152 MICRO-ORGANISMS AND FERMENTATION. 



The lowest limit of temperature for the six species first 

 investigated was found to be 0'5 to 3 C., and the highest 

 limit 37'5C. HANSEN also determined the intermediate 

 temperature and time relations for these six species, and found 

 that when these two values are graphically represented, with 

 the degrees of temperature as abscissas and the time intervals 

 as ordinates, the curves obtained for all six species had 

 essentially the same form. They sink from the ordinates of 

 the lowest temperatures towards the axes of the abscissae, and 

 then rise from these ; at the same time, however, it is seen from 

 these curves that the cardinal points determined, more especially 

 from the highest and lowest temperatures, give characteristic 

 distinctions for the different species ; that is to say, that the 

 limits of temperature within which the formation of spores can 

 take place are different for the various species (compare classifica- 

 tion of the Saccharomycetes). 



With regard to the time required for the appearance of the 

 first indications of spore-formation in the six species investigated, 

 when maintained at the same temperature, the following was 

 observed : At the highest temperature the time required for 

 the development was for every species about 30 hours; at 25 

 there was also no great difference in the time required ; at the 

 lower temperatures, however, marked differences occurred. Thus, 

 in the case of Sacch. cerevisice I., the first indications of spore- 

 formation at 11 '5 C. are only found after ten days; but in 

 the case of Sacch. Pastorianus II. after 77 hours. 



In all determinations of this kind a very considerable in- 

 fluence is exerted l)y the conditions of the cells, the results vary 

 according to the temperature at which they have grown, their 

 age and vigour (compare the section in Chapter V. on the 

 Variation of Yeast-cells). It follows from this that the com- 

 position of the nutrient fluid also exercises an influence. In 

 methodical, comparative experiments of this nature, it is 

 necessary, therefore, that the previous cultivation 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 corresponding to such varied conditions must like- 

 wise be determined. 



By means of these experiments HANSEN has established an 



