BACTEBIA OF THE SOIL 255 



The nodules of different legumes show some differences 

 in structure. In general, however, there is an epidermal 

 layer with a portion of the interior consisting of large cells 

 more or less compactly filled with bacteria. Scattered 

 among these cells there may be uninfected cells, in some 

 cases packed with starch granules. Fibrovascular bundles 

 leave the main root and pass from base to tip of the nodule 



Fig. 59. — Section op nodule of hoot op leguminous plant. Cells 

 containing bacteria shaded black. 



just underneath the epidermis. It is interesting to note 

 that in these bundles the arrangement of the conduction 

 tissues, that is, of the phloem and xylem, is the reverse to 

 that found in the normal root. The nodules in some species 

 of legumes grow rapidly to full size and then stop growing. 

 There is, in such nodules, no permanent meristematic 

 tissue. In others, a group of growing cells remain active 

 near the tip of the nodule. This meristematic mass con- 



