2l8 



THE SPORE-PRODUCING ORGANS 



[CH. 



this ; for the sporophylls of the Ferns with marginal sori or sporangia are 

 relatively narrow, while those of the Marattiaceae are broad, and those of the 

 Gleicheniaceae and Matonineae share this feature, though in less degree. 

 The sporophyll of Todea barbara also is broad compared with the normally 

 attenuated fertile region of Osmunda. Thus on prima facie evidence the 

 general view may be stated as a working hypothesis that the primitive posi- 

 tion of the sporangium or sorus was distal or marginal, and that the superficial 

 position is derivative, having been acqicired in relation to an increase in surface 

 of the sporophyll. But before acceptance as a substantive view relating to 

 the evolution of Ferns, it will have to be tested by a comparative study of 

 development, and by preference the test should be made in genera or species 

 nearly related one to another. 



UlL 



Fig. ^lo. /. Osiminda regalis, part of a meta- 

 morphosed sporophyll with aborted sporangia 

 attached superficially. //. Todea pellucida, 

 part of a pinnule with sporangial stalks. 

 ///. Transverse section of a fertile pinnule 

 of Osmtmda showing normal insertion of 

 sporangia. (All after von Goebel.) 



Fig. 211. Vertical section 

 through young sorus 

 of Dicksonia Scheidei. 

 ( X 200.) j!? = recepta- 

 cle; 6''= upper, and 

 Z = lower, indusium. 



The marginal origin of the receptacle and of the sporangia has been 

 demonstrated developmentally in many examples. Prantl showed it in the 

 Hymenophyllaceae and Schizaeaceae {Unters. zur Morph. d. Gefdsskrypt. I, 

 II, Leipzig, 1875, 1 881). It may be demonstrated fully for the Dicksonieae. 

 A section through the very young sorus of Dicksonia {Cibotium) Scheidei 

 shows how the regular marginal segmentation is continued directly into the 

 receptacle (Fig. 21 1, R). The upper and lower indusial flaps {U, L) arise by 

 intramarginal surface-growths : neither of them is a continuation of the leaf- 

 surface itself At first the receptacle, which is here the actual leaf-margin, 

 is flattened, and lies evenly between the two indusial flaps (Fig.' 2 12). Upon 

 its extreme ridge a row of sporangia arises simultaneously {B, C): they are 

 followed by others successively below the margin. This succession is more 



