AN INTRODUCTION TO PLANT BIOLOGY 105 



appears from the leaf at night is that it goes down into 

 the stem or root, or into the developing flowers or fruits. 

 That this is what occurs in part is proved by the fact that 

 starch and other foods (e.g., sugar, with the same elements) 

 are found in these parts of the plant, while all experiments 

 ever tried have failed to show that these substances can be 

 formed in cells without chlorophyll. The carbohydrate 

 foods needed in all living cells of a plant must come from the 

 cells with chlorophyll, most of which are in leaves. 



Microscopic study shows that starch is in the form of 

 grains inside of cells in leaves. 



Starch in Cells. (D) Thin sections of a potato will show the 

 starch-grains in the cells. Stain some sections with iodine solution 

 before mounting, or by drawing iodine under the cover-glass with 

 blotting-paper. Note the form of the starch-grains. 



102. Digestion of Starch to Sugar. How can such solid 

 grains get through the cell-walls in going from the leaves 

 and then into cells in other organs? The answer to this 

 question is that starch is easily changed or digested to sugar, 

 which is soluble in water and can osmose from cell to cell. 

 This change from starch to sugar is caused by a substance 

 known as diastase, which is present in cells of the leaves 

 and other organs of plants. This substance may digest 

 starch at all times during the day and night, but the starch is 

 formed during sunlight more rapidly than it can be digested. 

 At night when starch is not being formed the diastase succeeds 

 in digesting all the starch which was left in the leaf at the 

 close of daylight. Hence, starch-grains are not found in 

 leaves in the morning after being in darkness for several 

 hours; but sometimes it is possible by chemical analysis 

 to find in leaf cells sugar into which the starch has been 

 changed by enzymes, and which either will be used in the leaf 

 or will pass down the petiole into the stem. 



Sugar Test. (D) Boil a few grapes, raisins, or prunes in a test- 

 tube with a little water in order to extract some of their sugar. 



