vi.] External conditions of vegetation. 59 



even while vegetation and growth are going on in a portion of the 

 vegetative cells, and therefore while the conditions of vegetation 

 are still in operation. In other and especially in the endo- 

 sporous species it is true to say, that the formation of spores 

 begins when the substratum is exhausted, that is, has become 

 unsuitable for the vegetation of the species. Whether the latter 

 condition is really due in every case to a consumption of the 

 requisite nutrient substances or to an accumulation of checking 

 products of decomposition, or whether the formation of spores is 

 induced in this case as in others by internal causes when the 

 vegetation has reached a definite height, are all questions which 

 require more precise investigation, though they may perhaps be 

 of only subordinate practical importance. 



Vegetation proceeds with great rapidity in most Bacteria 

 under the co-operation of the most favourable conditions. 

 Brefeld determined in the case of Bacillus subtilis, that with a 

 good supply of food and oxygen, and a temperature of 30 C., a 

 rod divides once in every thirty minutes, which means that it 

 doubles its length every thirty minutes, the thickness remaining 

 the same, and then separates transversely into two equal parts. 

 The process goes on more slowly in proportion as the condi- 

 tions recede from the optimum. If we assume that the increase 

 directly observed in the way here described is accompanied 

 by a corresponding increase in the mass, especially of the 

 dry substance, an assumption which is not strictly proved 

 but from the indications before us is certainly approximatively 

 correct, then we have growth to double the former size in the 

 full sense of the expression once in every thirty minutes. 

 Similar results are arrived at from observations on many other 

 species, as Bacillus Anthracis, B. Megaterium, &c. But here, 

 too, there are exceptions. The Bacterium of kefir, for example, 

 in the cases which I examined, required more than three weeks 

 for growing to about twice its weight, more than 500 times the 

 period observed in Bacillus subtilis. I am not able to say 

 whether the conditions were absolutely the most favourable ; at 



