04 COMPOUND ORGANS OE PLANTS. 



appearance only shows itself in the more advanced stages 

 of development, the thecje being at first concealed by the 

 cuticle, which they raise up like a blister, and ultimately rup- 

 ture and detach from the parenchyma of the frond in a variety 

 of ways, which serve as characteristics of the genera. It is 

 necessary, however, to except Polypodium vulgare, or the 

 common polypody, the thecse of which are formed above the 

 cuticle. 



119. The cuticle which is thus ruptured from the frond is 

 called the indusium. Generic and specific characters de- 

 pending on the peculiarities of its form, and the nature of its 

 rupture from the parenchyma of the frond, can only be detect- 

 ed in early spring. Later in the season, the indusium withers 

 away, and the sori or heaps of theoaa are alone visible. 



120. In Fig. 35,* at a, we have represented two of the pin- 



Fis. 35. 



a- 



nse of Polypodium thelypteris covered with sori, having an 

 indusium ; 5 is a magnified view of one of these sori, show- 

 ing the clusters of annulate thecaj and the partially withered 



* Richard's "Precis de Botanique," pt. ii., Paris, 1852. 



