194 



REPRODUCTION 



in some lichens, such as Cerania ( Thamnolid) vermiailaris, they are the only 

 sporiferous organs known. Not unfrequently crustaceous thalli bear sper- 

 mogonia only, and in some Cladomae, more especially in ascyphous species, 

 spermogonia are produced abundantly at the tips of the podetial branches 

 (Fig. 109), while apothecia are exceedingly rare. Usually they occur in 

 scattered or crowded groups, more rarely they are solitary. Very often they 

 are developed and the contents dispersed before the apothecia reach the 

 surface of the thallus; hence the difficulty in relating these organisms, since 

 the mature apothecium is mostly of extreme importance in determining the 

 species. 



Fig. 109. CladoniafurcataSchxzA. Branched 

 podetium with spermogonia at the tips 

 (after Krabbe). 



Fig. 110. Physcia hispida TncVexm. Ciliate 

 frond, a, spermogonia ; b, apothecia. x ca. 5 

 (after Lindsay). 



In a very large number of lichens, both crustaceous and foliose, the 

 spermogonia are scattered over the entire thallus (Fig. 1 10), covering it more 

 or less thickly with minute black dots, as in Parmelia conspersa. In other 

 instances, they are to some extent confined to the peripheral areas as in 

 Parmelia physodes; or they occur on the extreme edge of the thallus as in 

 the crustaceous species Lecanora glaucoma (sordidd). In Pyrenula nitida 

 they grow on the marginal hypothallus, usually on the dark line of demar- 

 cation between two thalli. 



They tend to congregate on, and indeed are practically restricted to the 



