SOME TYPES OF FLOWERLESS PLANTS. 229 
280. Substances which compose the Yeast Cell.—The cell wall is 
composed mostly of cellulose, the protoplasm consists largely of water 
together with considerable portions of a proteid substance,’ 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. 
281. Food of the Yeast Cell, Fermentation. — Yeast cannot grow 
much in pure water nor in pure solution of sugar. The diluted 
molasses ‘in which it was grown in Exp. 33 contained all the mineral 
substances mentioned in § 280, together with sugar, proteid materials, 
and water. The addition of a little nitrate of ammonium would prob- 
ably have aided the growth of the yeast in this experiment, by supplying 
more abundantly the elements out of which the yeast constructs its pro- 
teid cell-contents. A great dealof sugar disappears during the growth of 
the yeast.2 Most of the sugar destroyed is changed into carbonic acid gas 
(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 carbonic acid and alcohol by aid of yeast is 
called 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. 
282. 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 carbonic acid gas, nitrates, 
_17t may be found troublesome to apply tests to the yeast cell on the slide, under 
the cover-glass. Testing a yeast cake is not of much value, unless it may be assumed 
that compressed yeast contains little foreign matter and consists mostly of yeast 
cells. Still the test is worth making. Millon’s reagent does not work well, but the 
red or maroon color which constitutes a good test for proteids is readily obtained by 
mixing a teaspoonful of granulated sugar with enough strong sulphuric acid to 
barely moisten the sugar throughout, and then, as quickly as possible, mixing a bit 
of yeast cake with the acid and sugar. A comparative experiment may be made at 
the same time with some other familiar proteid substance, e.g., wheat germ meal. 
2 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. 
3 For example, tartrate of ammonia, 
