88 REPORT OF THE AGRICULTURAL RESEARCH INSTITUTE 



in the conditions of the bacteria and bacteroidal tissue. 

 Nodules in all these various stages of development could be 

 found at the same time on the same plant. Indications 

 were obtained that nodule formation did not take place 

 at that soil level at which most vigorous root growth was 

 found, but tended to occur chiefly where the ratio of air 

 to water was higher than was consistent with maximum 

 root development. A characteristic difference in the char 

 acter of growth was observed between roots in clay and 

 those in sand, with intermediate variation in mixtures of 

 the two, and similar changes where the plants were grown 

 in alternate layers of these soils. It was remarkable that 

 even in pure sand, nodule development was restricted to a 

 comparatively shallow surface layer very slightly deeper 

 than that found in the case of pure clay. 



Sporadic development of nodules, insignificant in 

 number, occurred at deeper levels. 



The most universally prevalent characteristic was the 

 fact that at least 90 per cent, of the nodules present at 

 any one time were found on, or very closely adjacent to, 

 the main stem, this being apparently due partly to the 

 fact that such nodules had a longer life than those formed 

 on more distal portions of the root, showing indeed the 

 specific morphological characters associated with their host, 

 and also in part to the fact of this position being coinci- 

 dent with the conditions of aeration which appear to favour 

 their growth. An alternative hypothesis might suggest 

 the importation of the specific radicicola organism by the 

 seed and its consequent occurrence in the soil only in the 

 neighbourhood of the latter; this, however, does not appear 

 a probable explanation in view of the indigenous character 

 of the legumes under observation. The general suggestion 

 would be that nodule formation took place more readily 

 in the earliest stages of the growth of the plant owing to 

 the lower power of resistance to bacterial invasion which 

 the latter possesses at that time. Rootlets of a similar age 

 but formed at a later period of growth appear to be less 

 readily invaded. 



