310 BIOLOGY: GENERAL AND MEDICAL 



off carbon dioxide that is useful to the plant. Here 

 both symbionts are benefited by the association. 



The symbiosis of the alga and fungus constituting a 

 lichen is a form of mutualism that may be of physiological 



-~ / 



FIG. 110. Tubercles, on the roots of red clover. I, Section of ascending 

 branches; 6, enlarged base of stem; t, root tubercles containing bacteria. (From 

 Bergen and Davis" "Principles of Botany." Ginn & Co., publishers.) 



benefit to both symbionts, the fungus furnishing nitro- 

 gen and the alga carbohydrates. 



Again, the bacteria that form tubercles upon the root- 

 lets of the leguminous plants are of great benefit to them 

 by fixing nitrogen. At the same time they probably 

 derive their nutrition from the juices of the plant with- 

 out damaging it. 



The bacteria that live upon the skin, in the mouth, and 

 in the intestines of animals are for the most part harm- 

 less mutuals that derive their nourishment from the 

 host without causing any harm. 



Certain bees live under conditions of mutualism as 



