FEETILIZEES 



109 



fixing of nitrogen, however, cannot go on without lime. Owing 

 to the power of their bacterial symbiont to fix nitrogen from 

 the air, legumes are able to thrive in soils too poor in nitrogen 

 to support other crops. In sandy regions, where the loose and 

 open soil permits the loss of nitrates almost as fast as formed, 

 legumes are usually abundant. 



Mycorrhizas. In a considerable number of plants, among 

 which are various trees and shrubs, the older parts of the 

 roots are inhabited by fungi known 

 as mycorrhizas, which enter into sym- 

 biosis with them. Such associations 

 are common, or possibly the rule, 

 among woody plants, but are espe- 

 cially abundant in the heath family, 

 to which the rhododendron, cranberry, 

 and blueberry belong. The mycor- 

 rhizas extend out into the soil and 

 function like root hairs. They appear 

 to have the power to fix nitrogen 

 from the humus in the soil and ab- 

 sorb sugars derived from fallen leaves 

 by other soil bacteria. Certain flow- 

 ering plants, like the Indian pipe and 

 the pinesap, which lack chlorophyll, ab- 

 sorb all their food in this way. Mycor- 

 rhizas are also frequently associated 

 with plants that transpire slowly; 

 otherwise these plants would find 

 difficulty in getting sufficient food. 



Soil inoculation. In many soils it is difficult to get a good 

 crop of legumes because the necessary bacteria for symbiosis 

 do not occur there. Experiments seem to show that each 

 species of legume, if it does not have its own special bacterial 

 species, has at least a special form with which it is associated, 



FIG. 86. The snow plant 

 (Barcodes sanguined) 



A saprophytic heathwort 



(allied to the Indian pipe, 



Monotropa uniflora) from 



the Pacific Coast region 



