LICHEN ALGAE 



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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. 1 8). Dense gelatinous colonies of definite 



Fig. 18. Examples of Nostoc. N. Linckia Born. A, nat. size ; B, small portion x 340 : 

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



Fig. 19. Example of Scytonema alga. S. mirabile^hur. 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 Peltigerd)-, 

 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 



