Ch.XJ 



THE YEASTS 



459 



phytically in the sweet juices of plants, especially such as 

 ooze from over-ripe fruits. These they absorb into the cell, 

 and use the sugar as material for both growth and anaerobic 

 respiration, the incidental products being carbon dioxide, 

 alcohol, and some minor substances (page 170). They repro- 

 duce asexually by the budding of new cells from the old 

 ones, new and old remaining attached for a time in irregular, 

 fragile chains. When the food supply approaches exhaus- 

 tion the contents of some cells segregate into four spores 

 which develop thick walls 

 and go into the resting 

 condition. In this state 

 they float with the dust 

 in the air, and reach other 

 fruits. Thus the wine 

 yeasts are carried into 

 the vats where grapes are 

 crushed, and produce the 

 fermentation which un- 

 derlies the making of 

 wine. It happens that 

 Yeasts are easily suscep- 

 tible to cultivation by 

 man, who uses them for various purposes, including the 

 making of beer and the production of alcohol used in the 

 arts ; and some .of the kinds thus propagated are not now 

 known in the wild state. 



At first sight the Yeasts seem little related to Ascomy- 

 cetes, but good reason exists for the view that the cell which 

 forms four spores is an ascus, even though its structure is 

 so simplified. In a few kinds its formation is preceded by a 

 conjugation of two cells, while in some allied forms a simple 

 mycelium develops. 



Fig. 321. — The bread Yeast, Saccharo- 

 myces cerevisiw. 



Left, the ordinary vegetative state, in 

 process of budding; X 625. Right, the 

 formation of spores ; X 825. 



Order 7. Lichenes: the Lichens. These are among 

 the most prominent of all Fungi, for they comprise the 



