3i8 



ECOLOG Y. 



598. Others regard the lichens as autonomous plants, that is, the two or- 

 ganisms have by this long-continued community of existence become unified 

 into an individualized organism, which possesses a habit and mode of life 



Fig. 419. 



Section of fruit body or apothecium of lichen (parmelia), sliowing asci and spores of the 

 fungus. 



distinct from that of either of the organisms forming the component parts. 

 This community of existence between two different organisms is called by 

 some mutualism^ or symbiosis. 



Nitrogen ' gatherers. 



599. How clovers, peas, and other legumes gather nitrogen. — It has long 

 been known that clover plants, peas, beans, and 

 many other leguminous plants are often able to 

 thrive in soil where the cereals do but poorly. 

 Soil poor in nitrogenous plant food becomes richer 

 in this substance where clovers, peas, etc., are 

 grown, and they are often planted for the purpose 

 of enriching the soil. Leguminous plants, espe- 

 cially in poor soil, are almost certain to have en- 

 largements, in the form of nodules, or "root 

 tubercles." A root of the common vetch with 

 some of these root tubercles is sliown in fig. 420. 



600. A fungal or haoterial organism in these 

 root tubercles. — If we cut one of these root tuber- 

 cles open, ami mount a small porti(.in of tlie in- 

 terior in water for examination with the micro- 

 scope, we shall find tmall ■ rod-shaped bodies, 



Fig. 420. 

 Root of the common vetch, 

 showing root tubercles. 



