SYMBIOTIC NITROGEN FIXATION 



115 



and the organisms appear to become attracted to these roots by 

 some substance which is excreted by the young roots. The infect- 

 ing organisms are probably coccoid cells, either non-motile (pre- 

 swarmers) or motile (swarmers). Once within the tissues the 

 bacteria multiply rapidly at the expense of the food material of 

 the plant. They occur at this stage as short rods enclosed in a 

 mass of mucoid material. The mass of bacterial growth continues 





^ ^-_V 



Fig. 53. — Infection of root cells of a legume by the legume bacterium: (1) cells 

 from the young nodule tissue showing small rod-shaped cells in the infecting 

 strands; (2) cells from older nodule tissue; many bacteria have separated from 

 the infecting strand and are still existing as thin rods; (3) cells from quite old 

 parts of the nodule tissue; the cells have become filled with bacteria in the 

 swollen "bacteroid" condition (from Brenchley and Thornton). 



to pass through cells of the cortex of the root in the form of a fila- 

 ment with many branches, called the infection thread or strand. 

 The penetration of the microorganisms results in growth and mul- 

 tipHcation of the inner cortical cells, which push outward to form 

 the young nodule. These cells, in turn, become invaded by the 

 infection threads (see 1, Fig. 53). As the nodule develops, vas- 

 cular tissue which conducts nutrient materials in the plant grows 



