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. XX XIX contained all the 
mineral substances mentioned in Sect. 328, 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.’ 
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. 
