ИСТИНА

Flowering shoot systems in Apiaceae are ususally interpreted as `inflorescences´ with umbels. Their architectural similarity suggests homology among species. However, ontogenetic studies reveal that `inflorescences´ in Apiaceae originate from different developmental pathways rendering similar patterns analogous. Umbels originate from reproductive meristems which pass through repeated steps of fractionation. First, umbellet meristems, then, flower meristems, and only in the third step of fractionation, floral organs appear. After each step the meristem expands providing space for the next fractionation. While umbel development appears to be uniformous in Apiaceae, `inflorescences´ with umbels differ in their meristematic origin. If each umbel originates from its own reproductive meristem (e.g. Orlaya grandiflora), the whole plant is in the vegetative stage except the umbels which appear solitarily at the shoot tips. This mode is usually combined with the formation of leaves among the umbels and with the successive production of higher order umbels (ordinal gradient). The youngest umbels are often formed when the first umbels are already fruiting allowing the plant to plastically respond to herbivors and fruit set. If, in contrast, several umbels originate from the same reproductive meristem (e.g. Dorema aucheri), they occur in true inflorescences. These are characterised by bracts instead of leaves and a fast development resulting in mass flowering and little developmentral plasticity. We conclude that the largely overlooked characters of leaf production and flowering sequence may help to identify the underling developmental pattern.