CONDITIONS OF LIFE OF THE BUDDING FUNGI. 523 



a special influence on the concurrent growth of the 

 yeast. For example, if a very minute quantity of yeast 

 is introduced into a neutral saccharine nutrient solution, 

 and if no care is taken to exclude bacteria, the latter 

 generally multiply readily, and a very impure cultivation 

 of yeast is obtained, or one in which the bacteria are 

 in greatest numbers. If, however, we increase the 

 amount of yeast introduced up to a certain point 1*7 

 grammes of dry yeast, or 10 ccm. of thick yeast, for 

 every litre of nutrient solution the yeast alone multi- . 

 plies, and the bacteria scarcely develop at all. It can be 

 shown that this phenomenon does not depend on the 

 excretion of materials hurtful to the bacteria by the 

 yeast, and we must therefore suppose that it is the 

 fermentative movement which hinders the multiplication 

 of the bacteria. In accordance with this we have the 

 observation that we are the more likely to obtain a 

 relatively pure cultivation of yeast the quicker and more 

 completely the fermentation occurs after the organisms 

 are introduced, and that it is thus independent of the 

 number of the fission fungi which have obtained 

 admission at the same time. Thus we have the ex- 

 planation why the ordinary beer yeast is usually fairly 

 (but never quite) free from bacteria, and a why, when the 

 brewing process is properly carried out, we need not fear 

 disturbance by the development of bacteria (Nageli).* 



4. Conditions of Spore Formation and Spore Germina- Conditions of 

 tion. In contrast to the higher plants, and also to the ( 



mould fungi, the yeast and fission fungi are distinguished 

 by the possession of a very great tendency to utilise the 

 nutrient materials for the continuous production of 

 purely vegetative cells without the formation of any true 

 fructification. In suitable nutrient media the yeast fungi 

 constantly form new cells by budding, and the bacteria 

 divide continuously like an enormously developed tree 

 without fruit. It is only when the nutrient conditions 

 become markedly unfavourable, and when one of the 

 most important nutrient substances begins to disappear, 

 that there is an interruption in the ordinary mode of 



* Nageli, Theorie der Gdhrung, 1879, p. 77. 



