The Physiology of Flowering Plants - KHAM PHA MOI

286 REPRODUCTIVE DEVELOPMENTFig: 11:10 Inflorescence andflowers of Antirrhinum majus.(A) Flowers are borne on aninflorescence. (B) The wild-typeflower exhibits bilateral symmetry.(C) Mutations in the cycloideavariety result in the development ofradially symmetrical flowers. Images(A) and (B) taken by R. Carpenter(courtesy of John Innes Centre,Norwich), (C) by D. Bock (suppliedby W. E. Lönnig, Max-PlanckInstitute, Köln).ABCArabidopsis flowers have four green sepals, four white petals, sixstamens, two of which are shorter than the others, and a gynoeciumwhich consists of a two-chambered ovary capped by stigmatic papillaeand containing 30–50 ovules. The Arabidopsis flower is radiallysymmetrical (except for the stamens) and is said to be actinomorphicas it possesses more than one plane of symmetry. TheAntirrhinum flower also consists of four concentric whorls of organsarranged in the same order. There are five green sepals, five petalsfused to form a tube with five lobes, five stamens, the uppermost ofwhich aborts early in development, and an ovary composed of twocarpels. The arrangement is bilaterally symmetrical and is said to bezygomorphic, possessing a single plane of symmetry.Zygomorphy is thought to have arisen from actinomorphy severaltimes during evolution, and most probably helps to maintain pollinatorspecificity, often in flowers pollinated by bees. It is found inmany families of angiosperms including the Fabaceae (includes peaand soybean), Lamiaceae (mints), Scrophulariaceae (Antirrhinum andthe common toadflax, Linaria vulgaris) and, perhaps most elaborately,in the Orchidaceae. Zygomorphic development is controlled by just afew genes, mutations in which lead to a reversion to an actinomorphicpattern of development (Fig. 11.10C). One such gene,