138 PLANT PHYSIOLOGY 



carry some of the liquid over into the tubes. This, however, can be pre- 

 vented by a plug of glass wool in the neck of the flask, or even, it is said, by 

 a piece of paraffin in the liquid. The temperature of greatest activity of 

 the process is about 28 . 



Fermentable Solutions. Best of all these for yeast fermentation is 

 Pasteur's, made as follows: 838 cc. of pure water, 150 g. pure grape- 

 sugar, 10 g. ammonium tartrate, 2 g. potassium phosphate, .2 g. calcium 

 phosphate, .2 g. magnesium sulphate. For greater practical convenience in 

 use, the mineral constituents may be powdered together (in a mortar) and 

 kept in a tightly stoppered bottle. An optimum quantity of compressed 

 yeast for the fermentation is 1 g. to about 15 cc. of solution. Instead of 

 the grape-sugar one may use cane-sugar which, while not itself fermentable, 

 is readily inverted to fermentable glucose and dextrose by an enzyme in the 

 yeast cake; the cane-sugar is considerably slower in fermentation than 

 the grape-sugar. Another excellent solution is molasses (which contains 

 about 35% of cane-sugar and about 32% of grape-sugar, with 18% of water 

 and some miscellaneous substances) diluted to about 20%. No solution 

 of over about 30% sugar should be used, since yeast is inhibited, apparently 

 by osmotic pressure, from development in solutions of higher strength. 



While the above solutions are best for studies in which the fermentation 

 is to be prolonged, they are by no means necessary for demonstration pur- 

 poses, since a solution of either grape- or cane-sugar to which yeast has been 

 added will ferment actively for an hour or two without the mineral con- 

 stituents. 



While Fleischmann's compressed yeast cakes are the most convenient 

 source of yeast, starting fermentation as they do almost immediately after 

 addition to the solution, other forms of the yeast are available, including 

 the cakes of dried yeast which, however, is much slower in beginning action. 

 Demonstration. Yeast fermentation can very readily be demonstrated 

 to a class, and even as a lecture experiment. For this I have found the 

 arrangement figured herewith (Fig. 37) very effective. The solution hav- 

 ing been placed in the flask and the yeast added, the outlet tube, of the form 

 shown in the figure, is dropped to near the bottom of a slender cylindrical 

 tube containing clear lime-water, the whitening of which as the gas comes 

 off (commencing almost immediately) is very effective. Then on another 

 occasion, after the fermentation is complete, the liquid in the flask is dis- 

 tilled, which can best be done by gently heating the flask with the Bunsen 

 burner to about 8o° (alcohol boils at 78°), and sending the vapor through 

 a glass still, the distillate being collected in the test-tube beneath. After 

 the collection of some of the distillate, the iodoform test for alcohol may 

 be applied. A convenient arrangement of the apparatus, which may all 

 be supported permanently on one support-stand, is shown by the accom- 

 panying figure (Fig. 37). 



For elementary or make-shift purposes the demonstration of the alcohol 

 is too difficult, though an application of the iodoform test to the fermented 

 solution yields a faint iodoform odor; but the proof that carbon dioxide 

 is evolved is very easy along the line of the experiment above recommended. 



