36 PLANT BIOLOGY 



plant to another, as well as the processes by which the blood 

 of animals obtains and gives off food to various cells of the 

 body. 



By the preceding experiments we have proved that there are 

 two classes of food substances. One kind (including water and 

 grape sugar) will readily pass through a membrane by osmosis ; 

 the other kind (represented by starch) will not. In our study 

 of cells we learned that the protoplasm or living substance is 

 inclosed by a cell-wall which separates one cell from another. 

 Now if cells are to make use of the food materials manufac- 

 tured in other parts of the plant, each food substance must be 

 in such a form that it can pass through these cell-membranes. 

 It is evident that water and grape sugar can do this. We 

 find, however, large quantities of starch stored in cells 

 (Fig. 9). Hence, to be available for use in other cells, some 

 change must be made in this food substance before it 

 can be transferred from cell to cell. We shall now show by 

 experiment what this change is. 



49. How starch is made ready to pass through a membrane. 



Laboratory Study No. 26. 



Into each of two test tubes put a small amount of starch 

 (arrowroot starch if it can be obtained), add some water, 

 shake, and boil. To the starch mixture in one test tube add 

 some diastase, equal in amount to one-half the size of a pea. 

 (Diastase is a chemical substance produced or secreted by 

 the protoplasm of plant cells.) Put the two test tubes side 

 by side in a warm place for 5 minutes if arrowroot starch, 

 24 hours if corn starch is used, then test a small amount of 

 the mixture in each test tube by adding a few drops of iodine. 



1. Describe in your own words what has been done. 



2. In which test tube do you find starch present ? 



3. Now test with Fehling's solution a small quantity of 



each mixture. In which tube do you find grape sugar ? 



