300 BIOLOGY: GENERAL AND MEDICAL 



radiolaria not infrequently harbor a small green alga, 

 the zooxanthella. The alga gives off oxygen that is 

 advantageous to the little animal, which in turn gives 

 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 



Fio. 106. 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 



