptosporangiatae : Osmundaceae 



The Osmundaceae are a small family of Ferns, of great interest both on 

 account of their peculiar morphology and of their fossil history^ which is 

 unusually well known. There are two genera, Osmunda with nine species 

 and Todea with only one, though a third genus, Leptopteris, with seven species 

 is sometimes united to Todea. 



The outstanding character of the group is the structure of the sporangium, 

 which has only a rudimentary annulus consisting of a group of thick-walled 

 cells at one side, while the dehiscence follows a line across the sporangial 



