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GENERAL MORPHOLOGY OF PLANTS 



lichen body it is prevented from injurious desiccation and is held 

 on even to steep slopes or perpendicular rock faces. 



438. The relation of the algae and fungus in the lichen 

 thallus. In the early history of the study of lichens it was thought 

 by some that the lichen was an individual plant (autonomous 

 plant), and that the algal cells were special spores (gonidia) 

 which were cut off from the colorless threads of the mycelium. 



Lichen tundra, showing the "reindeer moss" or lichen, Alaska. (Copyright by E. H. 

 Harriman.) 



Opposed to this theory another one came to be accepted by some 

 students, i.e., that the alga and the fungus are distinct plants, and 

 this theory has finally prevailed. The most important arguments 

 presented for this theory were furnished by some investigators 

 who separated the spores of the lichen and grew them free from the 

 algae, and also separated the algal cells and grew them as separate 

 algae. In this way it has been possible to identify some of the algae 

 with those which are found in a free state. Pleurococcus vulgaris, 

 so common in cool shady places, is one of these. The blue-green 



