601 



inner colls of the root to prodnco the infections there. He therefore 

 calls them " infection threads." 



The first essential point in which his theory differs from that of Praz- 

 mowski is, thus, in regarding" the filaments as products of the root cells 

 instead of the bacteria. lie thinks that in some cases the infe(!tion 

 occurs without the development of the filaments. After the infection 

 the cells of the root are stimulated into growth to form the tubercle, as 

 already described, and bacteroids appear in the central cells. P''rank, 

 however, regards the bacteroids as peculiar formations of the plant tis- 

 sue and not as distinct organisms or degenerate bacteria. According 

 to him the presence of the bacteria produces abnormal changes in the 

 plasma of the root cells, causing it to l)ecome separated into numerous 

 irregular masses which contain the bacteria inside of them. These 

 masses are the bacteroids which fill the central cells. They are subse- 

 quently absorbed by the plant in the manner described by Prazmowski. 



The most recent work on the subject is that of Laurent.* In a series 

 of experiments performed at the Pasteur Institute in Paris he made 

 new observations as to the relation of the tubercles to organisms in the 

 soil. In his experiments he successfidly made use of water cultures, 

 and succeeded in obtaining tubercles in abundance by direct inocula- 

 tion. His method of inoculation was a new one. He stuck the end of 

 a needle into the tubercle of a leguminous plant and then pricked the 

 young root of the plant growing in his water culture with the needle. 

 This sort of inoculation was in all cases followed by the growth of a 

 tubercle at the point of inoculation. The effect was not wholly con- 

 fined to this point, however, but was commonly somewhat difiuse, a 

 fact easily explained by the diffusion of the organism through the 

 water. Laurent found that he could produce the tubercles on the root 

 of the pea plant by inoculating it from tubercles in any one of thirty- 

 six different species of leguminous plants, although not all species 

 would produce them in equal numbers. These and further observations 

 convinced him that there are a large number of varieties of the organ- 

 isms associated with the different leguminous plants, and that these 

 varieties may live side by side in the soil. They are not, however, pres- 

 ent in soil in groat abundance unless leguminous plants have been pre- 

 viously growing there. Laurent shows that the ordinary soil bacteria 

 have no power to produce tubercles. 



In these results the work of Laurent confirms that of Prazmowski 

 ami Frank; but in his study of the organism itself Laurent reaches a 

 different conclusion. In his studies of pure cultures of the tubercle 

 organism he finds that in gelatin the organisms spontaneously assume, 

 by a sort of budding, the irregular forms which have been called 

 bacteroids. The bacteroids are therefore not a degenerate form of the 

 bacteria, resulting from the deleterious action of the plant tissue, nor 

 are they aggregations of the root plasma, but they are normal forms of 



*An. «1. riust. r.isfcnr, 181>1, No. 2. 



