298 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



favorably, I, too, failed to get satisfactory results from 

 microtome material, butowing to the success of my own 

 method I did not try hers very long. 



II. Origin and Morphology of Root-tubercles. 



Although most that I can say about the entrance of the 

 tubercle bacteria into the roots of leguminous plants has 

 already been reported by others, I wish to describe the 

 infection in Bur Clover {Medicago denticulata Willd.) from 

 the beginning, and to discuss some of the stages in the 

 process. I must also frankly admit that I do not know all 

 that has been written on the subject, for the literature is 

 copious and scattered, and I have been able to see only the 

 papers herein referred to. I therefore bespeak lenient 

 criticism of my acquaintance with the literature, remote- 

 ness from the centres of scientific and other work making 

 it very difficult to secure the papers, and even references 

 to the papers, of my subject. 



How the tubercle bacteria in the soil come into contact 

 with the root-hairs of the leguminous plants which they 

 attack is not known. The majority of authors consider 

 these bacteria {^Bacillus radicicola Beyerinck, Rhizobium 

 leguniinosa?'U7n Frank) only slowly motile if motile at 

 all. In artificial cultures they are usually quite motion- 

 less (Migula, 1900, p. 772). Zinsser (1897, p. 447) says 

 they are small and actively motile. Miss Dawson (1900, p. 

 59) reports that in drop-cultures, a week or more old, the 

 chains become motile, the shorter moving more rapidly than 

 the longer, but none actively, and the motion resembles the 

 pendulum movement of Oscillatoria filaments. In younger 

 drop-cultures, containing 2.5 per cent, gelatine, I have seen 

 this movement. The movement of the chains formed in 

 artificial cultures may be only a feeble index of a much 

 more active movement of the separate bacilli when these 

 occur in natural conditions. Artificial cultures are unsatis- 

 factory at best, and it may very well be that the tubercle 

 bacilli are actively motile, for a time at least, in damp soil. 



