60 CONSTITUENTS OF THE LICHEN THALLUS 



There is some confusion as to the genera of algae that form the gonidia 



of these epiphyllous lichens. Phyllactidium 

 given by Zahlbruckner as the gonidium of 

 all the Strigulaceae (except Strigula in 

 part) is classified by de Toni 1 as probably 

 synonymous with Phycopeltis Millard, and 

 as differing from Mycoidea parasitica in the 

 mode of growth. 



Earn. PRASIOLACEAE. Thallus filamen- 



Fig.-32. Phycopeltis exfansa Jenn. tous , often expanded into broad sheets by 

 much magnified (after Vaughan the fusion of the filaments in one plane. 



Jennings). 



PRASIOLA Ag. Thallus filamentous, of one- to many-cell rows, or 

 widely expanded (Fig. 33). The gonidium of Mastoidiaceae (Pyreno- 

 carpeae). 



Fig- 33- Prasiola parietina Willex 500 (after West). 



B. CHANGES INDUCED IN THE ALGA 



a. MYXOPHYCEAE. Though, as a general rule, the alga is less affected 

 by its altered life-conditions than the fungus, yet in many instances it 

 becomes considerably modified in appearance. In species of the genus 

 Pyrenopsis small gelatinous lichens the alga is a Gloeocapsa very similar to 

 G. magma. In the open it forms small colonies of blue-green cells surrounded 

 by a gelatinous sheath which is coloured red with gloeocapsin. As a 

 gonidium lying towards or on the oulside of the granules composing the 

 thallus, the red sheath of the cells is practically unchanged, so that the 

 resemblance to Gloeocapsa is unmistakable. In the inner parts of the thallus, 

 the colonies are somewhat broken up by the hyphae and the sheaths are not 



1 De Toni 1889. 



