heterostyly(redirected from Pin and thrum)
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heterostylya combination of the morphological and physiological mechanisms that promote cross-pollination in flowering plants. Structurally, there are usually two flower forms (e.g. the ‘pin’ and ‘thrum’ forms of the primrose) which ensure that the pollen collected from the stamens of one type is deposited on the stigma of the other (see ENTOMOPHILY). There is also SELF-INCOMPATIBILITY between pollen and stigma from the same flower type, although there is compatibility between flowers of different types. Compare HOMOSTYLY.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005