68 PRESIDENTIAL ADDRESS — SECTION C. 



liOticed. Some bacteria arc not able to effect an entrance at all; 

 others enter and provoke a reaction in the tissues which leads to 

 advanta.s^eous nodule formation; and still others injure the host 

 plant. In brief, the bacteria behave toward the plant, at least 

 m the beginning, like true parasites, against which the plant 

 strives to protect itself with all possible means of defence. 

 Eventually a state of equilibrium or armed truce is brought about, 

 in which both bacterium and plant benefit by the association. . . 



" Broadly considered, the process of nitrogen fixation by 

 leguminous plants, consists, first, in the penetration of the root 

 tissues by certain bacteria, which establish themselves there in a 

 sort of half-parasitical, half-symbiotic relation; second, in the 

 accumulation of nitrogenous substances by the bacteria under 

 the influence of the abundant carbohydrate food supply available 

 in the ])lant tissues, the nitrogen used in the constructive process 

 being derived from the atmosphere ; and, finally, in the appro- 

 priation by the plant of the nitrogenous compounds contained 

 v/ithin, or diffusing out from, the nutritionally and structurally 

 modified bacteria (bacterioids)." 



It is even claimed that in certain cases the nodule bacteria 

 become actively parasitic. In a recent paper by van der Wolk 

 (6i), besides a discussion of symbiotic relationships, the author 

 gives an account of a soy bean disease, which, causing first an 

 etiolated condition, may finally result in the death of the plants. 

 He considers that the trouble is due primarily to the lowering of 

 resistance and decrease of protective products in the single layer 

 of cells lying between the nodule and the rootlet proper. This 

 layer normally acts as an absorbing organ, and also as a barrier 

 to the nodule bacteria. The absence or inefficiency of this 

 barrier results in a condition described as a sticcessful parasitism 

 of the plant by its own nodule bacteria. This trouble appeared 

 to be frequent in Arachis and Glycine plants growing under 

 unfavourable conditions. 



Something of a similar nature to the relation which nor- 

 mally obtains between the nodule bacteria of the Leguminosce 

 and their hosts occurs in the tropical East Indian plant,- Ardisia 

 (36). Here there is no external evidence of the presence of 

 the bacteria, which are most abundant in the leaf-serratures. In 

 the buds they occur in nodules or pockets at regular intervals 

 30-50 mm. apart, at a short distance from the edge of the leaf. 

 They multiply and have the effect of making the leaf margins 

 appear somewhat yellowish and slightly swollen, but never 

 cause any serious damage. The bacteria are also present in 

 smaller numbers in the inner parts of the seed, from which they 

 pre transferred to the seedling; they are apparently always pre- 

 sent in plants of A. crispa, but it is not known whether, as in the 

 case of Leguminoscc, the plant derives any compensating benefit 

 from the presence of the bacteria in its tissues. 



