In bright sunshine, temperatures within the apex of the keel of flowers of Lotus corniculatus were consistently higher in dark-keeled than in light-keeled individuals. Temperature differences may increase With higher ambient temperatures and averaged 5.7°C at 23.2°C ambient, In overcast conditions, no significant temperature differences between dark-keeled and light-keeled flowers Were observed, or light keel temperatures were slightly greater. In general, temperatures within keels were lower than ambient. Within sites, dark-keeled plants showed a tendency to inhabit colder microsites than did light-keeled plants. Contrary to some earlier reports, we believe that most clinal variation for keel colour is temperature related. We suggest that there may be an optimum Temperature far pollen germination and pollen rube growth. In cold microsites. dark-keeled plants would be more fecund, but in warm microsites tight-keeled plants would set. more seed, thus maintaining the polymorphism found in most sites.
Keywords: Lotus corniculatus; ecology; keel colour; temperature.