TYPES OF CRYPTOGAMS; THALLOPHYTES 269 



some fat, and very minute portions of sulphur, phosphorus, potash, 

 magnesia, and lime. It is destitute of chlorophyll, as would be 

 inferred from its lack of green color, and contains no starch. 



324. Food of the Yeast-Cell ; Fermentation. The diluted molasses 

 in which the yeast was grown in Exp. XXXIX contained all the 

 mineral substances mentioned in Sect. 323, together with sugar, 

 proteid materials, and water. The addition of a little nitrate of 

 ammonium would probably have aided the growth of the yeast in 

 this experiment, by supplying more abundantly the elements out 

 of which the yeast constructs its proteid cell-contents. A great deal 

 of sugar disappears during the growth of the yeast. 1 Most of the 

 sugar destroyed is changed into carbon dioxide (which the student 

 saw rising through the liquid in bubbles) and alcohol, which can 

 be separated from the liquid by simple means. The process 

 of breaking up weak syrup into carbon dioxide and alcohol by 

 aid of yeast is one kind of fermentation; it is of great practical 

 importance, in bread-making and in the manufacture of alcohol. 

 Since grape juice, sweet cider, molasses and water, and similar 

 liquids, when merely exposed to the air soon begin to ferment and 

 are then found to contain growing yeast, it is concluded that dried 

 yeast-cells, in the form of dust, must be everywhere present in 

 ordinary air. 



325. Yeast a Plant; a Saprophyte. The yeast-cell is known 

 to be a plant, and not an animal, from the fact of its producing 

 a coating of cellulose around its protoplasmic contents and from 

 the fact that it can produce proteids out of substances from which 

 animals could not produce them. 2 



On the other hand, yeast cannot live wholly on carbon dioxide, 

 nitrates, water, and other mineral substances, as ordinary green 

 plants can. It gives off no oxygen, but only carbonic acid gas, and 

 is therefore to be classed with the saprophytes, like the Indian pipe, 

 among flowering plants (Sect. 180). 



sugar. A comparative experiment may be made at the same time with some 

 other familiar proteid substance, e.g., wheat-germ meal. 



1 The sugar contained in molasses is partly cane sugar and partly grape 

 sugar. Only the latter is detected by the addition of Fehling's solution. 

 Both kinds are destroyed during the process of fermentation. 



2 For example, tartrate of ammonia. 



