Diastase can be extracted from "malted" barley (that is, barley kept moist 

 until the grains sprout), from rice, and from many other seeds. Malt is pro- 

 duced in quantities from sprouting seeds, and is used in making beer. A 

 substance similar to diastase is found in human saliva and in the digestive 

 juices of many other animals. The digestion of starch into sugar makes it 

 possible for carbohydrates to pass through cell walls by osmosis. 



Enzymes Substances like diastase and the active part of the saliva are 

 called ferments, or enzymes. Many different kinds are known. Like vita- 

 mins and hormones, enzymes induce chemical changes in other suhstanc-es 

 out of proportion to their amounts. These substances resemble what the 

 chemists call a "catalyst" — something that seems to induce or accelerate 

 chemical changes in other materials while remaining apparently un- 

 changed itself. 



Food Transportation Sugar formed in leaves during daylight diffuses 

 out of the pulp cells and moves down through the bast or phloem tubes. 

 When sugar is produced faster than it can be carried away, the excess is 

 converted into insoluble starch. Starch thus accumulates in the leaf during 

 the day. When darkness sets in, diastase converts starch into sugar, and this 

 is then carried down into the stem or roots (see illustration opposite). That 

 accounts for the fact that green leaves are full of starch in the late after- 

 noon, but have no starch at all before dawn. 



In the cells of potato tubers and of other organs that do not contain 

 chlorophyl, starch is formed from sugar by the action of an enzyme. This 

 process is just the reverse of digestion. The dissolved sugar in the leaves 

 passes at first from cell to cell by osmosis, then in the sap by way of the bast 

 tubes. In the root or tuber the sugar passes from the vessels to the pulp cells 

 by osmosis, and is then converted into starch. 



Digestion Universal The process of digestion seems to go on in nearly 

 all living things. The ameba, which consists of a mass of naked protoplasm, 

 swallows a solid particle into itself at any point and then digests the "food" 

 inside the cell. Among the bacteria, which are the smallest living things 

 known, each individual is a single cell consisting of protoplasm and cell 

 wall. These tiny plants can get food only in a liquid state; yet many of 

 them live on solid food that is not soluble in water. When meat or cheese 

 rots, it becomes fluid. The rotting in such cases is the work of the digestive 

 ferments secreted by the bacteria (see illustration, p. 166). When certain 

 bacteria get established in the nose, for example, or in the throat or the 

 appendix, the digestive action of their enzymes destroys living tissue, pro- 

 ducing inflammation and soreness. 



In higher animals like ourselves, a similar process of digestion takes place. 

 But not every cell pours out digestive juices into its immediate neighbor- 

 hood: only certain portions of the body produce and secrete such enzymes. 



164 



