Several families of plants have representatives whose leaves move in a regular and predictable pattern during day and night. Generally, the leaf surfaces are horizontal and face the sun. during the day, then fold together in a vertical position at sunset. This process, referred to as nyctinasty (night movement), was described more than 2,300 years ago by Androsthenes, who noted nyctinastic legumes during the march of Alexander the Great. Charles Darwin, fascinated by these movements, hypothesized that nyctinasty provides protection against thermal radiation, since vertical leaves facing each other lose less heat at night than do horizontal leaves facing the open sky (Darwin 1881). Biinning and Moser (1969) wondered why bright moonlight did not disturb photoperiodic time measurement in short-day species, which require a long uninterrupted dark period for floral initiation. They noted that many shortday plants have nyctinastic leaves and that leaves cannot absorb light from the zenith efficiently when in the night position. After careful measurements, they concluded that nyctinastic movement reduces the absorption of moonlight below threshold intensity for interfering with floral induction, and accordingly concluded that protection of photoperiodic time measurement was the main function of nyctinasty under natural conditions. Neither of these