XLI] 



DRYOPTERIS 



123 



Two pinnules of the Male Shield Fern bearing sori are shown in Fig. 655. 

 Each sorus is inserted singly upon the anadromic branch of a forked vein, 

 which continues its course beyond the recep- 

 tacle. This position corresponds to that in the 

 Gleicheniaceae (Vol. II, Figs. 486, 488), and 

 in Lophosoria (Fig. 548), while in Cyathea all 

 the branch-veins may bear sori (Fig. 557). In 

 Diacalpe and Pei^anema also the insertion is 

 upon the back of a vein, and it is usually so in 

 Woodsza,though there it is sometimes almost 

 terminal. In all of these except in Perancvia 

 the sorus is radially constructed: but the 

 well-known kidney-shaped indusium oi Dry- 

 opieris at once reveals the lop-sidedness of its 

 sorus, which is best elucidated by comparison 

 with those of Peranevia and Heinitelia. The 

 structure of the sorus in the Male Shield Y\g.(>^i,.Dryopterisfiiix-mas{^>i^c\\o\.i. 

 Fern is often misunderstood owing to the j^'^lTf tofeT.)*''"'"""^'"'^'^' 

 circumstance that sections are more easily 



cut transversely to the vein than longitudinally. The former sections appear 

 as in Vol. I, Fig. 14, with the indusium inserted by an apparently central stalk 

 which overarches the receptacle equally on either side. A much better concep- 

 tion is gained by sections following the course of the vein, and cutting the sorus 



Fig. 656. Vertical sections of young sori of Dryopteris filix-nias, cut in the direction 

 following the vein. A is the younger, and it includes the leaf-margin. The indusium 

 precedes the appearance of the sporangia. ( x 250.) 



in the median plane of its dorsiventrality. If such a section be taken from 

 a very young sorus, it will be seen that the receptacle which arises not far 

 from the leaf-margin is distinctly lop-sided from the first, and begins to be 

 overarched by the young indusium before the first traces of any sporangia 

 appear (Fig. 656, A, E). Later the sporangia arise upon the slightly convex 

 receptacle, and there may be clear indications of a basipetal sequence of 

 those first formed (Fig. 654, (T): but this is soon superseded by a mixed 



