PTEROPSIDA 165 



The stipe receives numerous leaf traces, and the venation is 

 open. The sori are superficial on the veins, or at vein endings, 

 and are covered by an indusium which in Dryopteris is 

 reniform (Fig. 25L) and in Polystichum is peltate (Fig. 25M). 

 Of these the reniform type is probably the more primitive, 

 for it is not far removed from the condition figured for 

 Nephrolepis (Fig. 25N). From this type, it is easy to 

 imagine the evolution of the radially symmetrical indusium 

 of Polystichum, by the extension of the 'shoulders' round the 

 point of attachment, followed by a 'fusion' to form a disc, 

 with a central point of attachment. 



Athyrioideae 



Some species of Athyrium have indusia that are identical in 

 shape with those of Dryopteris, but most have two types on 

 the same frond, as does the British A.filix-femina (Fig. 25R). 

 Here, there are some sori with reniform indusia and some in 

 which the indusium is extended along the lateral veins. The 

 vascular supply to the stipe of the frond consists of two leaf 

 traces, which unite into a single gutter-shaped strand higher up. 



Lomariopsidoideae 



All the members of this subfamily are acrostichoid. There 

 has been much discussion as to their affinities, but they 

 probably fie with the Davallioideae, for the stele of Elapho- 

 glossum is very similar to that of Davallia, in having two 

 large meristeles connected into a cyfinder by a network of 

 smaller bundles. 



Asplenioideae 



This subfamily, too, is befieved by Holttum to have affinities 

 with the Davalfioideae. The sorus of Asplenium (Fig. 25W) 

 is extended along the lateral veins and is protected by an 

 indusium which is usually acroscopic (i.e. its free margin is 

 directed towards the apex of the pinna). In this, it resembles 

 most of the sori of Athyrium. However, the vascular supply 



