254 FILICES. 



hence the term circinate applied to the characteristic 

 vernation of ferns. 



Upon the under surface of the fertile fronds, the fructi- 

 fication is arranged in small, round clusters, brown when 

 ripe. These clusters are termed sori (each cluster ' a 

 sorus), and there are two rows of the sori upon, at least, 

 the lower lobes {pinnules) of each of the pinna of the 



FIG. 197. Pinnule of Male Shield Fern, with six sori. 



frond. These sori are protected at first by a reniform, 

 pale-coloured membrane (indusiuni)^ which at length 

 withers up, exposing the minute stalked sporanges of 

 which each sorus is composed. 



The sporanges require examination with a magnifying 

 glass. They originate from the superficial cells of the 

 frond ; they are wholly free from each other, and, when 

 fully developed, are found to be globose or obovoid 

 cellular stalked capsules around which longitudinally a 

 a thickened row of cells forms a prominent anmtlus(\g. 

 196, a). This annulus contracts hygroscopically when the 

 sporange is ripe, and so splits open the sporange by a 

 transverse chink through which the spores which they 

 contain are set free. The spores may easily be obtained 

 by allowing the mature frond of a fern in autumn to 

 wither up upon a sheet of paper in a dry place, when 

 abundance of the spores will be shed from the sporanges 

 as fine dust. 



Some few British Ferns deviate from this type in the 

 form of the frond, in the absence of an indusium, in the 

 mode of arrangement, as well as in the form and 

 dehiscence, of the sporanges. 



