STRUCTURES PECULIAR TO LICHENS 



127 



b. DEVELOPMENT OF CYPHELLAE. The cortex of both surfaces in the 

 thallus of Sticta is a several-layered plectenchyma of thick-walled closely 



Fig. 72. Lobaria pulmonaria Hoffm. Showing pitted surface, a, under surface. 

 Reduced (S. H., Photo.'). 



packed cells, the outer layer growing out into hairs on the under surface of 

 most of the species. Where either cyphellae or pseudocyphellae occur, a 

 more or less open channel is formed between the exterior and the internal 

 tissues of the lichen. In the case of the cyphellae, the medullary hyphae 

 which line the cup are divided into short roundish cells with comparatively 

 thin walls (Fig. 73). They form a tissue sharply differentiated from the 



- 73- Sticta damaecornis Nyl. Transverse section 

 of thallus with cyphella x 100. 



loose hyphae that occupy the medulla. The rounded cells tend to lie in 

 vertical rows, though the arrangement in fully formed cyphellae is generally 



