STUDIES OF SPORE-PLANTS 271 



Tubes 6 and 7 may be omitted if materials are not at hand. 

 Carefully label each tube with the numbers (on paper labels, or 

 with wax-pencil). Put one drop of washed yeast * into each, 

 shake the tubes thoroughly, tightly plug the mouth of each with a wad 

 of clean cotton-batting or absorbent cotton, and set them in a warm 

 place near a stove or radiator. 



Examine the tubes 1 to 7 after several hours and again on the next 

 day and judge from the turbidity, and if possible also from micro- 

 scopic examination, (1) in which fluids the yeast grows best, 

 and (2) in which fermentation takes place. In which are the most 

 bubbles of gas formed ? Does the formation of gas bear any relation 

 to the growth ? (Compare 2 and 4.) Keep careful notes, and at the 

 close of the experiments write an account of them, giving your 

 observations and conclusions regarding the substances necessary 

 for growth of yeast and for fermentation. 



Experiment 2. (D) Effect of heat on yeast. Three test-tubes 

 half full with molasses-solution. Put equal amounts of yeast into 

 each, and plug tubes with cotton. Label 1, 2, 3. Heat No. 1 to 

 the boiling temperature by holding tube in flame of gas-burner, 

 and leave Nos. 2 and 3 unheated. Keep tubes No. 1 and No. 2 to- 

 gether in a warm place (between 25 and 35 C.), and No. 3 on ice in 

 an ice-box. After a day or two examine first as to turbidity and 

 indications of fermentation and then open tubes and examine mi- 

 croscopically. Write conclusions as to effect of high and low 

 temperature on yeast. 



Experiment 3. (D) Gas evolved by yeast plant. (1) Take one 

 large test-tube, about by 5 inches, and pour in lime- or barium- 

 water about inch deep. Take a smaller tube, about I by 3 inches, 

 and tie a thread around its neck. Provide a cork for the larger tube. 

 Pour vinegar into smaller tube (\ full), get it ready to lower into 

 the larger tube. Then drop into the vinegar a small lump of baking 

 soda, and by the thread quickly lower the small tube, and stopper 

 the mouth of the larger one. Results ? Compare change of lime- 

 water with that produced by breathing through a straw or glass tube 

 into a bottle containing lime-water. 



* The washed yeast may be obtained by taking with a long pipette some 

 white sediment from bottom of bottle with molasses-solution in which yeast 

 has been growing for several days. Wash this sediment by placing on filter- 

 paper (preferably in a funnel) and pouring water through it to wash out the 

 soluble food (sugar, etc.). Then place the paper in a small bottle of water and 

 shake to dislodge the yeast cells. They settle to the bottom and may be 

 easily transferred by a pipette to tubes of solutions mentioned above. 



