THE NATURE OF SEEDS 351 



much oil is changed to sugar. This is doubtless due to 

 the action of an enzyme, but no one has as yet been able 

 to isolate the enzyme which does this. If any one did so, 

 he would probably have in this enzyme the basis for an 

 enormously useful digestive medicine, for the transforma- 

 tion of fats is one of the things which very frequently 

 causes trouble in human digestion. 



The first step in the germination of seeds appears to be 

 the absorption of water. This water causes the cells to 

 enlarge. The seed swells. The next step is the digestion 

 of food ; that is to say, it is the action of an enzyme upon 

 the food stored in the seed with the result that this food 

 is made soluble and moves into the growing parts of the 

 embryo. 



This process may be readily studied in germinating 

 grains, as in wheat. The aleurone layer (see Figure 154) 

 first shows signs of life. The protoplasm in it produces an 

 enzyme called diastase, which is the kind of enzyme most 

 commonly found in seeds. It is one which has the power 

 to transform starch to sugar. It is the starch next to the 

 aleurone layer which, in wheat, is the first to be changed. 

 Digestion after that proceeds rapidly, and liquid food is 

 supplied to the seedling as fast as needed. 



d. Assimilation. In germinating seeds the dissolved 

 foods pass by osmosis from cell to cell until they reach those 

 cells which are growing. Here the protoplasm is very 

 active, and here the foods must be built up into new pro- 

 toplasm. It is this transformation of foods by protoplasm 

 into new protoplasm, or into other substances, which is 

 called assimilation. Assimilation involves numerous and 

 intricate chemical changes very few of which are under- 

 stood. One of the simplest of them is the transformation 



