36 Minnesota Plant Diseases. 



of lack of leaf-green, they are unable to manufacture starch for 

 themselves they are forced to depend on the products of other 

 plants or animals. Such elaborated food stuff is found in many 

 different forms both in animal and plant remains. Saprophytes 

 which are adapted to growth on special substances often re- 

 quire such materials both for development of the mycelium and 

 also for the germination of spores. The following are the more 

 common habits of saprophytes : 



The yeast habit. The yeast are fungi which grow luxuri- 

 antly in sugar solutions of one kind or another. In nature they 

 occur, for instance, on the ripe berries of grapes, especially 

 where a berry has broken open and the sugary juice exudes. 

 In this 'juice the yeast plant thrives. Again in the slimy fluxes 

 of tree trunks, yeasts often grow well. The yeast plant is 

 microscopic in size and propagates with great speed. This 

 speed is often facilitated by the fluid condition of the medium 

 in which the yeast is placed, because the new plants when bud- 

 ded off from the old can easily separate. This is not true of 

 yeast growing in solid starch paste. The yeast usually exerts 

 a peculiar effect upon the medium in which it lives. It exudes 

 at the surface of its cells a chemical substance known as a fer- 

 ment and this substance has the power of splitting up the 

 sugar into two substances, carbonic acid gas, which escapes as 

 tiny bubbles, and alcohol, which remains in the solution. The 

 escape of these bubbles is the well known effect which is pro- 

 duced in fermentation, though not all yeasts cause fermenta- 

 tion. Preserved fruits sometimes "work," gas bubbles arising 

 to the surface. Such may be caused by yeast plants which were 

 allowed to get into the preserves before sealing. Two great 

 industries are founded upon this fermenting power of yeasts. 

 The raising of dough in bread-making is caused by the produc- 

 tion of gas bubbles in the action of growing yeast plants upon 

 sugary solutions, and thus bread-making is dependent upon 

 this process. The second is the process of brewing. The abil- 

 ity of yeast to break up sugars into alcohol and carbonic acid 

 gas is again utilized, but the alcohol is here the chief object of 

 the employment of the yeast. 



Water-mold habit. .Almost all water molds and fish molds 

 live in a submerged condition. Manv of the fish molds are 



