TEAST 31 



f. Iodine test for detecting the presence of starch. 



1. Mix a little corn-starch or laundry starch in 



water in a watch-glass and add a drop of di- 

 lute iodine. "What is the effect on the starch ? 



2. Put on a slide a drop of water containing starch 



and examine under the microscope. "What is 

 the color of the starch grains? Eun a drop 

 of dilute iodine under the cover-glass. "What 

 change takes place in the grains ? 



3. Mount a drop of yeast on a slide and treat with 



a drop of dilute iodine as above. "What is the 

 effect on the cells ? Is starch present in the 

 fluid yeast ? Is there any starch in the cells 

 themselves ? 

 Make drawings of the yeast cells showing the effect 

 of the reagents. 



PHYSIOLOGY 



In the following experiments the amount of growth 

 which has taken place may roughly be measured by the 

 increase of turbidity of the liquid in the test-tubes. It 

 may be tested microscopically by the number of buds 

 to which the cells give rise, by the amount of proto- 

 plasm, and the number of vacuoles in each cell. To 

 Ibegin with, the test-tubes must be as clean as possible. 



a. Effect of food-supply upon growth.^Take five test- 

 tubes each one-third full of the solution named : 

 (1) distilled water ; (2) ten per cent, solution of 

 cane sugar in water ; (3) Pasteur's solution with- 

 out sugar; (4) Pasteur's solution with sugar; 

 (5) Mayer's pepsin solution. With a glass rod 

 put a drop of yeast into the fluid in each tube, 

 being careful not to lose part of the drop by 



