SOIL ORGANISMS 435 



the production of tubercles on other leguminous species. Cer- 

 tain cross inoculations are, however, very successful. The 

 organisms seem to be interchangeable within the clovers, the 

 vetches and the bean family. The organisms from sweet 

 clover and burr clover will inoculate alfalfa, while the bac- 

 teria may be transferred from vetch to field pea or from cow- 

 pea to velvet bean. 



It has been shown by several investigators that bacteria 

 from the nodules of legumes are able to fix atmospheric nitro- 

 gen even when not associated with leguminous plants. There 

 would seem to be no doubt, therefore, that the fixation of 

 nitrogen in the tubercles of legumes is accomplished directly 

 by this organism, not by the plant itself nor through any com- 

 bination of the plant and the organism. The relationship is, 

 therefore, parasitical rather than strictly symbiotic, although 

 the host plant benefits from the relation. The part played 

 by the plant is doubtless to furnish the carbohydrates which 

 are required in considerable quantities by all nitrogen-fixing 

 organisms and which the legumes are able to supply in large 

 amounts. The utilization of large quantities of carbohydrates 

 by the nitrogen-fixing bacteria in the tubercles may also ac- 

 count for the small proportion of non-nitrogenous organic 

 matter in the plants. 



How the plant absorbs this nitrogen after it has been 

 secured by the bacteria is not well understood nor is it known 

 in exactly what form the nitrogen is at first fixed, although 

 amino and amide nitrogen very soon appear. 1 Early in the 

 growth of the tubercle, a mucilaginous substance is produced, 

 which permeates the tissues of the plant in the form of long 

 slender threads containing the bacteria. These threads de- 

 velop by branching or budding, and form what have been 

 called Y and T forms, known as bacteroids, which are peculiar 

 to these bacteria. The threads finally disappear, and the 



1 Strowd, W. H., The Forms of Nitrogen in Soybean Nodules; Soil 

 Sci., Vol. XI, No. 2, pp. 123-130, 1921. 



