Light, Time, and the Signals of the Year

Suppose the seasons came as a surprise. If heat, or cold, or drought were always unexpected, how much harder it would be to cope with them and to survive. So from the earliest times, men have watched the height of the sun, even celebrating such nearly abstract events as equinoxes and solstices. But long before man's festivals, evolution made a similar discovery: The length of the day reflects the course of the year. With that information, a species can do more than just react to the seasons as they occur; it can, in effect, anticipate them. The value of this is suggested by the many life-cycles, both plant and animal, in which some crucial phase is controlled by photoperiodism response to the timing of light and darkness. The most conspicuous examples involve reproduction flowering in plants or breeding seasons in animals. But, depending on the organism, all sorts of processes, dormancy, senescence, and even death, may be caused or prevented by photoperiodism. An example of flowering controlled by photoperiodism is in the familiar ornamental vine, morning glory. This is a "short-day plant," flowering only when exposed to a certain number of hours of light per day. The "critical daylength" is approximately 17 hours in one variety. Under usual cultivation, early development occurs when the days (including dawn and twilight) are longer than 17 hours, and flowering begins as the days shorten in July and August. The drastic capacities of photoperiodic control become evident in comparing plants grown continuously during days longer than the critical value with those germinated and grown entirely during days shorter than the critical value. The former are long vines completely devoid of flowers. The latter, on the contrary, flower precociously as seedlings before becoming vines at all, then cease growth quickly at a height of a few inches. Having been converted to flowers, all the growing points set seed and die. The precision of the timing is considerable, since a difference of 15 minutes in the