[Chap. XIX SUBSTANCES MADE FROM FOOD 171 



compounds. Changes that occur in pectic compounds in plants may be 

 represented briefly. 



Pectose > Pectins > Pectic acid. 



Pectic acid + Calcium salt > Calcium pectate. 



Fungi that live parasitically within green plants are often found only 

 in the layer of pectic compounds between the cells. There they digest 

 and use these compounds as food. The isolated cells of the host die and 

 disintegrate. A similar process may result in overripened fruits and vege- 

 tables wherever the enzymes that digest pectic compounds are present. 

 The retting of flax fiber is merely a process of breaking down the pectic 

 compounds that hold the fibers together. 



Large quantities of pectin are extracted annually from plants and 

 used in the preparation of jellies, salads, creams, and emulsions desired 

 as foods and drugs. Most of the commercial pectin, such as Certo, is 

 extracted from apple pomace or cull lemons. A smaller amount is ob- 

 tained from the pulp of oranges, sugar beets, carrots, and algae. 



Other cell wall constituents. Suberin, the substance that is character- 

 istic of cork, apparently occurs in a definite layer in the cell walls of 

 cork tissue. It may also become deposited on or within cellulose walls, 

 a process called suberization. Several other substances, such as lignin, 

 cutin, wax, resins, and tannins, may become intermingled with cellulose 

 and pectic compounds as the walls increase in age. The cutinized layer 

 of the exposed surfaces of epidermal cells is called the cuticle. Suberin, 

 cutin, wax, and resin when present in or on cell walls make them less 

 permeable to water and restrict the entrance of parasites. These sub- 

 stances often constitute a relatively large part of the vegetable material 

 that ultimately becomes transformed to coal, for only a few kinds of 

 organisms can digest them. 



Lignin is characteristic of woody cells and of hard plant tissues in 

 general. Its presence is easily demonstrated by treating plant tissue with 

 a dilute solution of phloroglucin followed b\' strong hydrochloric acid; 

 lignified walls become brilliantly red with this treatment. One of the 

 problems of obtaining cellulose from wood is the difficulty of removing 

 lignin and other substances that are associated with it in the cell walls 

 of wood. 



Pigments. In addition to the chlorophylls, carotenoids, and antho- 

 cyanins discussed in Chapter IV, many other pigments are formed from 

 food in plants. Some of them are important as co-enzymes in oxidation- 



