BOTANY OF THE LIVING PLANT 



initially sterilised and if other precautions are taken to prevent 

 infection and nodulation, the leguminous plant develops signs of 

 starvation and makes poor growth (Fig. 165). Evidently it is the 

 presence of the bacterium which enables nitrogen fixation to proceed 

 in nodulated plants, and there is general agreement that the bacterium 

 itself effects the fixation within the nodules, though the chemical 

 stages in the process are at present uncertain. In some way the 

 nitrogenous products of fixation are transferred from the bacterium 

 to the surrounding cytoplasm of the nodule cells, perhaps by a diges- 

 tion of bacteria under the influence of plant enzymes, or through 

 excretion of nitrogenous compounds by the bacteria. There is a 

 steady passage of these products of fixation from the nodules into 

 the rest of the plant, to its great advantage, especially when the soil 

 is deficient in nitrogen. No doubt the bacteria appropriate to their 

 own use certain carbohydrate materials from the plant. The environ- 

 ment provided by the nodule cells of the Leguminosae seems to be 

 particularly favourable to the process of nitrogen-fixation, for al- 

 though the bacterium can readily be grown on a variety of prepared 

 nutrient media, there is as yet no satisfactory evidence that appre- 

 ciable fixation of nitrogen proceeds under such conditions. 



The significance to the organic world in general of this fixation 

 of nitrogen by B. radicicola in association with a leguminous plant 

 has already been stressed. After the completion of the life-cycle 

 of the plant the nitrogen fixed during its development is added to 

 the soil, and in due course it becomes available for uptake by plants 

 in general. We have here a process whereby soil fertility is main- 

 tained. In agriculture the manurial effect of leguminous crops 

 such as clover and lupins has been known for centuries. It is, 

 however, possible that leguminous plants confer benefit not only on 

 future generations of other plants, in the manner explained, but 

 also on contemporary plants of other families that may be growing 

 in the vicinity. For there is evidence that at least under some con- 

 ditions there is a leakage from the nodules into the soil of nitrogenous 

 compounds, available for absorption by other neighbouring plants. 



It is frequently advantageous in agricultural practice to inoculate the 

 seeds of a leguminous crop with the correct bacterium, in order to 

 ensure plentiful nodule-formation. This is especially the case when 

 a new crop is being introduced into a district, since the correct bacter- 

 ium may not be present in the soil. It should be mentioned that a 

 number of different races of the nodule bacterium exist, each capable 

 of infecting only a limited number of different leguminous species. 



