Patterns of floral construction in ontogeny and phylogeny

Among the flowering plants flowers are relatively closed systems of a considerable complexity and therefore more suitable for comparative developmental studies than other organs. They work with a few kinds of structural elements that occur in variable or fixed numbers and are arranged in patterns more or less constrained by developmental conditions. Diversity of floral structure occurs at different levels, with emphasis either on number and arrangement or on synorganization and form of the structural elements. The first level is shown here with examples of several representatives of the primitive subclasses Magnoliidae and Hamamelididae, the second with Asclepiadaceae representing the most advanced subclass of dicotyledons Asteridae. Although early ontogenetic patterns are fairly constant among larger groups, especially the sequence of initiation and position of the different classes of structural elements, there is no strict stability in any of the parameters studied.