Xllj 



INDIVIDUALITY OF THE SORUS 



227 



linear leaf-form, and the series of superficial or ultimately marginal sori on 

 either side of the midrib; nevertheless we copclude that this was an 

 ancestral feature for them all. It is thus seen that the individuality of the 

 sorus may be lost hy fission, and especially in Ferns where the leaf-area has 

 been widened, as it so often is in relation to grrowth under forest shade. 



Fig. 222. A. Part of a fertile pinna of Dipteris Lobbiana, witli sori left on the one side of the 

 midrib : they have been removed on the other side, and show that there is here no special 

 vascular supply to the receptacle, only a rather dense plexus of the usual veins. ( x 8.) 

 B. Fertile area of leaf of Dip/en's conjugata : the sori have been removed on the left, 

 showing no marked vascular development under the receptacle. (x8.) 



Fig. 223. Part of a pinna of Akophila bkcluioides (Rich.), Hk. {^Metaxya rostrata, 

 Presl), showing the relation of the sori to the veins: more than one sorus may be 

 borne on a single vein. ( x 2.) 



On the other hand, where sori are numerous their individuality is apt 

 to be lost also by fusion, a change which has appeared in several distinct 

 phyletic lines, and especially in Ferns where the sori are borne in linear 

 sequence. Von Goebel {Organographie, II. Aufl. p. 1143) has shown early 

 steps in lateral fusion of the intramarginal sori of Saccoloma, where the 

 receptacles and lower indusia remain distinct, but the upper are united to 



15—2 



