otherwise processing primary metabolites and it is the unusual chemical that circumvents these mechanisms to cause toxicity.
Unlike primary metabolites, which are practically universal constituents of cells, tissues, and organs, secondary compounds are generally idiosyncratic in distribution, both taxonomically and ontogenetically. Chlorophyll, for example, the principal photosynthetic pigment, is found in virtually all species of angiosperms, in virtually all life stages of virtually all individuals. In contrast, the furanocoumarins are secondary compounds known from only a handful of angiosperm families (5). Within a species (e.g., Pastinaca sativa), there is variability in furanocoumarin content and composition among populations (6, 7); within an individual, there is variation among body parts during any particular life stage (8) and temporal variation in the appearance of these compounds over the course of development (9); there are even differences in the content of individual seeds, depending upon their location in an umbel (10), fertilization history (11), and their position within the schizocarp (12).
Secondary chemicals are by definition taxonomically restricted in distribution, yet despite this fact there are patterns in production and allocation that transcend taxa (13). Their presence in an organism is generally characterized by specialized synthesis, transport, or storage. Levels of abundance are subject to environmental or developmental regulation and, unlike primary constituents, which may be present in virtually all cells of an organism, chemical defenses are typically compartmentalized, even in those cases in which the chemicals are acquired exogenously, as when sequestered from a food source. There often exists a system for external discharge, delivery, or activation, not only as a means of ensuring contact with a potential consumer but also as a means of avoiding autotoxicity until a confrontation arises; and of course these compounds are almost invariably, by virtue of structure, chemically reactive (e.g., able to be taken up by a living system, to interact with a receptor or molecular target, and to effect a change in the structure of the molecular target). The remarkable convergence of structural types in plant and insect secondary metabolites is at least suggestive that the processes leading to biological activity in both groups share certain fundamental similarities (14).
Secondary chemicals can be said to be defensive in function only if they protect their producers from the life-threatening activities of another organism. Distinguishing between offensive and defensive use of chemicals is difficult, and present terminology does little to assist in making that distinction. The term "allomone" is frequently used synonymously with "chemical defense," yet allomones are not necessarily defensive in function. An allomone has been defined as a chemical substance beneficial to its producer and detrimental to its recipient (15), so chemicals used