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The plant alkaloids constitute a group of nitrogenous substances which possess great interest for the organic chemist because of their inltricate molecular structure and for the pharmacologist because of the extraordinary physiological effects that many of them exert wheni introduced into the animal body. It is a regrettable fact, however, that so little is known of their physiological functions and of the mechanisms by which they are synthesized within the plant cell. Concerning these problems, the lack of an obvious experimental approach has led to extensive speculation. For instance, PICTET, who first svnthesized nicotine, assuimied that the alkaloids may represent degradation products of proteins, nucleic acids, and chlorophyll (35). The great JusTus voN LIEBIG imiiplied their use as substitutes for the mineral bases within the plant body when he developed his "mineral theory" (3, 26). The most prevalent view at the present time has been well expressed by CROMWELL (5) and by VICKERY (42). This hypothesis holds that the alkaloids represent by-products of nitrogen metabolism in which irreversible reactions are involved and that they differ from the end-products of nitrogen metabolism in animals by their retention within the body of the organism. Within the past decade a serious attempt has been made to ascertain the nature of nicotine production in the tobacco plant. Several factors have contributed to this choice, but the most important have undoubtedly been the

DEVELOPMENT OF SOME RECENT CONCEPTS IN THE PHYSIOLOGICAL CHEMISTRY OF THE TOBACCO ALKALOIDS