With the winter solstice upon us, we're now experiencing the shortest daylight periods of the year. Some of us like the change, and some of us lament it. Plants and animals are affected, too: it's these variations in day length that help them set their internal clocks.
Migratory birds are a prime example. A Slate-colored Junco nesting in northern Canada responds to the first shortening days of summer with a series of physical changes: its reproductive organs become inactive and shrink in size, hormones stimulate the rapid growth of a new set of feathers (its non-breeding plumage), and fat deposits develop to provide fuel for the long migratory flight ahead.
Thus the preparation for migration starts as soon as the days begin to shorten. And the process must operate in reverse when the bird is in its winter habitat in the United States. As soon as days begin to lengthen, the Slate-colored Junco must gear up physically for the flight north and breeding season. If it fails to do so, it likely won't survive a long-distance migration.
Plants in temperate zones must also set their calendars accurately in order to flower and, for deciduous species, develop and drop leaves at the optimal time. Plants set their internal calendars using several attributes from the sunlight they receive. In fact, the angle of the sun may be more important to a plant than day length.
That's because plant cells produce compounds called phytochromes in response to different portions of the light spectrum. Direct sunlight is higher in red light, while indirect sunlight contains more far-red light. During late fall and early winter, when the sun remains low in the southern sky, the indirect light produces an increase in far-red phytochromes. As spring approaches and the arc of the sun rises in the sky, direct sunlight triggers the production of red phytochromes. The ratio of these two compounds mediates the hormones involved in flowering, leaf drop, and bud development. Even seeds below the soil are affected. It's true: the amount of red and far-red light that penetrate the soil is sufficient to govern germination.