LICHEN ALGAE 



53 



two genera form the gonidia in the family Pyrenopsidaceae. Gloeocapsa 

 polydermatica Kiitz. has been identified as a lichen gonidium. 



Fam. NOSTOCACEAE. Filamentous algae unbranched and without base 

 or apex. 



NoSTOC Vauch. Composed of flexuous trichomes, with intercalary 

 heterocysts (colourless cells) (Fig. i8). Dense gelatinous colonies of definite 



Fig. i8. Examples of Nostoc. N. Linckia Bom. A, nat. size; B, small portion X 340 ; 

 C, N. coerulescens Lyngbye, nat. size (after West). 



Fig. 19. 'Ex&mple of Scjf tone ma sdga. S. 7nirabileT'h\xx. C, apex of a branch ; D, organ 

 of attachment at base of filament." x 440 (after West). 



form are built up by cohesion. In some lichens the trichomes retain their 

 chain-like appearance, in others they are more or less broken up and massed 

 together, with disappearance of the gelatinous sheath (as in Peltigera); 

 colour mostly dark blue-green. 



Nostoc occurs in a few or all of the genera of Pyrenidiaceae, Collemaceae, 

 Pannariaceae, Peltigeraceae and Stictaceae, and N. sphaericum Vauch 



