|Tommy LaVergne/Rice University)|
Plants and animals have circadian rhythms as well. Ever since one of Alexander the Great's captains noted the repositioning of leaves during the day, we have been aware that plants respond to daily cycles. Later studies showed that the rhythm continues even when the plant isn't exposed to the sun.
An interesting article and video posted by our Master Naturalist Bob Korpella at freshare.net describes research at Rice University on circadian responses of plants which produce defenses against insect attack. The cyclic release of a plant hormone makes a common cabbage looper caterpillar leave the plant alone. Sounds a little like an attack of botanical PMT.