THE BIOTIC ENVIRONMENT OF ORGANISMS 



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of the legumes, such as clover, alfalfa, and peas, and the nitrogen-fixing 

 bacteria that live within their root nodules. The legumes provide protec- 

 tion and the requisite food supplies for the bacteria and receive, in return, 

 a rich supply of available nitrogen for their own growth. 



Comparable examples are also common among animals. The termites 

 (Fig. B.21G), notorious devourers of wood, are unable to digest the 



Fig. 33.3. An encrusting lichen, Parnelia species, growing on rock in Chester County, Pa. 

 Lichens are made up of two simple plants, one a green alga and the other a fungus, growing 

 intermingled in close symbiotic relation with each other. Encrusting lichens are character- 

 istic pioneer plants on exposed rock surfaces; by their aid to the weathering process they 

 help to make soil and to produce conditions suitable for other organisms. {Photo by Prof. 

 A. M. Laessle.) 



woody tissues that they chew and ingest. Instead, they are dependent 

 upon the unicellular organisms that inhabit their intestines for the 

 enzymes that will make the woody pulp available for their own tissues. 

 A termite deprived of its intestinal symbionts will starve in spite of a 

 plentiful supply of wood, whereas the unicellular organisms that provide 

 the enzymes depend upon the termite for protection, moisture, and a 

 continuous supply of wood in the proper physical condition for digestion. 



