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Class. — LICHENS. 



Type.-COLLEMA PULPOSUM. 



" Lichens are true fungi belonging to the Ascomy- 

 eetes ; they are distinguished by a peculiar parasitism. 

 Their hosts are Algce which grow in damp places, but 

 not actually in water. Lichens are not found in any 

 other form but as parasites on Algse. The Algae 

 when combined with the Fungus, are called Gouidia, 

 and these Algse are known in a free condition without 

 the fungus. The gonidia of Collema has been identi- 

 fied with the alga Nostoc. The algee grow as stains 

 and incrustations on damp ground, the bark of trees, 

 and stones. The separate cells and groups of cells 

 become so involved by the tissue of the fungus that 

 they at last are only interspersed here and there in 

 the dense hyphal tissue of the fungus." 



1. GENERAL CHAEACTERS. 

 Collema grows on moist earth and rocks. Its 

 thallus (the whole plant — so called because it shows 

 no differentiation into root, stem, and leaf) may be 

 aU but ground to a powder without destroying its 

 vitality. When saturated with water it becomes slimy 



