BIOLOGICAL RELATIONSHIPS OF YEAST. 273 



as the true winter habitat of the fungus. It preserves its 

 usual appearance throughout the long winter time, and is 

 then carried up into the air by the combined agency of insects 

 and of wind, and by these means of transport it is distributed 

 from fruit to fruit. 



It is obvious that during the period when a large number 

 occur on ripe fruit the currents of air may carry the fungus 

 to other places, and also on to unripe fruit. Hansen stated 

 in his first memoir that the rare occurrence on unripe fruit 

 must be due to the fact that the organism quickly dies off, 

 partly through want of nourishment, and partly through the 

 drying up of the cells. He subsequently proved by experiment 

 the correctness of this view. He distributed both old and 

 young cells in water, and placed them either in a thin layer 

 on an object glass or on a tuft of thinly spread cotton- wool ; 

 thus allowing evaporation to go on while the cells were pro- 

 tected from the sun. In less than twenty-four hours the 

 whole of the cells were killed. It is quite obvious that the 

 individual cells spread over the surface of unripe fruit are ex- 

 posed to more unfavourable conditions than in his experiments. 

 If, however, thicker layers of the cells are covered by cotton- 

 wool or filter paper, they remain living just as they do in the 

 soil for a long time. Thus they live for more than eight 

 months in filter paper. 



It was then possible for Hansen to demonstrate that the 

 greater number of yeast species must pass through a similar 

 cycle in nature. Their most important breeding places are 

 the sweet juicy fruits. Their winter habitat is the soil, and 

 they are carried by wind, rain, insects, and other creatures on 

 to the fruit. They then multiply once more on sweet fruit, 

 and obviously more particularly where the juice oozes out 

 from the fruit. Hansen further found that these yeast species 

 often occur in the ground at places far removed from orchards, 

 where 8. apiculatus can no longer be found. 



Miiller-Thurgau arrived at the same results as Hansen with 

 regard to 8. apiculatus during an examination of the wine 

 species. He found that grapes are their chief breeding places, 

 and that their presence may be distinguished in the soil 

 throughout the year. On the other hand, they seldom occur 



18 



