FIXATION OF NITROGEN BY BACTERIA 195 



becomes elongated. The infection-threads soon extend 

 into the new tissue, and although they do not always 

 come into direct contact with the nuclei of the cells 

 through which they pass, the latter appear to be in- 

 fluenced by the presence of the bacteria in the thread, 

 since they increase in size and develop an abnormal 

 number of nucleoli, after which they finally degenerate. 

 In clover and some other plants the infection-threads 

 remain visible for some time 

 before becoming obliterated. 



The organisms present in the 

 infection-threads and youngest 

 parts of the nodules are short 

 straight rods. In the older parts 

 they become distributed through 

 out the interior of the cells, 

 grow considerably in size and 



thickneSS, and Undergo change FIG. 3 6.Bactero"ids from red clover 

 . , , . j j T, nodule ( X 1000). 



into branched, curved or 1 



and Y shaped forms, commonly termed barter aids, in 

 which state they stain irregularly, and appear to consist 

 of vacuolated protoplasm (Fig. 36). The bacteroids 

 soon lose their power of division, and after a time are 

 dissolved by a proteolytic enzyme secreted from the 

 surrounding protoplasm, the compounds produced being 

 transferred to the flowers and ripening seeds of the 

 leguminous plant. The nodules eventually shrivel and 

 decay, but some of the small rods remain unchanged 

 and find their way into the soil ready to infect a 

 subsequent crop. 



The relationship between the bacteria and the legumi- 

 nous plant is one of symbiosis or mutual assistance, 

 At first the bacteria live as parasites, deriving all their 



