296 



SORI 



vulgare, Fig. 159, B, s.), where they appear as small circular 

 brown patches situated over the lateral veins of the pinnae. A 

 similar arrangement obtains in the Male Fern (Nephrodium, 

 Fig. 163, B), but here each sorus is protected by a kidney-shaped 

 outgrowth (indusium, In.) of the placenta, which withers when 

 the sporangia are mature. In the Maidenhair Fern (Adiantum, 

 Fig. 159, C, s.) and the Wall Rue (Asplenium rula muraria) the 

 sori are near the edges of the pinnules, and are covered by 

 indusia taking the form of little flaps projecting inwards from 



the margin, whilst in the 

 Bracken (Pteris, Fig. 163, 

 D), where the sporangia 

 form a continuous fringe 

 all round the margin of 

 the pinnule, the incurved 

 edge of the latter functions 

 as an indusium. 



A peculiar type of sorus 

 is found in the Filmy 

 Ferns (Hymenophyllacese), 

 which are so-called be- 

 cause of their delicate 

 fronds, consisting of but a 

 single layer of cells. 

 Though mainly encoun- 

 tered in the Tropics, they 



FIG. 163. Sori of various Ferns. A, 

 Hart's Tongue Fern (Scolopendrium). 

 B, Male Fern (Nephrodium). C, As- 

 plenium. D, Bracken (Pteris). In., 

 indusium. 



are represented in Britain 

 by one or two species (e.g. 



Hymenophyllum tunbridgense, Fig. 164) occurring in damp caves and 

 other humid situations in rocky districts. Here the sporangia arise 

 from rod-like placentae (s.) projecting from the leaf-margin, and 

 each sorus is enveloped at its base in a cup-shaped indusium (.). 

 In transverse sections through a fertile pinnule of Nephrodium 

 filix-mas (Fig. 165), the bulging placenta (pi.) on the underside 

 is seen to be supplied by tracheids (t.) from the overlying vascular 

 bundle. The indusium ('.) appears as an umbrella-shaped out- 

 growth, one layer of cells thick, arising from the top of the 

 placenta. On the sides of the latter are borne numerous 

 sporangia in various stages of development, 



