THE FOOD OF PLANTS 



103 



[Chap. XH 



leaves, tubers, and roots already contain substances from which new 

 protoplasm and new cell walls can be made, and from which chemical 

 energy can be derived. What are these substances? 



Foods in cells. Thin sections of seeds or tubers observed through a 

 microscope are seen to be composed of numerous cell walls surrounding 

 rather dense masses of granules and oil droplets ( Fig. 51 ) . Similar 



C D 



Fig. 51. Illustrations of various accumulated foods in plant cells: A, protein and 

 starch in cells of a wheat grain; B, oil droplets in a cell from a coconut seed; 

 C, small starch grains in chloroplasts; D, crystals of inulin in cells of salsify root; 

 E, starch grains and cubical crystals of protein in cells of a potato tuber. 



granules and oil droplets may be found in most living plant cells, but 

 usually in much smaller quantities. The oil droplets may be distinguished 

 by their appearance or by their bright red color in the presence of dilute 

 solutions of certain dyes, such as Sudan III. The presence of fat-like sub- 

 stances in plant parts may also be detected by crushing a small piece of 

 the tissue on paper and warming it. The fat-like substance melts and 

 produces a translucent oil spot on the paper. 



The remaining granules in the cells may be starch grains, protein 

 granules, or granules and crystals composed of certain organic com- 

 pounds combined with inorganic elements such as calcium and phos- 

 phorus. Organic substances when combined with phosphorus are fre- 

 quently referred to as phosphatides. Starch grains may be detected b\ 

 applying a drop of a dilute solution of iodine and potassium iodide to a 

 section of the plant. When treated with this reagent, starch grains may 

 be recognized by their various shades of blue and purple; the other 

 substances in the cell remain colorless or are stained light brown. 



