346 THE STRUCTURE OF ECONOMIC PLANTS 



Invasion of the root by the tubercle bacillus is effected through 

 the root hairs. The bacteria have very little, if any, motility. 

 In nature, they are probably disseminated mainly by the disinte- 

 gration of the nodules of the host plant, the liberation of the bac- 

 teria into the soil and their subsequent spread by such agencies as 

 wind, rain, soil organisms, or by tillage of the soil. When a root 



Fig. 176. Transection of stelar portion of nearly mature root : en, endodermis ; mx, meta- 

 xylem ; pel, pericycle ; ph, phloem ; ph fib, phloem fibers ; px, protoxylem ; xy x, secondary 

 xylem. 



hair does contact the organisms, they aggregate at its tip and pene- 

 trate it by dissolving the cell wall. Thornton and McCoy (16) 

 report that 



"there is evidence that ... a substance is secreted from the roots which 

 assists the entry of the bacteria. Infection of the root hairs is always 

 accompanied by a characteristic curling or deformation of the tip of 

 the hair, which is caused by a filterable secretion of the bacteria. There 

 is some evidence that the root hairs on seedlings are not curled until 

 after the opening of the first true leaves, suggesting that the root secre- 

 tion also plays a part in producing the curling. The composition 

 of the cell wall in the deformed region appears to be modified and it 

 is perhaps due to this that the organisms can enter at this spot. 



"The bacteria penetrate the tissues of the root cortex in the form 



