PHYSIOLOGICAL STUDIES ON RHIZOBIUM IV. UTILIZATION OF CARBONACEOUS MATERIALS,

Although extensive experiments have been conducted with the legume root-nodule bacteria, no acceptable theory has been ad-vanced to explain the specificity of the organisms in nodule pro-duction. Different species may show similar morphological and cultural characteristics, and yet be unable to bring about nodula-tion upon any plants except their particular hosts. Little is known of the materials presumably furnished the organisms by the host plant during the period of the symbiotic growth, nor is it known whether or not the compounds supplied by the plants of the same cross-inoculation groups are similar. It has been shown that the grouping of legume bacteria on the basis of cross-inoculation is similar to that indicated by serological reactions in cross-agglutination. The suggestion has also been made that certain materials given off by the roots of leguminous seedlings at a certain stage of development stimulate the forma-tion of nodules. Hence, attempts have been made to show a relationship between the composition of plant roots and tops and the ability of certain organisms to bring about nodule formation. It has been shown further that the different species of Rhizobium differ in their ability to utilize carbonaceous materials. In view of all these facts it seems probable that studies of carbohydrate utilization or dissimilation may yield information of value in explaining the specificity of the several species