Flower development

Plants that bear flowers are called Angiosperms. With around 300,000 species, they represent the largest and most successful group in the green kingdom. Flowers contain one or more female organs, or carpels, in their center. Carpels enclose the ovules, and later the seeds. Stamens, the male pollenbearing organs, surround the carpels. Sexual organs are in turn surrounded by vegetative organs (the perianth) usually of two types — petals to attract pollinators, and sepals to protect all other organs in the bud. In flowering plants, each shoot carries a cluster of undifferentiated pluripotent cells at its growing tip called a meristem. As the shoot grows, the meristem generates the stalk that supports it, and a succession of leaves on its flanks. Eventually the shoot produces flowers, at which stage it is called an inflorescence. Flowers develop where bracts (small leaf-like organs) join the stem, or directly on the flanks of the shoot meristem. They arise as small meristems that are similar in many ways to shoot meristems. However, instead of producing leaves on their flanks, they produce floral organs which arise as undifferentiated primordia that grow and differentiate into one of the four types of mature organ. Each organ type arises in defined positions and numbers that are relatively constant within the flowers of related species. As recognised by Linnaeus, this constancy means that floral organ numbers can be used as key characters in plant classification. In the last decade we have begun to understand how gene products control the mechanisms of flower development. Success has depended upon the use of representative model species with convenient molecular genetics. These species span the range of Angiosperm diversity, and include the grasses maize and rice, the weedy mustard species Arabidopsis thaliana, the decorative Petunia hybrida and the edible tomato (both members of the tobacco family), and the snapdragon Antirrhinum majus (Figure 1).