STUDIES OF SPORE-PLANTS 275 



The usual method of reproduction is by budding, but 

 under certain conditions a yeast cell may form spores within 

 itself, and later each of these spores may grow into an ordinary 

 yeast cell capable of budding. 



As to relation to temperature, the yeast plants grow best 

 at about 20 to 30 C. They are quickly killed at the boil- 

 ing temperature, and hence it is quite easy to bottle sweet 

 (unfermented) cider or grape juice. There is apparently 

 little growth at the temperature of ice-boxes, and hence 

 fermentable foods may be preserved at low temperatures; 

 but only temporarily, for other organisms may develop, 

 except at the freezing temperature (which is colder than in 

 common ice-boxes). 



Yeast plants can withstand partial drying. This explains 

 their living in the cakes of dry yeast ; but after some months 

 all the cells are dead in such cakes. It also explains how the 

 wild yeasts can live in the dust on fruits, or on the surface 

 of the soil in orchards and vineyards, ready to begin growth 

 as soon as the bruised or crushed fruit allows the yeast plants 

 access to the juices. In the dry condition, the yeast cells 

 float with dust in the air, and this explains why sterile fruit- 

 juices (e.g., boiled cider) or sugary solutions will soon ferment 

 after exposure to ordinary air. 



253. Economic Relations of Yeasts. It is only neces- 

 sary to allude briefly to the vast industries of manufacturing 

 bread and alcoholic beverages which are dependent upon pro- 

 duction of alcohol and carbon dioxide by yeast plants. So 

 great is the importance of pure yeasts in these two industries 

 that enormous factories, costing immense sums of money, are 

 devoted to raising yeasts. 



Apart from alcohol in beverages and in medicine, it has 

 long been invaluable in varnishes and numerous other com- 

 pounds used in mechanic arts. Recently it has been coming 

 into prominence as a cheap source of energy for engines, 

 burners, lamps, etc. It is possible to make alcohol cheaply 



