INTRODUCTION

Plants growing in those areas of the world where there are definite seasons synchronize their activities to suit regular variations in the weather. It makes little sense for a seed in temperate regions to begin germinating as soon as it is produced if it is late summer. Onset of winter would likely freeze and kill the fragile seedling before it establishes itself. A tree or shrub faced with an approaching cold season begins, in advance, to make provision to protect tender growing points and leaves within buds (see Chapter 9).

In another example, annual herbs develop vegetatively before diverting their energies to forming flowers, fruits, and seeds. Leaves are most often produced first to supply enough food reserves through photosynthesis before energy-consuming tasks like reproduction begin. Such sequences of events are timed to occur in synchrony with the seasons.

For these, and many other reasons, plants have evolved ways to sense the passage of time; to measure the lengths of days and the onset of seasons. Rhythms in plant life are linked to such cycles in their environment through a number of remarkably exact time-measuring mechanisms.

DAILY AND SEASONAL RHYTHMS IN PLANT LIFE

Sleep movements

It has been recognized for a long time that rhythms in plants exist. Androsthenes, historian to Alexander the Great, was one of the first to observe, over 2300 years ago, that the leaves of some plants adopt different positions during the day than at night.