HOW FOOD IS USED BY PLANTS 331 



the veins, along which it travels to the stem and so through- 

 out the plant. But what about other food materials ? Most 

 kinds of food material in a plant are not soluble. For instance, 

 if we soak a potato or a quantity of rice in water, the valuable 

 materials in it do not dissolve in the water, else we should 

 find it of more advantage to drink the water than to eat the 

 rice or -potato. The fact is that in most foods comparatively 

 little of the carbohydrates, fats, and proteins dissolve, no mat- 

 ter how long we soak the foods in water. In the examples 

 mentioned the most abundant material is starch, and this 

 is quite insoluble. Since food material is usually found in 

 plants in the form of starch and other insoluble compounds, 

 we must ask ourselves how it is that it can be transferred 

 to the various parts of the plants where it is needed. Cer- 

 tainly the solid grains, such as the grains of starch, cannot 

 travel far through the plant and remain unchanged. 



334. Digestion of starch. In discussing how insoluble 

 materials are moved through the plant, we shall take the 

 specific example of starch and remember that the process for 

 other insoluble foods is somewhat similar. 



If we examine with a microscope some starch grains from 

 a sprouting seed, where the food material is being carried 

 away to be used in forming now parts, we rind that the 

 grains of starch are not smoothly rounded, as they would be 

 at any other time, but rough, with many holes on the sur- 

 face, as if some of the starch had been dissolved from each 

 grain. Of course starch does not ordinarily dissolve in water, 

 but it is rather easily changed into sugar (Chapter VII), 

 and the sugar will dissolve. In plants there is a substance 

 (called diastase) which has the peculiar property of causing 

 starch slowly to change into sugar if the diastase is brought 

 in contact with the starch. This substance is in the plant 

 sap. It acts upon the starch at the surface of the grains, 

 and the rough, pitted appearance shows where the starch 

 has been so changed and dissolved. 



