A discussion on symbiosis involving micro-organisms : Introduction. The symbiosis between Rhizobium and leguminous plants and the influence on this of the bacterial strain

The term symbiosis has been used with different meanings, and the question of its correct meaning and even of the desirability of its use at all has been debated. The term, indeed, raises the question as to how far it is possible to distinguish a definitely beneficial association between two or more organisms from certain states of parasitism on the one hand and from complex ecological associations on the other. For the purpose of the present discussion, it seems desirable to define the term symbiosis as implying some evidence that the partners in the association each receive some benefit from it. We can then see how far the examples that will now be described satisfy this criterion. I propose to begin by considering an association in which there is clear evidence of such mutual benefit, that of the nodule-forming bacteria of the genus Rhizobium with their leguminous host plants. This association provides exceptional facilities for studying symbiotic adaptation, first because the bacteria can readily be grown in laboratory media, and in some cases the host plant can easily be grown on agar, either aseptically or supplied only with a pure culture of a bacterial strain. Thus a study can be made of the effect on the symbiosis not only of different naturally occurring strains of Rhizobium on different species and varieties of legume, but also of genetically determined changes both in the bacterium and in its host plant. Secondly, because the environmental conditions of the host plant can readily be varied and their effects on the symbiosis can be studied, and thirdly because we have, in the number of nodules, a measure of infection and in the quantity of nitrogen fixed, a measure of the effectiveness of the symbiosis
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