66 



CONSTITUENTS OF THE LICHEN THALLUS 



of Peltigera had found their way into the tissue, the underlying cortical 

 cells having degenerated. The blue-green cells of the normal gonidial layer 



Fig. 35. Pure culture of protonema of Mnium hornum L. with spores and hyphae of 

 Lecidea vernalis Ac\i. a,a,a, buds forming x 150 (after Bonnier). 



had died off before their advance but no zone was formed by the invading 

 algae; they simply withdrew nourishment and gave seemingly no return. 

 The phenomenon is somewhat isolated and accidental but illustrates the 

 capacity of the alga to absorb food supply from lichen hyphae. 



An instance of epiphytic growth has also been recorded by Zahlbruckner'. 

 He found an alga, Trentepohlia abietina, covering the thallus of a Brazilian 

 lichen, Parmelia isidiophora, and growing so profusely as to obscure the 

 isidiose character towards the centre of the thallus. There was no genetic 

 connection of the alga with the lichen as the former was not that of the 

 lichen gonidium. Lichen thalli are indeed very frequently the habitat of 

 green algae, though their occurrence may be and probably is accidental. 



' Zahlbruckner 1902. 



