LEPTOSPORANGIAT^ HETEROSPORE^ 



437 



The cavities rapidly become larger with the expansion of 

 the growing sporocarp, but the space between the inner surface 

 of the lobes and the columella remains narrow, owing to the 

 growth of the sorus, w^iich almost completely fills it from the 

 first. The sorus forms an elongated cushion, extending nearly 

 the whole distance from the apex to the base of the lobe, along 

 the median line of its inner face. In origin and position it 

 corresponds closely to that of the Schizseacese. 



Fig. 256. — Transverse section of an older sporocarp of P. Americana, showing the four 

 sori (j); jh, vascular bundles, X85; B, section of the wall of a nearly ripe sporo- 

 carp, X255. 



The vascular bundle of the peduncle divides into four 

 branches, where it enters the sporocarp, and one branch goes 

 to each lobe, of which it forms the midrib lying below the 

 sorus. From each of these two smaller branches are given 

 off near the base, following the margin of the lobe (Fig. 256, 



