Ch. Ill, 13] USES OF THE PLANT'S FOOD 109 



simply incidental by-products of other processes, as in 

 foliage plants (page 88), in autumn leaves, in the heart 

 wood of trees, in the colored saps of roots and stems, and 

 in the highly colored Fungi, though in some of these cases 

 investigators have found suppositional explanations of their 

 presence. These pigments are mostly too unstable in light 

 to serve any useful purpose to man, unless we consider pleas- 

 ure a utility, for he takes great delight in assembling them in 

 gardens. Some pigments, however, are stable, including 

 a few which lack color in the plant but acquire it on ex- 

 posure to air (e.g. indigo and madder), making them useful 

 dyes. But chemists can now make such dyes artificially, 

 and more cheaply than we can obtain them from plants. 



The Alkaloids are best known to us in Morphine (from 

 the Poppy), Nicotine (from Tobacco), Quinine (from a tree 

 bark, Cinchona), Strychnine (from seeds of Nux vomica), 

 Cocaine (from the leaves of a shrub, Erythroxylon Coca) ; 

 while Caffein or Thein (from Coffee and Tea), and Theo- 

 bromine (from the Cacao tree) are related, if not actually 

 in the same class. They occur mostly in special cells or tubes 

 (often in the "milk" system, or latex), but their signifi- 

 cance to the plant is very uncertain. Some investigators 

 hold that they are semi-poisonous waste products which the 

 plant thus isolates, while others have thought that their 

 powerful bitter tastes form a protection to the plants against 

 animal foes. Chemically they are composed of C, H, O, N, 

 thus suggesting a derivation through the amides. They are 

 all endowed with active properties, which are the source of 

 their value to man, for, as the list above given will show, 

 they include some of the most efficacious stimulants and 

 powerful poisons which are contained in our materia medica. 

 In fact, the principal plant poisons and our most important 

 drugs belong in this class. The ptomaines, those well-known 

 poisons resulting from the action of Bacteria in animal 

 tissues, are also alkaloids. 



Related to the alkaloids in their active properties are some 



