226 



THE SPORE-PRODUCING ORGANS 



[CH. 



Gleicheniaceae, Hymenophyllaceae, and Dicksonieae. It may be retained 

 with modifications even where the leaf-surface is widened, as it is in the 

 Marattiaceae : but then the sori are liable to be extended along the course 

 of the veins. This is seen in slight degree in Aiigiopteris, and Marattia 

 (Fig. 202, A, C)\ but more plainly in ArcJiangiopteris, and Danaea {B, E), 

 where the sori stretch almost the whole distance from the midrib to the 

 margin, having apparently kept pace with the widening of the pinnae. But 

 Ckristensenia is a type with the leaf-blade greatly widened, probably in relation 

 to life under forest shade ; and the sori are dotted over the expanded surface, 

 having each a circular form (Fig. 202, D). At times the individuality of the 

 sorus appears to have been lost, owing to fission, as suggested by the frequent 

 occurrence of sori in pairs, or of forms pointing to incomplete fission 

 (Fig. 221, a — e below). This may account phyletically in part or in whole 



Fig. 221. a,b,c (above), Danaea alata. Smith. a = feitile pinna 

 with many normal sori : the arrow indicates an abnormal 

 fission ; b, c show more frequent abnormal fissions resulting 

 in irregularly formed sori distributed over a slightly enlarged 

 leaf-surface, (x 2.) a — e (below), sori of Ckristensenia aesciili- 

 folia, Blume, showing states of partial or complete abstriction. 



for the numerous scattered sori of the genus. Similar states may be seen 

 also in Danaea (Fig. 221, a — c). The genus Dipteris further illustrates the 

 relation of the sori to a widening leaf In D. Lobbiana the segments are 

 narrow, and a single row of sori lies on either side of the midrib. In 

 D. qiimqueftircata the segments are broader, and the sori are more widely 

 spread, with many signs of fission. In Z>. conjiigata the broad lamina is covered 

 with many minute sori, giving again frequent signs of fission (Fig. 222, A, B 

 and compare Land Flora, Figs. 344 — 346). A slightly different state is seen 

 in Metaxya, for there not uncommonly two or more sori may be borne on each 

 vein of the wide pinna. These suggest in each case a fission of the single sorus 

 usually present on each vein (Fig. 223). It is not asserted that such fission 

 is the only mode of increase of the sori borne upon an enlarged leaf-area : 

 but it is at least one factor. Those Ferns which are thus broad-leaved, with 

 many scattered superficial sori, appear to have diverged far from the original 



