Ch. Ill, 13] USES OF THE PLANT'S FOOD 



103 



lucency where sugar is the food, as, for example, in Beets. 

 Being insoluble in water and therefore not removable in that 

 form from storage cells, starch must be digested before use, 

 in which process it is converted by the action of enzymes 

 back into grape sugar, the change being marked, as familiar 

 in germinating seeds and growing potatoes, by a transition 

 from the dull white to a soft translucent appearance. 



Starch, stored by plants for their own uses, forms likewise 

 the best of food for animals, which take what they need, and 

 like plants digest it by enzymes back 

 to grape sugar, in which form it is 

 transferred for use to all parts of their 

 bodies. It is the principal constituent 

 of the ordinary foods of all herbivo- 

 rous and graminivorous animals. As 

 for man, starch is by far the most im- 

 portant of all the food substances 

 taken by him from plants. This is 

 sufficiently plain when we recall that 

 all of the grains, which constitute the 

 principal food of the human race, — 

 Wheat, Corn, Rice, Barley, Millet, and 

 others, — consist chiefly of starch. 



The Hemi-celluloses are much 

 less prominent than the sugars and starches. They are 

 modified forms of cellulose, having the same chemical for- 

 mula, but with the n indicating a different number. They 

 occur as extra layers of the cellulose walls (Fig. 64), espe- 

 cially in some tropical seeds, which thereby are made heavy 

 and hard, as well illustrated in the Date seed, or still better 

 the Ivory Nut, — a large seed of a Palm, hard enough to 

 serve as imitation of ivory. The hemi-celluloses are easily 

 digested by plants but only in part by animals. They merge 

 over gradually to the pectins, or fruit jellies (the ordinary 

 gelatin being an animal product), which are dissolved out by 

 hot water in making preserves, and these again merge over 



Fig. 64. — Thickened 

 cell walls (striated) in 

 the Ivory Nut. The 

 protoplasm (dotted) ex- 

 tends into pits persistent 

 in the walls. 



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