522 



BIOLOGY OF THE MICRO-ORGANISMS. 



Inflmnce of 

 tempt rature. 



Influence of 

 fermentation 



Concurrence 

 with other 

 fungi. 



experiments which are much less open to objection, that 

 the beer yeast multiplies more readily when constantly 

 shaken than when kept at rest. The temperature has 

 an important influence on the development of the yeast. 

 The optimum appears to lie between 25 and 30 C. ; 

 but it depends to a certain extent on the composition of 

 the nutrient solution. Above the optimum the rapidity 

 of growth rapidly diminishes, and ultimately ceases at 

 about 53 C. ; below the optimum the diminution of the 

 growth occurs more slowly, and even slight vegetation 

 may take place a little above the freezing point. 



The fermentative activity of those yeast fungi which 

 are able to excite fermentation forms a special factor of 

 considerable importance. It is a matter of experience 

 that the development of the yeast cells runs parallel 

 with the energy of the fermentation; farther, yeast 

 appears to multiply much more rapidly when sugar is 

 present in the nutrient solution than when materials, 

 such as glycerine, which cannot be broken up by the 

 yeast, are present. On the other hand, the latter sub- 

 stance is quite as good a nutrient material as sugar in 

 the case of other species which cannot excite fermenta- 

 tion ; and hence we may draw the conclusion that the 

 fermentative activity itself is able to supply a certain 

 amount of energy to the yeast cells which can be utilised 

 for their vegetative life (Na'geli.) 



The simultaneous presence of other fungi, and the 

 concurrent growth with these, plays an important part 

 in the cultivation of the yeast fungi. The bacteria 

 more especially can, as the result of their more rapid 

 multiplication, very readily limit the development of the 

 yeast fungi ; here, however, the result is also dependent 

 on the sum total of the conditions of life which may be 

 more favourable for the yeast fungi, and may thus form 

 a substitute for the less energy of their growth. The 

 concentration and reaction of the nutrient medium is 

 here also of the greatest importance ; at times also the 

 temperature, higher degrees of which, for example, may 

 protect the yeast against many mould fungi, such as 

 penicillium. Fermentative activity also seems to exert 



