STRATOSE THALLUS 



89 



into short cells and a kind of plectenchyma is formed, as in Lecanora 

 (Psoroma) hypnorum, in Endocarpon, etc. 



Fig. 49. Parmelia caperala Ach. (S. H., Photo.}. 



The felted medulla is characteristic of most lichens and is formed of 

 loose slender branching septate hyphae with thickish walls. This interwoven 

 hyphal texture provides abundant air-spaces. 



Hue 1 has noted that the walls of the medullary hyphae in Parmeliae are 

 smooth, unless they have been exposed to great extremes of heat or cold, 

 when they become wrinkled or scaly. They are very thick-walled in Pelti- 

 gera (Fig. 50). 



Fig. 50. Hyphae from lower medulla of Peltigera canina DC. x 600. 

 1 Hue 1898. 



