300 CALIFORNIA ACADEMY OF SCIENCES. [PROC. 3D Ser. 



bacteria are not motile or move only slowly, or that it is 

 chance which governs the infection of the root-hairs. This 

 latter inference is encouraged by a statement of Miss Daw- 

 son's (1899, p. 21), that she saw " on a very small piece of 

 a lateral root from one of the plants no less than twenty- 

 seven hairs, side by side, with well grown infection tubes 

 within them. This observation may serve to show how 

 successful the attacks of this organism may be, provided 

 suitable conditions can be arrived at." 



To ascertain whether the number of infections in root- 

 hairs in nature is smaller than it migrht be under other 

 conditions, I tried the following experiment. Three layers 

 of filter paper, moistened with tap-water, were laid in each 

 of four small saucers and covered by tumblers. These 

 were sterilized in an Arnold steam sterilizer on three suc- 

 cessive days. A half dozen Bur Clover seeds were then 

 placed under each tumbler on the damp filter paper. The 

 seeds had just been removed from the little bur-like pods, 

 dipped in corrosive sublimate solution of one to one thou- 

 sand, and rinsed in distilled water which had been repeat- 

 edly boiled. The filter-papers were watered daily with 

 boiled distilled water. In a few days the seeds germinated. 

 When the roots had grown to an inch or so in length and 

 had developed many root-hairs, they were watered with 

 boiled distilled water in which had been ground healthy 

 growing tubercles. The water therefore held tubercle 

 bacteria in suspension. The next day nearly every hair 

 in the field on one side of a root was found to be enlarged 

 and twisted at the ends and showed the beginning of an 

 infection thread. Given the contact or close proximity of 

 the tubercle bacteria with the root-hairs, infections may 

 take place in great numbers simultaneously, at least when 

 the roots are very young. Whether the roots are always 

 susceptible, or whether older root-hairs or root-hairs on 

 older roots are susceptible, is another question. Any change 

 in the composition of the walls of the root-hairs may affect 

 their solubility or at least their permeability by the bacteria 

 (cutinization?). 



