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We investigated diurnal patterns of petal movement (upright and reflexed) and sensitivity of pollen to moisture in a winter-flowering flora from the desert coast of Namaqualand, South Africa. Specifically, we tested the hypothesis that nocturnal flower closure associated with upright petal movement affords protection to pollen from winter precipitation. The proportion of open flowers in eight species from seven genera and three families, increased rapidly above air temperatures of about 20 degrees C. Flower temperature explained most of the variance in petal status. About 90% of the variance in flower temperature was explained by air temperature while radiation, wind speed and relative humidity had no significant independent effect. Petal opening was more closely correlated with temperature than the closing response, which may be under the additional control of endogenous factors. Pollen exposed to moisture overnight had a significantly higher frequency of damaged grains than control pollen in the majority of study species within the Aizoaceae and Neuradeceae. We found no evidence that pollen of asteraceous species exhibiting flower closure is sensitive to moisture. We conclude that pollen damage and the reduction in male fitness that may result from exposure to rain, dew and fog has provided an important selective impetus for the widespread evolution of upright petal movement and associated flower closure found among more than 3500 species in the predominantly winter and spring flowering Cape fynbos and succulent karoo floras.