58 General Botany 



commercial rubber. Whether it has a definite function in the 

 living plant is unknown. 



Alkaloids. Under this general name may be grouped a large 

 variety of chemical substances that seem to be of little importance 

 in the economy of the plant, but which have been of great impor- 

 tance in medicine. They are nitrogen compounds, are generally 

 odorless, and have a bitter taste and marked physiological effects 

 upon animals. They 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 cinchona bark; morphine and codeine, 

 from the poppy ; and caft'eine from coffee, tea, and cacao seeds. 



Essential oils. The odors of flowers and the taste of many 

 fruits and vegetables are due to minute quantities of these sub- 

 stances. Because of their medicinal uses their composition is 

 well known. You are familiar w^ith menthol, the characteristic 

 oil of mint, camphor, oils of lavender, bergamot, bitter almonds, 

 and vanilla. Some of the oils contain sulfur. These produce 

 the odor and taste of onions, garlic, water cress, radishes, and 

 many kinds of mustard. 



Vitamins. These substances, which are essential in the nutri- 

 tion of animals, occur only in minute quantities. We cannot 

 test for them or find them in the cell, and we know of their occur- 

 rence only through feeding experiments with animals. If they 

 are destroyed by prolonged boiling before the food containing 

 them is fed to animals, the animals fail to grow properly and 

 gradually weaken and die. Scurvy, beriberi, and rickets are 

 diseases produced by lack of the essential vitamins. Vitamins 

 are manufactured mostly by plants, and accumulate in certain 

 animal tissues and in milk, from the plants eaten by the animals. 



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

 and certain unripe fruits like the persimmon, contain bitter 

 astringent substances known as tannins. These substances 

 react with gelatin or raw hides, forming insoluble compounds, 



