326 NATURE STUDY AND AGRICULTURE 



In the cells of the very young plant (embryo) within the 

 seed the protoplasts are in the quiescent condition, and 

 food is also stored in them. We will assume that this 

 stored food is in the form of starch, but it must be re- 

 membered that starch is only one form of stored food. In 

 this condition the seed awaits the suitable conditions of 

 temperature, moisture, and air which will stimulate its 

 protoplasts to activity again. When the proper combina- 

 tion of these conditions arrives, water passes into the seed 

 in relatively great abundance, and the seed may be ob- 

 served to swell. This means that the protoplasts are re- 

 gaining water and with it the structure for work. 



Then the starch food is attacked, and this involves the 

 very important process known as digestion. Starch is not 

 soluble, and therefore cannot pass through cell walls and 

 into protoplasts. It must be transformed into a soluble 

 substance, and this transformation of insoluble food into 

 soluble or diffusible food is digestion. Digestion puts food 

 into such form that it can be carried through the bodies of 

 plants and animals to the places where it is to be used. In 

 the case of starch the transformation is into a sugar, and 

 the active agent in causing this transformation is a peculiar 

 substance called an enzyme. Enzymes are manufactured 

 by protoplasts. There are many different kinds of enzymes 

 in living things, just as there are many different kinds of 

 substances in foods which are to be transformed in this 

 way. The usual enzyme which converts starch into sugar 

 in seeds is called diastase, and all the enzymes that have 

 been separated out so as to be studied have also received 

 names. 



When the digestion of starch has transformed it into 



