248 ELEMENTS OF GENERAL SCIENCE 



various parts of the plants where it is needed. Certainly the 

 solid grains, such as the grains of starch, cannot travel far 

 through the plant unchanged. 



269. Digestion of starch. In discussing how insoluble ma- 

 terials 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 new parts, we find that the grains 

 of starch are not smoothly rounded, as they would be at any 

 other time, but rough, with many holes on the surface, 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 to slowly 

 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 appear- 

 ance shows where the starch has been so changed and dissolved. 



It is not difficult to secure a solution containing diastase 

 by crushing in water some sprouting seeds, as those of barley. 

 If this solution is mixed with starch in a test tube, it will 

 change starch into sugar as it does in the plant. The use of 

 diastase to digest starch is a common trade process. Brewer's 

 malt is simply slightly sprouted barley which has afterwards 

 been killed by heat and drying. The diastase in it digests 

 the starch, forming sugar, which may afterwards be fermented 

 to form the alcohol which is present in beer. 



The process, such as we have just been describing, of 

 changing an insoluble substance into a soluble one is called 

 digestion. It is possible to transfer insoluble foods from place 

 to place through the plant only after they have been made 

 soluble by digestion. 



