33'- 



MOSSES AND FERNS 



CHAP. 



in great numbers, and the lamina of the leaf much contracted. 

 One of the simplest cases is seen in Polypodium. Here thfe 

 sporangia develop late upon ordinary leaves, and form scattered 

 round sori, bearing, however, a definite relation to the veins — 

 in this case forming above the free end of one of the small 

 veins. Where there are special sporophylls, the development 

 of the sporangia begins before the leaves begin to unfold. 



St. 



B. 



Fig. ie6.—Poly/>odium/alcatum (Kellogg). A, Cross-section of a sterile leaf, cutting across one of 

 the smaller veins, x 260 ; st, section of a stoma ; B, similar section of a sporophyll, showing the 

 position of the sorus above the vein, X 85. 



In Polypodium (Fig. 167) the first evidence of the formation 

 of sporangia is a series of minute depressions upon the lower 

 side of the leaf, much as occur.s in Angiopteris. The bottom of 

 this depression is occupied by a low elevation, the placenta, and 

 upon this the sporangia form in an analogous way, but are not 

 all developed at the same time, so that a single sorus may 



