8oo BACTERIA : THEIR WORK 



In the annual Leguminos^, such as peas and lupins, the greatest 

 development of bacteroids is usually met with about the time 

 when the plants reach the flowering stage. From this point 

 onwards analysis shows that the nitrogen-content of the nodule 

 decreases rapidly and the bacteroids diminish in numbers ; most 

 of the latter become dissolved and their component nitrogenous 

 compounds leave the nodule by way of the small vascular strands 

 i^b. Fig. 265) which are organically connected with the main 

 vascular strands of the root on which the nodule is produced. 



At the time when the seeds of the leguminous plants are ripe 

 the bacteroidal tissue of the nodules is empty and shrivelled. 



Although by far the larger number of bacteria produced in the 

 nodules undergo degeneration and absorption, a few remain 

 unaltered and are set free in the soil when the roots and 

 nodules become disorganised. To these residual organisms 

 is attributed the infection of leguminous crops subsequently 

 grown on such soil. 



When one organism lives upon another and both render each 

 other mutual service, the relationship is spoken of as symbiotic. 



The combined life of nodule bacteria and leguminous plant is a 

 case of symbiosis, for the carbon and some of the other elements 

 which the bacteria need for their development are supplied 

 in the form of sugar and other compounds by the green 

 plant, and in return for these the latter obtains a supply of 

 nitrogen from the bacteria. 



The practical utilisation of the powers of these nodule- 

 bacteria is a matter of considerable importance to the farmer. 

 A profitable leguminous crop can be obtained on soils devoid of 

 nitrogen provided that the requisite mineral constituents of 

 plant food-materials and the suitable nodule-bacteria are present ; 

 moreover, soils after yielding such a crop are found to be largely 

 enriched by the combined nitrogen contained in the roots 

 left after the produce is removed, and this nitrogen is readily 

 yielded to subsequent crops such as the cereals which specially 



