XI THE HOMOSPOROUS LEPTOSPORANGIAT.K 377 



the latter is present, it is cither formed by the mari^nii (jf llie 

 leaf, as in Adiantuvi or Ptcris, or it may be a special scale- 

 like oiitfjrowth of the lower side of the leaf. In such cases it 

 forms a membraneous covering of characteristic form. Thus 

 in Aspidiuni (Fig. 194, A) it is kidney-shaped, in Aspknhan 

 elongated, and free only along one side. Where, as in Onoclea 

 (Fig. 194, B), the margins of the sporophyll are involute, so as 

 to completely enclose the sori, the indusium is wanting or very 

 rudimentary. 



