IRREGULAR NUTRITION 



237 



leave the infection thread and by continued multiplication eventually 

 fill the cells of the developing nodule, though they are restricted to 

 its tissues and do not infect any other part of the plant. The bacteria 

 within the nodule-cells have at first a spherical or rod form, but 

 eventually most of them pass into an enlarged, frequently forked 

 condition, known as the Bacteroid form (Fig. 164, 4, 5). The formation 



Fig. 165. 

 Plants of Soya Bean (Glycine) growing in nitrogen-free sand. Those on the left 

 bore root-nodules, those on the right were free of nodules. Photo. G. B. ( x {.) 



of nodules usually begins during the first few weeks of development 

 of a young leguminous plant, and the nodules persist in an annual 

 until the close of the life-cycle of the plant approaches, when their 

 tissues decay. 



Unlike the great majority of plants, nodulated leguminous 

 plants are able to make good growth in rooting media free of nitrate 

 or other nitrogenous compounds, although other necessary salts are 

 provided : a result which indicates their ability to utilise atmospheric 

 nitrogen in protein synthesis. On the other hand, if the medium is 



