270 APPLIED BIOLOGY 



the cells of a chain become isolated, and each may form a chain of 

 new cells when food for growth is abundant. An abundance of long 

 chains indicates that the yeast has been growing rapidly ; compare 

 that taken directly from the compressed yeast with that which has 

 been in molasses-solution. 



The yeast cells are variable in size and shape. The con- 

 tents consist of protoplasm, cavities containing a clear 

 fluid, small droplets of oil, and a nucleus which is invisible 

 in ordinary living yeast cells. The appearance is different in 

 the " resting " cells, as in the yeast-cake, from that of the 

 actively growing cells. This is visible evidence that changes 

 occur in protoplasm when it is especially active. Of course, 

 slight changes are taking place constantly as long as the 

 yeast cells are living, but during the great activities attendant 

 upon growth the changes become distinctly visible. Many 

 such observations on living transparent cells of lower organ- 

 isms have helped biologists gain very important knowledge 

 of cell-life. 



Experiments with Yeast. (D or L) In the following experiments 

 the amount of growth may be roughly estimated by the turbidity or 

 cloudiness in the liquid. It may be determined accurately by 

 microscopic examination, especially by the number of buds to 

 which each cell has given rise. 



Experiment 1. Effect of food-supply upon growth and fermenta- 

 tion. Take seven test-tubes, or small bottles, and fill one-half full as 

 follows: (1) distilled water or rain-water ; (2) 10 per cent solution of 

 sugar in water (water 500 cc. or nearly 1 pint ; granulated sugar 50 

 grams) ; (3) 10 per cent sugar, and add to the tube a mass of commer- 

 cial beef-extract as large as a pea ; (4) molasses-solution (any dark 

 colored molasses, such as New Orleans, 1 part to water 10 parts) ; 

 (5) wheat flour in water to make very thin paste; (6) Pasteur's 

 solution ; * (7) Pasteur's solution without sugar. 



* Pasteur's solution is water containing sugar (C, H, O), ammonium 

 tartrate (N), potassium phosphate (K, P), calcium phosphate (Ca, P), and 

 magnesium sulphate (Mg, S). The symbols in parentheses show the ele- 

 ments (total nine) which the yeast plant gets from each compound used in 

 making the solution, and the amount of these used was originally determined 

 by chemical analysis of yeast. 



