310 MICRO-ORGANISMS AND FERMENTATION. 



They sink from the ordinates of the lowest temperature towards 

 the axes of the abscissae, and then rise. At the same time, 

 however, these curves indicate that the cardinal points deter- 

 mined, more especially for the highest and lowest temperatures, 

 give characteristic distinctions for the different species i.e., 

 that the limits of temperature within which the formation 

 of spores can take place differ for the various species (see- 

 systematic description). 



In a course of years a number of investigators have carried 

 out similar researches, including Holm, Will, Aderhold, Kayser, 

 Seyffert, Marx, Schionning, and the author. 



The following observations were made regarding the time 

 required for the appearance of the first indications of spores 

 in the six species maintained at the same temperature. At 

 the highest temperature thirty hours were required for the- 

 development of each species ; at 25 there was again no great 

 difference in the time required ; at the lower temperature, 

 however, marked differences occurred. Thus, in the case of 

 S. cerevisice I., the first indications of spore-formation appear 

 at 11-5 C. after ten days, but in the case of S. Pastorianus II. t 

 they appear within seventy-seven hours. 



In all such determinations a considerable influence is 

 exerted by the state of the cells, and the results vary with the 

 temperature at which they have been grown, with their age, 

 vigour, etc. (compare section on Variation of yeast cells). It 

 follows that the composition of the nutrient fluid also exercises 

 an influence. Thus in methodical, comparative experiments of 

 this nature, it is a necessary condition that the previous culti- 

 vation of the cells should always be carried out in the same 

 manner. If these external conditions are varied, the limits for 

 the reactions of the species must be determined in each case. 



By these experiments Hansen has established an important 

 character for the determination of the Saccharomycetes. It 

 is also of great interest to note that the spore-formation has 

 a lower temperature maximum than budding, but a higher 

 temperature minimum ; in other words, spore-formation takes 

 place within a narrower range of temperature than budding. 



The method given below for the practical analysis of low 

 brewery yeast was based by Hansen on the temperature curves 



