174 TEXTBOOK OF BOTANY 



the rapid change of sugar to starch in the harvested product. The higher 

 the temperature the more quickly the change occurs. 



Alkaloids. Under this general name may be grouped a large variety 

 of chemical substances apparently of little or no value to the plant in 

 which they are made, but which have been of great importance in 

 medicine. They are nitrogen-containing organic compounds, derived 

 from proteins, generally odorless but bitter to the taste. They have 

 marked physiological effects upon animals, and are extensively used as 

 stimulants and narcotics. The best known are nicotine, from tobacco; 

 atropin, from nightshade; strychnine, from strychnos; cocaine, from 

 coca leaves; quinine, from chinchona bark; morphine and codeine, from 

 the poppy; and caffeine, from coffee and cacao seeds. 



Tannins. The bark of many trees, the galls occurring on oaks, and 

 certain unripe fruits, such as the persimmon, contain bitter astringent 

 substances known as tannins. These substances coagulate proteins to 

 insoluble compounds. The coagulation of proteins in raw hides is the 

 basic process in the tanning of leather. With iron salts, tannins become 

 black or green in color. Ink was formerly made in this way. The freshly 

 exposed surfaces of many fruits and vegetables become dark because of 

 the oxidation of tannic acid, especially in the presence of a trace of iron 

 from a knife. 



Enzymes. The importance of enzymes in all metabolic processes in 

 plants has already been indicated. Their chemical composition is un- 

 known, but many of them seem to be protein-like. We know of their I 

 presence only through the effects they produce. Likewise, they are 

 named either on the basis of the kind of substance upon which they act 

 or the kind of action which they initiate or accelerate, or both. For 

 instance, sucrase acts upon sucrose, and oxidases accelerate the process 

 of oxidation. Many enzymes are very specific, acting only in one kind 

 of process. In all, there must be several hundred, perhaps thousands, 

 of them. They are primary factors in numerous biological processes. A 

 few of the common digestive enzymes in plants, the substrate ( substance 

 acted upon), and the end product are presented in Table 6. 



Vitamins. These organic substances, which are formed in plants in 

 minute quantities, were first discovered through their influence on the 

 health of man and other animals. The chemical composition of some 

 of them has been discovered. Vitamins are like enzymes rather than 

 food; that is, they are necessary not as building material or as a source 

 of energy, but rather as catalysts of certain basic biological processes 

 underlying health, growth, and reproduction. Scurvy, beriberi, and 



