THE PTERIDOPHYTA : FILICALES, THE FERNS 517 



periclinal divisions of the lamina cells. This suggests that these thickened 

 leaves have been secondarily evolved from the usual one-layer type. 



SoRUS AND Sporangia 



The sori are marginal and stand at the tips of certain of the leaf veins, 

 which are prolonged beyond the edge of the leaf in the form of a receptacle, 

 the equivalent of the placenta in Dryopteris, on which the sporangia develop 

 in basipetal succession. The sorus is enclosed in an indusium (Fig. 513), 



Annulus in section 



Indusium 



1 15^4 



Sporangium 





Annulus 



e 





Receptacle 









K/ t '"'■" ' ■ * 



Fig. 513. — Hyyjienophyllum tunbridgense. 

 Section through sorus showing cup- 

 shaped indusium and sporangia 

 developing successively from apex 

 to base of the receptacle. 



consisting of two equal and distinct lobes or valves, which arise from the 

 upper and lower sides of the lamina respectively, on each side of the receptacle. 

 In some species the latter extends by intercalary growth beyond the indusial 

 covering, so that only the lower part of the sorus, where the young sporangia 

 are forming, is protected by the indusium. The older sporangia near the top 

 of the receptacle project into the air and are thus better situated for spore 

 dispersal. The indusium is hygroscopic and closes against the sporangia in 

 drv weather. Finally it withers and dries up at the time when the lowest 

 sporangia are mature and ready to shed their spores. 



