Substances Made from Foods 



59 



and this reaction is the basic one in the tanning of leather. With 

 iron salts, tannins produce black or green colors. Ink was 





Fig. 28. Structures and substances occurring in plant cells : A, cell from pulp of ripe tomato, 

 showing chromoplasts in which the red pigment arises ; B, vertical section of upper cells 

 of petal of yellow lupine — a yellow pigment forms in the chromoplasts ; C, cells from green 

 bark of grapevine, some of which contain needle crystals (r) and other crystal aggregates 

 of calcium oxalate ; D, cells from castor bean containing aleurone (protein) grains ; E, part 

 of a vertical section of leaf of rubber plant showing crystal aggregate of calcium carbonate ; 

 F, compound starch grains of oats ; G, sphaero-crystals of inulin in cells of dahlia roots after 

 storage in alcohol ; H, calciuni oxalate crystals in cells of spiderwort ; I, cells from seed of pea 

 containing starch grains and protein granules. {After Frank.) 



formerly made in this way. Many fruits are discolored when 

 cut with a steel knife, because of black compounds formed by 

 tannic acid and the iron in the knife. 



Enzymes. The enzymes make up another group of very im- 

 portant substances found in all living cells. Their composition 

 is unknown, and we know of their presence only through the 

 effects that they produce. They are usually soluble in water, 

 in dilute salt solutions, or in glycerin, and are insoluble in alcohol. 



