This ill-defined group of plant compounds [alkaloids] includes many that are both useful and toxic. Though most of them strongly affect human physiology [eg. caffeine, nicotine, morphine, quinine, cocaine, turbocurarine, atropine, aconitine], their functions in plants are still obscure.
…Our self-centered view of the world leads us to expect that the alkaloids must play some comparably important role in the plants that make them. It comes as something of a surprise, there fore, to discover that many of them have no identifiable function whatever. By and large they seem to be incidental or accidental products of the metabolism of plant tissues. But this conclusion somehow fails to satisfy our anthropocentric concern. The pharmacological potency of alkaloids keeps us asking: What are they doing in plants, anyway? Investigators have found that a few alkaloids actually function in the life processes of certain plants. But this research has served principally to illuminate the subtlety of such processes.
…Some ingenious grafting experiments have furnished additional evidence that many alkaloids, once synthesized, become inert and play no further role in the plant’s metabolism. The tobacco plant, for example, manufactures nicotine in its roots, whence the alkaloid migrates to the leaves. However, if we graft the top of a tobacco plant to the roots of a tomato plant, which produces no nicotine, the tobacco flourishes despite the absence of the alkaloid. Conversely, a tomato top grafted to a tobacco root becomes impregnated with nicotine with no apparent ill effects. [Success of the grafting in introducing nicotine to the tomato plant apparently is not guaranteed & sporadic.]
…Though all alkaloids come from plants, not all plants produce alkaloids. Some plant families are entirely innocent of them. Every species of the poppy family, on the other hand, produces alkaloids; the opium poppy alone yields some 20 of them. The Solanaceae present a mixed picture: tobacco and deadly nightshade contain quantities of alkaloids; eggplant, almost none; the potato accumulates alkaloids in its foliage and fruits but not in its tubers. Some structurally interrelated alkaloids, such as the morphine group, occur only in plants of a single family. Nicotine, by contrast, is found not only in tobacco but in many quite unrelated plants, including the primitive horsetails. Alkaloids are often said to be uncommon in fungi, yet the ergot fungus produces alkaloids, and we might classify penicillin as an alkaloid had we not decided to call it an antibiotic. However, alkaloids do seem to be somewhat commoner among higher plants than among primitive ones.