3IO CALIFORNIA ACADEMY OF SCIENCES. [PROC. 3D Skr. 



to form lateral roots which, for some unknown reason, 

 aborted while still in the cortex of the mother root. Lat- 

 eral roots are known to do this, and if the abortion took 

 place early enough, the root character of the new formation 

 might be lost if it had already developed. It seems to me 

 much more probable, however, that both causes were in 

 operation; that the soil did not remain sterile; that the 

 plants were infected by so few or so feeble tubercle-bacteria 

 that the tubercles stimulated to begin to form aborted because 

 the infection was not strong enough. If the leguminous 

 plant, or its separate cells, and the bacteria are parasitically 

 associated, the plant would resist the entrance and growth 

 of the bacteria, and would be much more likely to succeed 

 in this if the attacking bacteria were few or feeble. Over- 

 coming the bacteria, the stimulus to tubercle formation 

 ceases, the tubercle remains rudimentary. That infection 

 of sterilized soil by the tubercle-bacteria is possible, and 

 even difficult to avoid, is known to all who have worked on 

 the subject. This, then, rather than inheritance, accounts 

 for the rudimentary tubercles which Frank describes. 



The bacteria in the infection thread, which grows through 

 the root-hair and the cortical parenchyma cells of the root 

 to the pericambium layer, multiply, but they multiply most 

 rapidly in the infected cells farthest from the surface of the 

 root. New threads form, which grow out into and infect 

 the cells of the mass of new cells composing the embryo- 

 tubercle. Thus a majority of the cells in the young tubercle 

 contain bacteria. 



Though infected cells do divide (see pp. 322-323), they 

 probably divide less often than the uninfected cells. The 

 primary infection is in a nearly straight line from the root- 

 hair inwards. The infection of the daughter-cells com- 

 posing the embryo-tubercle is accomplished by branching 

 infection threads growing in fairly straight lines radiating 

 from the base of the tubercle. In this way the cells near 

 the base of the growing tubercle are most infected, those 

 near the tip least. It may be in consequence of this that 

 the cells at and near the tip of the tubercle retain their 



