TYPES OF CRYPTOGAMS; THALLOPHYTES 238 
300. Food of the Yeast-Cell; Fermentation. — The diluted molasses 
in which the yeast was grown in Experiment XXI contained all 
the mineral substances mentioned in Sect. 299 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 grow- 
ing yeast, it is concluded that dried yeast-cells in the form of dust 
must be everywhere present in ordinary air. 
301. 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 coat- 
ing 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. 164). 
302. Multiplication of Yeast. — It is worth while to notice the fact 
that yeast is one of the few cryptogams which have for ages been 
largely cultivated for economic purposes. Very recently yeast pro- 
ducing has become a definite art, and the cakes of compressed yeast 
so commonly sold afford only one instance of the success that has 
been attained in this process. While yeast-cells are under favorable 
1 The sugar contained in molasses is partly cano 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. 
