11. Birds also have built-in timepieces which send them off on fall and spring migrations. What the birds really have is a clock-like mechanism which allows them to time hours of darkness or light in each day.

12. But what sends birds northward again in the spring? New research by Dr. Albert Wolfson of Northwestern University seems to indicate that the timing of return flight is extraordinarily complex. In the fall of the year the short days and long nights cause the "clocks" in migratory birds to undergo a kind of "winding" in preparation for their spring return and breeding. Then during the late fall and winter as the clock "ticks", certain physiological changes occur in the bird. The length of each day during the winter determines how fast the clock will run, and hence when the "alarm" will ring for the spring migration. The clock continues to run through breeding time, then stops-to be re-wound again the next fall.

13. Scientists are now learning that many of the clocks of nature can be reset, speeded up or slowed down-all for our benefit. Pioneering experiments at the U.S. Department of Agriculture's research center in Beltsville, Maryland, have shown that plants can be helped to develop faster in less time. By increasing or lessening the hours of darkness in each day, the scientists have been able to turn plant growth off and on like an electric switch.

14. New knowledge about nature's living clocks has practical applications. For man, too, seems to follow daily rhythms. The amount of sugar in our blood stream varies with the time of day, as does our temperature. More of the cells in our skin and muscles divide during the night hours than during the day. By tinkering with the clocks of plants and animals, scientists may learn more about the fascinating way our bodies work.