332 ELEMENTS OF GENERAL SCIENCE 



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. 



335. Digestion of other substances. Diastase will digest 

 only the carbohydrates, but there are other substances which 

 have a similar action on proteins and fats. Substances which, 

 like diastase, cause chemical changes to take place in other 

 substances while remaining unchanged themselves are called 

 enzymes. There are a great many enzymes in plants, and 

 digestion and many other changes are due to them. 



336. Transference in the plant. As pointed out in the 

 preceding paragraph, dissolved food materials may pass from 

 cell to cell throughout the plant. When food reaches the 

 large pores or tubes in the stem, it may pass upward with 

 the general movement of water through the plant, as men- 

 tioned in the discussion of absorption and movement of water 

 (sect. 327). Passage in a downward direction occurs through 

 tubes in the inner bark. Immediately within the tissues 

 through which the manufactured food passes downward is 

 the layer of tubes through which water passes upward in 

 the stem. The veins of the leaves are composed of tissues 

 similar to those of living wood and bark, and liquid material 

 moves in these as it does in the wood and bark. The veins 

 are the supporting framework of the leaves. 



