CHAPTER SEVEN 



DIGESTION, TRANSFER, AND ACCUMULATION OF FOODS 



We have seen that starch is formed in the leaves of a plant 

 when it is exposed to light. We have also learned by ex- 

 periment that starch disappears from leaves at night, but 

 that if a leaf is removed from a plant it will still contain starch 

 the next day. Furthermore, in many plants like the potato, 

 turnip, or corn, we find starch in parts of the plant far re- 

 moved from the leaves. These facts indicate that the starch 

 is transferred from the leaves and is accumulated in the 

 stems, roots, or seeds. In the present chapter we shall learn 

 how this is done. 



Digestion of starch. Starch is insoluble in water. It 'does 

 not dissolve in the cell sap, and the starch within the cells is 

 not divided into particles small enough to pass through the 

 cell walls. Before it can be moved from one part of the plant 

 to another, or even from one cell to another, it must be changed 

 into some substance that is soluble. The process of changing 

 starch into a soluble substance has been carefully studied ; 

 and we know that starch is first converted into maltose and 

 that the maltose is further split into glucose (page 28). Glu- 

 cose is readily soluble in water and consequently can be passed 

 from cell to cell and so transferred to any part of the plant. 

 The changing of insoluble substances like starch into simpler 

 soluble substances like glucose is called digestion. Unlike 

 animals, plants have no special organs of digestion. All 

 their living cells are capable of digesting the insoluble sub- 

 stances that are required for their nutrition. 



Digestion brought about by enzymes. Digestion is brought 

 about by substances called enzymes. These are produced by 

 the living protoplasm of the cells. A large number of differ- 



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