50 GUIDE TO THE STUDY OF LICHENS. • 



lighter-colored specks, somewhat smaller than a pin- 

 head ; these are the cyphellce and are functionally anal- 

 ogous to the stomata or breathing pores of true foli- 

 age leaves. Structurally there is no resemblance ; 

 they are simply breaks in the lower cortical layer, 

 filled in by a loose secondary cortical tissue. Upon 

 examining them with a hand-lens, it will be found 

 that they are of two kinds : one which appears as a 

 depression, that is, concave ; the other is somewhat 

 protruding or convex. They occur only in the genera 

 indicated. 



3. The Cephalodia. 



These are wart-like outgrowths, usually occurring 

 on the upper surface of the thallus. Sometimes they 

 also occur on the lower surface, as in Sticta Oregana. 

 They are simply neo-formations in the tissue of the 

 thallus, induced by a foreign alga. They occur upon 

 various lichens, but are, perhaps, most common among 

 the Peltigeras and Stictas. In Peltigera aphthosa 

 they are very numerous and are about the size of a 

 pin-head. In Sticta Oregana they vary from very 

 small to nearly the size of a pea.' These two plants 

 also typify the two kinds of cephalodia. In Peltigera 

 they develop upon the thallus, while in Sticta they de- 

 velop within the thallus. Nothing definite is known 

 regarding their origin and function. 



Jf.. The Spermagonia. 



These are very minute cup-shaped structures, im- 

 mersed in the upper surface of the thallus. To the 

 naked eye they appear as minute black specks. Noth- 



