BOT.-VOL. II.] PEIRCE-ROOT-TUBERCLES. 303 



or Other substances. In the one case we have a traumatropic 

 bending, in the latter a chemotropic. (See Spalding, 1894.) 

 This bending is entirely different in appearance and dis- 

 tinct in cause from that which carries the root-hairs closely 

 around particles of soil. This last is due to irritation by 

 contact (thigmotropism) and by water (hydrotropism). 



Since the majority of infected root-hairs show the bend- 

 ing at or near the tip, as shown in figs. 1-3, we may infer 

 that the bacteria enter uninjured hairs which are able by 

 growth curvatures to respond to mechanical or chemical 

 stimuh. If the hairs were broken, the ability to respond, 

 and the responses (curvatures) would be greatly lessened, 

 and instead of a short, close spiral (figs. 1-3), or a pro- 

 nounced bend, we should have little or no curvature. The 

 curvature of a broken hair is doubtful, and for mechanical 

 and physiological reasons certainly difficult to understand. 

 The roots of young Bur Clover grown in sandy soil in the 

 laboratory showed very few broken hairs when I dug up 

 the plants to search for infected hairs. The soil was very 

 friable, but even then I expected to find more hairs broken 

 as the result either of taking up the plants or of their growth 



in the soil. 



It would appear, then, that these bacteria are able to 

 soften or dissolve cell-wall, and when they come into con- 

 tact with a root-hair, enter it, whether it is broken or not. 

 The very slow movements, which are all that most observers 

 report having seen in these bacteria, their ability to soften 

 or dissolve cellulose, the small number of infected hairs, 

 and the small number of broken hairs, make Fischer's 

 graphic description (1897, pp. 91-2) of how the infection 

 of leguminous plants takes place at least doubtful though no 

 less graphic and interesting. He says: " Die feinen Wur- 

 zelharchen einer jungen, noch knoUchenfreien Legumino- 

 senpflanze schieben und drangen sich iiberall zwischen die 

 Bodentheilchen ein, um hier Wasser und mineralische Salze 

 aufzunehmen, ja sie scheiden sogar besondere Stoffe aus, 

 um die Erdteilchen, mit denen sie dicht verkleben, zu losen. 

 So wird schon die unverletzte Oberflache der Wurzeln 



