214 Mineral Nutrition of Plants 



the primary effect may be either on the plasma membrane or on a 

 "surface" metabolism intimately related to the permeability of proto- 

 plasm. 



Symbiosis 



There is one other relationship between inorganic solute supply 

 and accumulation, which is restricted to certain types of plants. A 

 symbiotic association is recognized between parasites and roots of cer- 

 tain plants. First, there is the association between certain bacteria and 



500 



CoBrJN MILLIEQUIV. PER LITER OF Q005N KBr CULTURE 



Figure 16. Effect of a divalent cation in the culture solution on the ac- 

 cumulation of KBr by abscised barley roots. Plants 4 weeks old; growth 

 in diluted nutrient solution. Experimental period 85 hours; temperature 

 20 C— Viets (87) 



the roots of legumes. This association is ultimately concerned with the 

 accumulation of nitrogen in these species. The activities of these organ- 

 isms relative to nitrogen fixation are related to the supply of nitrogen 

 in the external medium bathing the roots. Discussion of these interrela- 

 tions is beyond the scope of this review, but they should be recognized 

 in the general nitrogen economy of such plants. Again, there is a sym- 

 biotic association between the roots of host pine species and parasitic 

 fungi with the development of mycorrhizae. This association has been 



