SOME TYPES OP PLOWEELESS 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, i 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. 83 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 deal of 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, 



1 It 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 Eehling's solution, Both kinds are 

 destroyed during the process of fermentation, 



s For example, tartrate of ammopi^, 



