52 HOUSE, GARDEN, AND FIELD 



globe, but in one way or another they produce and set 

 free spores, which are the ordinary means of dispersal of 

 the moulds. The green threads that run through certain 

 kinds of cheese are moulds of a very common species, and 

 since they are thought to improve the flavour of the 

 cheese, these may be classed as useful moulds, but moulds 

 in general do far more harm than good to the owners of 

 the house. 



An undeniably useful mould, however, is yeast, which 

 has been used for thousands of years for making bread 

 and beer. Yeast may be said to be a mould reduced to 

 its lowest terms ; it has lost all definite arrangement, 

 sends out no branching fibres, and raises no columns into 

 the air, but consists merely of loose granules, which some- 

 times stick together in twos and threes. It is immersed 

 in its food, when the food is plentiful, and at other times 

 clings as fine dust to whatever objects it has happened to 

 alight on. In places where yeast has now and then a 

 good chance of feeding and multiplying, yeast-dust is 

 often widely disseminated. It is so in the house ; it is 

 so in the vineyard. Long before beer and bread were 

 thought of, the yeast-mould had adapted itself to one 

 particular mode of subsistence ; it had come to depend 

 upon the sugary juice of the ripe grape. When the sour 

 grapes appear, yeast-cells, dry and apparently dead, are 

 lodged by wind upon the grape-skin. During the whole 

 time of ripening they remain torpid ; they cannot pene- 

 trate the tough grape-skin, and as yet there is no sweet 

 juice in the grape, such as they require. But after long 

 waiting, the opportunity comes at last ; the grape ripens, 

 turns soft, and falls to the ground, bursting as it falls. A 

 quantity of sweet juice, enough to nourish millions of 

 yeast-cells, is suddenly put within their reach ; they drink 

 it in, and multiply with such speed that in a few hours 

 all the sugar of the grape is consumed and turned to 

 alcohol. A few hours more, and the grape is either 

 shrivelled by drought, or washed clean by rain. The 



