194 FIXATION OR ASSIMILATION OF NITROGEN 



it passes through the cell-walls it generally exhibits 

 peculiar trumpet-shaped expansions (Fig. 34). Here and 

 there upon it are minute swellings from which bacteria 

 appear to escape into the cell-cavity which they subse- 

 quently fill by repeated division and growth. 



The nodules arise endogenously from division of the 

 infected cortical cells near the endodermis of the rootlets. 





SL 



FIG. 35. a Transverse section of nodule from red clover root showing 

 dark bacteroidal tissue in centre, with cortical tissue and 

 vascular bundles round it ( X 20). 

 b Single cell of bacteroidal tissue ( X 250). 



Occupying the central part of a fully developed specimen 

 is a mass of parenchyma filled with bacteria or bacteroids 

 (Fig. 35). Surrounding the bacteroidal tissue, which is 

 often pinkish or greenish in colour, are small vascular 

 bundles connected with those of the root, upon which 

 the nodule is growing : outside is the cortex and 

 epidermis. The bacteroidal tissue is a meristem, which by 

 rapid division in all directions alike results in the pro- 

 duction of a more or less spherical nodule : frequently, 

 however, division goes on chiefly at the end of the tissue 

 farthest away from the root, in which case the nodule 



