Abstract: Studies on the closure behavior of the touch-sensitive stigmas in flowering plants are one of the hotspots in pollination ecology and evolutionary biology. Examples of closure behavior of bi- or multi-lobed stigmas in response to being touched by animal pollinators were mainly studied in some taxa of the order Scrophulariales . Such phenomenon had been studied at the beginning of 1990s. Generally, stigma closure has been attributed to the loss of turgor in cells comprising the stigmatic tissues, during pollen germination and subsequent pollen tube growth absorbing water from surrounding tissues in the pistil. Various hypotheses addressing the adaptive significance of stigma closure have been proposed. Recent years, studies indicated that stigma closure may increase pollen capture and receipt, improve pollen germination and pollen tube growth, and prevent intra-floral selfing. It was also suggested that stigma closure can reduce interference between pollen receipt and pollen dissemination. Stigma closure affects pollen deposition and reduces interference between pollen dispersal and stigmatic receipt as well . Foraging behavior of pollinator is strongly influenced by the status of the stigmas . Until now, the mechanism of stigma closure and the hypotheses of avoiding self-pollination have been still uncertain. Some experiments with male-female interference and the intra-floral selfing should be carried out to test the adaptive significance of stigma closure. In this paper, recent advances in the sensitive stigma are summarized , and some problematical issues are also discussed .

Key words: Adaptive significance