Biological Clocks

These videos from the Plants in Motion web site illustrate biological clocks in beans and velvel leaf.

This animation (No Audio) describes biological clocks.

Phytochrome, a blue-green pigment, is converted to an active form (Pfr) at sunrise and to an inactive form (Pr) at sunset. It influences seed germination, stem elongation, and formation of leaves, flowers, fruits, and seeds.

Circadian Rhythms

Circadian rhythms are regular cycles of plant activities that occur on a twenty-four hour basis. For example, plants may fold their leaves into "sleep" positions even when kept in constant light or darkness. In some plants, leaves are extended out during the day, and held close to the stem at night. Such leaf movements occur even in constant light or darkness. Experiments have demonstrated that phytochrome plays a role.

Photoperiodism

Photoperiodism is a biological response to a change in relative length of daylight and darkness as it changes throughout the year. Hormones such as phytochrome, and others not yet identified, probably influence flowering and other growth processes.

• "Long-day plants" flower in the spring as daylength becomes longer (spinach is an example).
• "Short-day plants" flower in late summer or early autumn when daylength becomes shorter (example: cocklebur).
• "Day-neutral plants" flower when they are mature.

REVIEW: Rhythms that are repeated every 24 hours are collectively and specifically known as
Answer circadian rhythms.