254 



THE SPORE-PRODUCING ORGANS 



b c 



[CH. 



Fig. 250. Sporangia of various Ferns, orientated so that the distal face is to the 

 left, the proximal or basal to the right. This brings clearly into view the 

 differences of proportion of those faces, and of the position of the annulus 

 and stomium. a = Lygodiwn ; b — Gleichenia ; c = Plagiogyria ; d= Loxsoma ; 

 e= Hymenophyllu7n. f=Leptochilus. j, j = stomium. 



lies at the side, while the dehiscence is by an oblique lateral slit (Fig. 250, c, e). 



But in the more advanced Mixtae, where the sorus is crowded, the annulus 



tends to a vertical position. The indu- f^ 



ration of the cells adjoining the stalk 



is omitted, and the continuity of the 



cells of the ring is interrupted by its 



insertion (Fig. 250,/"). In some of the 



less altered types, where the sorus is 



still irregularly gradate, the cells of the 



ring may be seen to be continuous past 



the stalk, as in Cheiropletiria{¥\g. 25 i,B), 



and in Platycerium {C). The like is also 



seen in Dennstaedtia apiifolia (Fig. 252, 



C)\ but in the related D. rubiginosa the 



series is clearly interrupted, as it is in 



all the more advanced Leptosporangiate 



Ferns (Fig. 252, Z^). It has been shown 



in several phyletically distinct groups 



that such a progression, from the oblique 



and continuous to the vertical but interrupted annulus, may be traced by 



Fig. 251. A, base of a sporangium oi Pteris 

 grandifolia, showing the interruption of the 

 annulus, and the three-rowed stalk ; B, base 

 of a sporangium of Cheiropleuria, showing 

 the ring continuous, and the four-rowed 

 stalk; C, sporangium of /'/a/j'f^;'/«w, show- 

 ing cells of the annulus almost interrupted, 

 though still actually continuous. In A, B, 

 the distal wall is directed upwards, in C it 

 is turned away from the observer. 



