The flowering process as an example of plastic development.

The field of flowering has been characterized by simplistic ideas, as exemplified by the attempts to classify plants on the basis of the kinds of environmental factors required for their transition to flower initiation and the claims that the process of flower formation in photoperiodic and cold-requiring plants is independent of correlative influences from various organs, e.g. the roots. Other examples of this simplified picture are the concept of a specific leaf-generated floral hormone and floral inhibitor and the search at the level of the meristem of a specific evocational event which would set in motion the whole sequence of other events and commit the meristem to flower initiation. Finally, there is the belief that because flower morphology is basis to species classification the morphogenesis of flowers is a rather invariable process. All these ideas are essentially erroneous and it is shown that all aspects of the flowering process are much more flexible than is usually believed. Floral induction may be completed by many, if not all, plants in several alternative sets of environmental factors. At least in some plants, the alternative inductive factors are perceived by different organs, indicating that these factors affect most probably entirely different processes. Thus, at induction, plasticity is extremely large and the fate of any shoot meristem appears to be controlled by a complex and flexible array of promoters and inhibitors arising from all plant parts. At meristem evocation, there are a number of events which are fundamentally the same in many plants, but so far no single initial critical event has been found. The various evocational changes appear to form sets of interconnected systems and this complex network seems to embody some plasticity since it has been possible to suppress experimentally some of the most universal evocational events or alter their temporal order without impairing evocation itself. At later stages, it has been observed that all the morphological characters of inflorescences and flowers may be experimentally altered. However, if the occasional and extreme malformations (monstrosities) caused by some growth substances are excluded, morphogenetic processes do not appear flexible to the point that the reproductive structures of one species are transformed into those of a taxonomically unrelated species. Thus, despite the fact that these processes are never absolutely fixed, plasticity at morphogenesis appears less than that at induction.(ABSTRACT TRUNCATED AT 400 WORDS)