A temporal and morphological framework for flower development in Antirrhinum majus

The entire course of flower development in Antirrhinum majus L., from initiation to maturity, is described in terms of regular time intervals. Floral meristem and bud morphology was determined by scanning electron microscopy for a sequence of 58 plastochrons. These can be grouped to define 15 stages or 7 phases of development, providing a temporal framework for gene expression and key morphological events, such as the formation of the complex corolla. The time course is also used to estimate overall growth rates of sepals and petals. Sepals initially grow at a constant rate, but growth rate gradually declines at later stages and sepal growth eventually arrests before flower development is complete. Petals initially grow at a similar rate to that of early sepals, but this growth rate is maintained for a longer period, accounting for the larger size of mature petals relative to sepals. Comparisons with Arabidopsis indicate that the duration of growth also makes an important contribution to variation in flower size.Key words: Antirrhinum, flower development, meristems, zygomorphy, developmental timing, petal.