PHEN OGAMS AND CRYPTOGAMS 



177 



it will be seen that the leaflets are contracted and are 

 densely covered beneath with brown bodies. These bodies 

 are collections of sporangia or spore-cases. 



Fig. 



256. — Common Polypode Fern. 

 Polypodium vulgare. 



Fig. 257. — Sori and Spo- 

 rangium of Polypode. 

 A chain of cells lies along 

 the top of the sporangium, 

 which springs back elasti- 

 cally on drying, thus dis- 

 seminating the spores. 



Fig. 258. — The Brake 

 Fruits underneath 

 the Revolute 

 Edges of the Leaf. 



The sporangia are collected into little groups, known as 

 sori (singular, sorus) or fruit-dots. Each sorus is covered 

 with a thin scale or shield, known as 

 an indusium. This indusium sepa- 

 rates from the frond at its edges, and 

 the sporangia are exposed. Not all 

 ferns have indusia. The polypode 

 (Figs. 256, 257) does not; the sori 

 are naked. In the brake (Fig. 258) 



and maidenhair (Fig. 259) the 

 edge of the frond turns over 

 and forms an indusium. The 

 nephrolepis or sword fern of 

 greenhouses is allied to the 

 polypode. The sori are in a 

 single row on either side the 

 midrib (Fig. 260). The indu- 

 sium is circular or kidney- 

 Fig. 259. — Fritting Pinnules j 



of Maidenhair Fern. shaped and open at one edge 



