Tropical plants grow cool flowers. (Botany).

By shifting the positions of their flowers, two tropical species keep their blooms at comfortable temperatures for pollinators, say researchers.

Ipomoea pes-caprae and Merremia borneensis in the morning glow family don't track the sun's passage exactly, but they do generally keep blooms facing sun-ward during the flowering season, say Sandra Patino of the Instituto Humboldt in Bogota, Colombia, and her colleagues. The researchers describe the motion and its ecological effects in the February Oecologia.

The scientists left some flowers alone but modified others by, for example, covering them with grease to prevent cooling by evaporation and mechanically preventing them from facing the sun.

Patino and her colleagues found that the undisturbed flowers maintained the lowest temperatures and were also the preferred blooms for pollinators. The researchers say their data suggest that a flower's temperature depends on both its position and the evaporative cooling of water leaving its tissues. --S.M.