Alkaloids, as we have seen, are vegetable substances which contain nitrogen, as well as carbon, hydrogen, and sometimes oxygen, and like alkalis form salts with acids. While certain of the alkaloids, as for example theobromine, which was referred to in the last chapter, are comparatively harmless in their action upon the human system, others, which are now to be considered, include some of the most powerful of poisons.

The plants which produce alkaloids or other poisons would seem to be protected against the ravages of herbivorous animals by means of these substances. All such animals, however, are not affected alike by them. Thus cattle eat poison-ivy without harm, and various insects are known to feed exclusively upon plants which are deadly poisons to higher animals. Commonly poisons are associated with rank odors or disagreeable tastes, but in some poisonous plants which are avoided by cattle and sheep there are no such warnings that we can discover. In the plant's economy the substances in question are to be considered simply as byproducts which are sometimes protective. It is a curious fact that many plants may be poisoned by their own alkaloids. For example, an opium poppy is killed if watered with a solution of morphine.

Out of the large number of drugs consisting of or containing poisonous alkaloids the few following may be taken as familiar examples: opium, tobacco, coca, atropine, quinine, strychnine, and aconite.