CHAPTER EIGHT 



SUBSTANCES MADE FROM FOODS 



All plants contain a variety of substances made from foods 

 that cannot properly be classed as foods. Some of them are of 

 great importance in plant processes ; others form the constituents 

 of cell structures. Some may be changed again into foods, and 

 others seem to be waste, or by-products, of cell activities. The 

 most important of these substances will be briefly described in 

 this chapter. 



Colorless plant tissues. One occasionally finds on plants 

 leaves that are wholly, or partly, white. This is simply the 

 natural color of living plant tissues that lack chlorophyll or other 

 pigments. The protoplasm, cell sap, and cell walls are trans- 

 parent and colorless. The presence of air spaces among the cells 

 makes these tissues appear white, just as ice is white when it is 

 filled with minute air bubbles. White leaves and flowers merely 

 show the natural appearance of plant tissues in the absence of 

 chlorophyll and other pigments. 



The pigments in green leaves. We can best approach the 

 matter of plant colors by inquiring into the composition of the 

 pigments that color the leaves of deciduous trees in summer and 

 the leaves of evergreen trees throughout the year. The most 

 abundant of these pigments is chlorophyll (Greek : chloros, 

 green, and phyll, leaf) , which is bright green in color. In addition 

 to chlorophyll, two other pigments, one yellow and one orange, 

 are found in a green leaf. These three pigments may exist quite 

 independently of one another.^ 



^ The coloring matter in a green leaf is composed of about 66 per cent green 

 pigment (chlorophyll) ; 2^ per cent yellow pigment (xanthophyll) ; and lo per cent 

 orange pigment (carotin ; so named because of its abundance in the carrot). The 

 green pigment is not a simple substance, however, but a mixture of two kinds of 

 chlorophyll, one of which is blue-green and the other yellow-green. The depth 

 of the green color in a leaf depends in part on the proportions in which these various 

 pigments are combined. Chlorophyll contains carbon, hydrogen, nitrogen, oxygen, 

 and magnesium. Carotin contains only carbon and hydrogen, while xanthophyll 

 contains in addition a small amount of oxygen. 



