X THE HOMOSPOROUS LEPTOSPORANGIAT^ 395 



The sporangia may almost completely cover the backs of 

 the sporophylls, as in Platycerhim (Fig. 232), or more com- 

 monly form definite sori, which may or may not have an in- 

 dusium. Where the latter is present, it is either formed by the 

 margin of the leaf, as in Adiantuin or Ptcris, or it may be a 

 special scale-like outgrowth of the lower side of the leaf. In 

 such cases it is a membranaceous covering of characteristic 

 form. Thus in Aspidhim (Fig. 230, A) it is kidney-shaped, 

 in Asplenmm elongated, and free only along one side. Where, 

 as in Onoclea (Fig. 230, B), the margins of the sporophyll are 

 involute, so as to completely enclose the sori, the indusium is 

 wanting or very rudimentary. 



