242 HYMENOPHYLLACEAE [CH. 



If the succession of the sporangia be strictly basipetal the result will be a 

 sorus as in Fig. 511. Meanwhile the indusium grows obliquely upwards 

 sheathing the base, and the youngest sporangia are thus protected in the 

 angle between it and the receptacle. This is in fact an extreme example of 

 the gradate sorus, the biological rationale of which has already been dis- 

 cussed (Vol. I, p. 212). It does not appear difficult to relate this with the 

 marginal origin of the sporangia in the Schizaeaceae, while the indusium 

 may find a correlative in the superficial protective flaps of Lygodiimi. 



The sporangia themselves are always short-stalked, and their insertion is 

 by a rosette of cells about six in number. The large sporangium of H. dila- 

 tatum is shown in four different aspects in Fig. 512, 95-98: from these it 



Fig. 514. Trichoinanes speciosuvi Willd. { = T. radkans). 

 Transverse section of the young receptacle, showing 

 various stages of early segmentation of the sporangia. 

 (After Prantl.) 



may be gathered that there is an oblique annulus formed of a single row of 

 cells, with lateral dehiscence by a long oblique slit: but the stomium is not 

 a definite cell-group. The peripheral or distal face consists of numerous 

 tabular cells, and is strongly convex. The central or basal face is also convex, 

 but in less degree. The whole structure resembles the sporangium of some 

 of the Simplices rather than that of advanced Leptosporangiate Ferns. In 

 T. radicans (Fig. 512, 99-102) the sporangium though small is of the same 

 type, but with fewer cells of the annulus, and a much simpler stomium. 



The origin of the sporangium of the Hymenophyllaceae is from a single 

 parent cell with a square base. It projects from the surface of the receptacle, 

 and the first segment-wall strikes its basal wall (Fig. 5 14). This is a feature 

 common in the sporangia of certain of the Simplices rather than in those 

 of the more advanced Ferns. In particular it is seen in the Schizaeaceae. 



