Studies of rain-wash effects on pollen have shown that flower structures can protect susceptible pollen from rain. It remains unclear, however, how a thin corolla can withstand external force and perform its protective function. The sclereids in petals of several species of Camellia (Theaceae) were anatomically investigated to determine their mechanical properties. To examine the effects of changing physical environment on the occurrence of sclereids in petals, sclereid density in petals of six species, including wild samples from different rainfall zones and samples from a greenhouse under mild conditions without wind and rain, were examined and statistically analysed. The results showed that the occurrence of sclereids in petals varied with physical environment. The number of sclereids in the same species increased with the increasing rainfall. There were abundant sclereids in petals of the wild species, but few or no sclereids in species cultivated in the greenhouse. Moreover, the anatomical features of sclereids, especially the unique distribution pattern that has not hitherto been described, were correlated with external environmental pressures. Our observations reveal a novel mechanical system in the corolla and provide further evidence for the hypothesis that flower structures may protect rain-susceptible pollen.