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



leguminous plants growing in the same soil and under the 

 same conditions. This possibility also deserves the test of 

 experiment. 



Returning now to the question of the motility of the tu- 

 bercle bacteria, we see that the experiments just described 

 indicate that fewer infections take place in nature than when 

 many bacteria are brought directly into contact with young 

 and sterile roots, but the experiments leave the matter of 

 the motility of the bacteria still undecided. The behavior 

 of the bacteria in artificial culture is inconclusive, and ap- 

 parently we cannot now imitate the conditions which prevail 

 in the soil. 



Figures 1-3 show root-hairs of Bur Clover plants infected 

 by tubercle bacteria. Figure 2 shows the lower and longer 

 of the two hairs in fig. i more highly magnified. Figure 3 

 is a hair on another plant drawn with the same magnification 

 as fig. 2 (X300). In these figures, as in those of Frank, 

 Dawson, and many others, it is noticeable that on the con- 

 cave side of the curved tip of the root-hair there is a small 

 mass of bacteria, this mass being continuous with the line 

 or strand of bacteria extending through the hair. The 

 wall of the hair seems intact and uninjured except where 

 the small mass of bacteria is. At this point there is no ap- 

 parent rupture of the wall. The wall may be actually per- 

 forated, though to see this with the mass and the strand of 

 bacteria in place would be very difficult or impossible even 

 in very thin sections. It is much more probable that the 

 wall is merely softened, the cellulose digested at the point 

 where the bacteria are — a soft place large enough in area 

 to permit the bacteria to enter either by actual locomotion 

 or by the formation and growth of new cells in this direction. 



The little group of bacteria on the surface and near the 

 tip of a root-hair is very often at the point of greatest curv- 

 ature of the hair. This curvature is due to the bacteria. 

 The bending is the evident response to irritation. The 

 irritation may consist in the softening and partial solution of 

 the cell-wall by enzyms formed by the bacteria — a mechan- 

 ical irritation — or in the stimulation of the cell by the same 



