STRUCTURES PECULIAR TO LICHENS 



139 



and lay like a cap over the top. The cephalodia described by Winter are 

 endogenous in origin, though the mature body finally emerges from the 

 interior and becomes either epigenous or hypogenous. Schneider 1 has fol- 

 lowed the development of a somewhat similar endotrophic or endogenous type 



Fig. 786. Pdtigera aphthosa Willd. Vertical section of thallus and 

 cephalodium x 480 (after Babikoff ). 



in Sticta oregana due also to the presence of a species of Rivularia. How 

 the alga attained its position in the medulla of the thallus was not observed. 

 Both the algal cells of internal cephalodia and the hyphae in contact 

 with them increase vigorously, and the newly formed tissue curving upwards 

 or downwards appears on the outside as a swelling or nodule varying in 

 size from that of a pin-head to a pea. On the upper surface the gonidial 

 zone partly encroaches on the nodule, but the foreign alga remains in the 

 centre of the structure well separated from the thalline gonidia by a layer 

 of hyphae. The group is internally divided into small nests of dark-green 

 algae surrounded by strands of hyphae (Fig. 79). The swellings, when they 



a 



Fig. 79. Nephroma expallidum Nyl. Vertical section 

 of thallus. , endotrophic cephalodium x 100 (after 

 Forssell). 



1 Schneider 1897. 



