MEDICAGO SATIVA 319 



the root hair at its distal extremity, although occasionally penetra- 

 tion may be effected through other epidermal cells. Within 

 the root hair, they form infection threads and these penetrate 

 to the inner layers of the cortex, but Thornton states that they 

 do not enter the endodermis. 



As a result of bacterial invasion, the cells become more densely 

 protoplasmic, their nuclei enlarge, and cell division ensues. The 

 actively dividing area involves two or three layers of cells on 

 either side of the infected cells, and this suggests that division 

 is induced by some substance diffused from the infection thread. 

 Cell divisions occur not only in the cortex but to some degree 

 in the endodermis and pericycle. The infection thread continues 

 to grow and penetrates the newly formed cells or it may form 

 zoogloeal masses from which the infection thread continues to 

 grow. There is a rapid multiplication of the bacteria in the 

 bacteroidal tissue and certain form changes occur in them so that 

 they increase in size and become banded. It is thought that the 

 banded rods represent a stage which is significant in nitrogen 

 fixation. 



When the infected cells cease to divide, they enlarge to approxi- 

 mately twice their original size, become vacuolated, and ultimately 

 form a large central vacuole. While cell enlargement is being 

 initiated in the central and basal regions of the nodule, a meristem 

 persists at its distal end which causes it to increase in length and 

 to assume a cylindrical form. Thornton (31) has measured nod- 

 ules at various ages and finds that those two weeks old aver- 

 age I mm. in diameter with a length of 1.3 mm. Increase in 

 diameter is rapid for the first two days, and the nodule reaches 

 a relatively constant diameter at the end of the first week, when 

 its length begins to exceed the breadth. The development 

 of vascular strands in the nodule is similar to that described by 

 Brenchley and Thornton (3) for Vicia Faba. (See Pisum, 

 Chapter XII.) 



Fred, Baldwin, and McCoy (10) have pointed out that the 

 nodule is 



"distinctly not a modified lateral root, for it has no central cylinder, 

 root cap, nor epidermis. Furthermore it does not digest its way out 

 from the cortex of the main root but remains covered with a consider- 

 able layer of cortical parenchyma. Anatomically then, it differs from 

 non-leguminous nodules, many of which are clearly modified roots." 



