BY R. GREIG-SMITH. 289 



fortnight, it was found that the threads had given place to coils 

 or threads of felted bacteroids. The cells in growing had 

 apparently pushed through the wall of the thread, the slime of 

 which had possibly dissolved, but more probably had swollen and 

 kept the bacteroidal cells together. Cultures on solid media 

 ("d neutral") contained short swollen vacuolated cells and few 

 bacteroidal branching forms, a condition which was in sharp 

 contrast with the acid fluid cultures. 



From finding most "infection threads" in the flasks containing 

 limiting amounts of acid, one would infer that the micro-organism 

 protects itself from the action of the acid by forming a denser 

 slime-capsule until it becomes accustomed to the acidity. While 

 this tolerance is developing, growth proceeds actively and at the 

 same time the capsular thread becomes diffuse and the bacteroids 

 spread out in all directions. At the same time it must not be 

 forgotten that acid makes the slime less soluble, that is less 

 diffusible. But it does not matter much whether the acid affects 

 the bacterium in the first place or the slime; the formation of an 

 infection thread is the result of the acidity. There is one inter- 

 esting point, infection threads are rarely found in the nodules of 

 the Lupin, and this race was obtained from such a nodule. 



It would appear that there is a relation between the acidity, 

 the bacteroids and the number of dead forms in the nodule. 

 The acidity brings about the bacteroidal condition, but an increase 

 in the amount is enough to kill the bacteria. Since, however, 

 some of the bacteria are alive, it is probable that the acidity 

 varies in different parts of the nodule, and it is only in the less 

 acid cells or vessels that the living Rhizobia may be found. 

 There is, on the other hand, the possibility that the cells which 

 have remained alive have been better protected by their slime 

 capsules than those which have perished. 



I believe it is assumed that a race obtained from one species 

 of plant is typical of that species. To obtain a culture for infect- 

 ing sterile soils or for the preparation of so-called "Nitragin," 

 one need only isolate a bacterium from the nodules of, say, grow- 

 ing Blue Lupins, in order to be able to infect other Blue Lupins. 

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