XII 



LEPTOSPORANGIAT^E HETEROSPORE^ 



417 



thicken, the first indication of the hard shell found in the ripe 

 sporocarp. 



The development of the sporangia corresponds almost 

 exactly with that of the Polypodiaceae. The surface cells of 

 the sorus protrude as papillae, in which the same divisions arise 

 as in other Leptosporangiata;. The first division wall is 

 usually strongly oblique, but may be transverse. The formation 

 of the archesporium is the same, but the apical growth of the 



Fig. 217. — Transverse section of an older sporocarp of P. Aviericaiia, showing the four sori (s) ; 

 /b, vascular bundles, X 85 ; B, section of the wall of a nearly ripe sporocarp, X 255. 



sporangia is checked sooner in the earlier ones, which have 

 consequently a very short stalk. In the later ones, which arise 

 between the others, the stalk is longer. The first sporangia are 

 formed at the base of the sorus, and their development 

 proceeds toward the apex ; but later secondary ones may arise 

 at any point in the sorus. 



The tapetum is well developed, and, as in most homosporous 



2 E 



