524 BIOLOGY OF THE MICRO-ORGANISMS. 



multiplication; the fungus then converts the remains of 

 the nutrient material at its disposal into a more durable 

 cell, which can survive after the exhaustion of the 

 nutrient material, and can grow again in new nutrient 

 soil even after a long interval. 



In the case of the yeast, the conditions necessary for 

 the formation of spores are furnished when the nutrient 

 substratum is very poor in nutritive materials, more 

 especially in sugar, or is very dilute, while at the same 

 time (as in the case of the mould fungi) free access of 

 the oxygen of the air is permitted, and evaporation is 

 limited. If yeast is spread out on thin slices of boiled 

 carrot, or on moistened plaster, and then kept in a moist 

 chamber, the formation of spores in the cells, as described 

 on page 147, occurs in the course of a few days; the 

 same result is obtained if a solution of sugar containing 

 yeast is more and more diluted by the daily addition of 

 water. According to Hansen,* spore formation is readily 

 observed on microscopic slides covered with a layer of 

 nutrient jelly, if the temperature is not too high; in 

 like manner yeast water containing air is also a favour- 

 able nutrient medium. The minimum of temperature 

 at which spore formation has been observed is between 

 + 1 and 3 C., the maximum at 37^ C. The spores 

 can be kept for a long time, and can be dried without 

 losing their power of sprouting. 



Germination The conditions of the germination of these spores are 

 similar to those mentioned in the case of the spores of 

 the mould fungi. The presence of nutrient materials is 

 not necessary for the commencement of germination ; on 

 the other hand, moisture and free oxygen are absolutely 

 essential, so that in respect of the latter there is a 

 marked difference between the vegetation and the fruc- 

 tification of yeast. Further, the temperature has an 

 important influence, but this has not as yet been quanti- 

 tatively determined, t 



* I. c., vol. ii., part 2. 



f Russ. Annalen der Oenologie, vol. ii. Botan. Zeit., 1873. 



