Effects of heliotropic movements of flowers of Dryas octopetala L. on gynoecium temperature and seed development.

Dryas octopetala has heliotropic flowers, whose petals reflect the light on the pistils, thus warming them up. The effects of this reflection on seed formation and development were studied by measuring the pistil temperature of intact flowers, of flowers with petals removed, of shaded flowers, and of flowers constrained in such a way as always to be directed towards the zenith. In August, the seeds of these flowers were collected, counted and weighed. The temperature differences between the differently treated flowers were greates about noon, the flowers ranking as follows in ascending order of temperature: shaded flowers, flowers without petals, constrained flowers and intact flowers. The mean temperature differences between the gynoecia and the air, in degrees Celsius, were: 1.1, 1.8, 2.5 and 3.2, respectively. The same ranking was obtained for the different treatments, when arranged in order of ascending weight per seed, except that the positions of the shaded flowers and the flowers without petals were interchanged. The mean values of the weight per seed in milligrams were: 0.42, 0.48, 0.53 and 0.61 for the groups flowers without petals, shaded flowers, constrained flowers and intact flowers, respectively.