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in flowers, the expansion of the corolla or corollalike perianth because of an increase in the number of petals. Doubleness is often the result of the transformation into petals of stamens (roses, peonies, ranunculuses, pomegranate) or pistils (Ranunculaceae, Rosaceae, some violets, clover, some petunias). More rarely, doubleness results from the splitting of petals (fuchsia) or stamens (some Caryophyllaceae) or the increase in the number of circles in a simple perianth (some tulips and lilies). Double inflorescences are encountered among the Compositae; these result from the conversion of the interior bisexual flowers into ligulate, usually infertile flowers (dahlia, aster, chrysanthemum) or peripheral ligulate flowers into tubular flowers.
Horticulturalists can induce doubleness by hybridization or by changing the conditions of cultivation (for example, abundant feeding promotes doubleness). Doubleness is often accompanied by profound changes in the organs of the flower. For example, in Primula anthers develop on the ovarian wall and on the stigma; in fuchsias and roses ovules arise on stamens. Incomplete doubleness is most often encountered. Complete doubleness, with all the stamens and pistils converted into petals (such flowers do not yield seeds), rarely occurs. Although viable pollen is formed in double flowers whose extra petals have been formed from stamens, it is located in deeply hidden tissues and is destroyed the moment the flower opens. If gathered in time, the pollen is suitable for artificial pollination. Vegetative reproduction is often used in cultivating double flowers.
REFERENCESFedorov, Al. A. Teratologiia i formoobrazovanie u rastenii. Moscow-Leningrad, 1958.
Zhukovskii, P. M. Botanika, 4th ed. Moscow, 1964.
L. V. KUDRIASHOV