20 CHEMISTRY OF PLANT LIFE 



PHYSIOLOGICAL USES OF THE ORGANIC COMPONENT GROUPS 



From the standpoint of the role which each plays in the plant 

 economy, the several groups of organic compounds may be 

 roughly divided into three classes. These are: (a) the frame- 

 work materials, including gums, pectins, and celluloses; (6) syn- 

 ergic foods, including carbohydrates, fats, and proteins; and (c) 

 the secretions, including the glucosides, volatile oils, alkaloids, 

 pigments, and enzymes. 



The framework material, as the name indicates, constitutes 

 the cell-wall and other skeleton substances of the plant. It is 

 made up of carbohydrate complexes, produced by the cell proto- 

 plasm from the simpler carbohydrates. 



The synergic foods, or " reserve foods " as they are sometimes 

 called, produced by the excess of synthetized material over that 

 needed for the immediate use of the plant, are accumulated either 

 in the various storage organs, to be available for future use by 

 the plant itself or by its vegetative offspring, or in the seed, to 

 be available to the young seedling of the next generation. Pro- 

 teins not only serve as reserve food materials but also make up 

 the body of the living organism itself. Carbohydrates and fats 

 serve as synergic and reserve foods. 



The secretions may be produced either in ordinary cells and 

 found in their vacuoles, or in special secretory cells and stored in 

 cavities in the secreting glands (as in the leaves of mints, skin of 

 oranges, etc.), or in special ducts (as in pines, milkweeds, etc.) 

 or on the epidermis (as the " bloom " of plums, cabbages, etc., 

 the resinous coating of many leaves, etc.). As a general rule, 

 the glucosides, pigments, and enzymes are the products of unspe- 

 cialized cells and have some definite connection with the metabolic 

 processes of the plant; while the volatile oils and the alkaloids 

 are usually secreted by special cells and have no known role in 

 metabolism. 



