200 HIGHER PLANTS AND SOIL MICROORGANISMS 



and favor plant development much the same as the legume sym- 

 biont does. With most fungi, however, fixation of nitrogen does 

 not occur. 



The effects exerted by the mycorrhiza differ with different 

 plants. In some cases there are definite parasitic effects, and the 

 invading fungus injures the cells which it penetrates without bene- 

 fiting the host in any way. In other plants the fungus is undoubt- 

 edly not parasitic, and produces no injury to the living cells 

 although it may penetrate these cells. The fungus causes a dis- 

 appearance of starch and other organic substances from the root 

 cells but does not destroy them ; on the contrary, the plant is much 

 favored by its development. The fungus mycehum may act as 

 root hairs absorbing water, inorganic nutrients, and organic sub- 

 stances, which become available either directly to the plant from 

 the penetrating parts of the fungus or subsequent to the digestion 

 of the mycelium, which takes place within the penetrated cells. 

 These effects, besides possible nitrogen fixation in certain few cases 

 and the role of decomposing soil organic matter, may explain some 

 of the more important possible benefits which the higher plants 

 derive from the fungus association. Undoubtedly in most cases 

 the mycorrhiza may be considered as symbiotic phenomena in 

 which both organisms, the fungus and the vascular host, derive 

 decided benefit from the association. 



Nodule Formation on Leguminous Plants. — By far the 

 best known instance of symbiosis between vascular plants and 

 microscopic organisms is the association between legumes and a 

 bacterium called Bacillus radicicola. This type of symbiosis is 

 apparent in most legumes and some few known non-leguminous 

 plants. Furthermore, the legume invader is of widespread occur- 

 rence in soils wherever legumes develop naturally. Domestic 

 cultivation of legumes frequently requires the inoculation of the 

 organism in order to insure its presence. By reason of this spe- 

 cific symbiotic growth, the legumes are particularly favorably 

 prepared to develop where many other plants could not grow; 

 they have, therefore, found wide use as cover crops to increase the 

 fertihty of soils, particularly with reference to nitrogen. This 

 relationship has been discussed at considerable length in preceding 

 pages. 



Bacteriorrhiza. — Bacterial invasion similar in nature to 

 fungus invasion by mycorrhiza has been observed in some instances 



