The problem of generalized flowers: morphological aspects

Abstract

Since Delpino (1868–74) recognized that flowers sharing similar physiognomies are preferentially visited by certain groups of animals, it has been commonly inferred that these biological groups (flowers and pollinators) have passed a more or less complicated evolutionary process of mutual specialization. The question, however, as to what were the initial angiosperm floral forms remains unanswered. Whatever the form, a broad spectrum of visitors was likely. Each organism needs to be adapted to its environment, for which reason the term “generalized” instead of “non‐specialized” is to be preferred. Generalized flowers occur even in otherwise highly evolved families, suggesting that these representatives have developed secondarily from a somewhat specialized pollination type; this points to the fact that “generalized” and “specialized” are relative terms. It also means that a generalized flower is not synonymous with a “primitive flower”! In terms of morphology, the original “primitive flower” form is a subject of debate. Neither in the fossil records nor among extant, archaic angiosperms is there a model organism bearing a flower with a broad spectrum of visitors, and which exhibits all organs in their plesiomorphic state. Nor is it likely that such a plant will ever be discovered. Thus, we must be content with a relative solution; there is one—hitherto only one—magnolioid family, the Winteraceae, some members of which approach the idealized model. Representatives of this family are characterized by many plesiomorphic features and have open flowers visited by a broad spectrum of pollen vectors. An explanation of how these plants could have remained in this generalized condition over long time periods is offered.