Yun-Yun Mao and Shuang-Quan Huang of Wuhan University in China studied the response to rain and water of 80 species of flowers. Their work revealed that many flowers have different shapes and structures to prevent their pollen from getting wet. Other flowers developed waterproof pollen instead.
Of the 80 species studied, 20 produce flowers that completely protect their pollen. Some plants shelter their pollen grains through a change in floral orientation or closing their corolla on rainy days. For example, tulip flowers close their petals rapidly when rains come. Some plants have flowers that droop downward, while others have outlets in the base of the flower that let water quickly drain away. But 44 of the 80 species expose their pollen completely, giving it no protection. Of these species, 13 produce pollen that is highly resistant to water, suggesting they have evolved an alternative way to deal with the rain.
“We have demonstrated that flower structures can protect susceptible pollen from rain, indicating that rain could interact with other factors governing the evolution of floral traits…Understanding plant reproductive strategies for protecting pollen from rain may provide a clue to the diversity of plant–pollinator interactions. In high-rainfall places or seasons, a high proportion of species were observed with nodding, pollen-concealed flowers or upward-facing flowers capable of temporary closure through perianth movements to avoid pollen loss (Dafni, 1996; Bynum & Smith, 2001; Huang et al., 2002; Hase et al., 2006). In bowl-shaped flowers exposed to rain, the petals are often separate or fused only at the base, so that raindrops flow quickly away between the petals, comprising a water drainage system. Sympetalous flowers are often pendulous and rarely erect. Erect sympetalous flowers usually have a narrow corolla. Morning glory flowers (Convolvulaceae) seem exceptional, with a wide open corolla, but they are generally ephemeral and secund. We observed that the corolla of Calystegia sepium closed on rainy days. Such floral changes may function to shelter pollen from rain damage before pollinators arrive. Extended pollen presentation duration in rain and the restoration of flower performance in fine weather involve reproductive costs that could otherwise have been invested in seed production. If we recognize that rain affects floral evolution, it is apparent that increased variability of precipitation may have potential effects on plant diversity and distribution in a given area.” (Mao and Huang 2009:898)