Steve Davis, age 11, of Newport News, Virginia, for his question:
How can a sunflower follow the sun?
In ancient Greece, they told the tale of romantic Clytie, who loved the sun god, Helios. Day after day she turned to watch his dazzling chariot ride over the sky. But proud Helios never noticed her. In time, a goddess took pity and changed doleful Clytie into a sunflower. This, said the Greeks, explained why a sunflower always turns to face the sun.
Sunflowers put on a grand show for you, even in a small backyard. They shoot up fast, sprout enormous daisy type heads and perform a remarkable feat every day. In the morning, they face toward the east. As the day wears on, the huge flowers inch around to keep facing the sun as it moves over the sky and down toward the west. They really do.
Scientists use two terms to explain this. One is "turgor pressure," which is the brimming moisture content that keeps plant cells stiff and sturdy. The other is "phototropism", which means bending toward the light. These two operations are run by water and light.
Water is absorbed by the roots and uses the plant's plumbing system to seep from cell to living cell. Meantime moisture evaporates through leaves into sunny air. Hence the streaming water content varies. When cells are stuffed full of moisture, they are stiff with turgor pressure. When they cannot replace enough evaporated moisture, they lose turgor and their plants sag like stalks of wilted celery.
Plant cells also gain size and vigor from auxins. These strange chemicals are affected by light. When sunbeams strike one side of a plant, they send auxins scooting to the shady side where they boost the cells. For example, let's place a potted plant on a window sill. The side facing the light loses moisture, turgor and auxins. The shady side loses less moisture and gains more auxins. Its extra stiffening pushes the leaves and stems toward the window. This bending toward the light is phototropism.
The sunflower is super sensitive to both turgor pressure and phototropism. Light from the morning sun reduces turgor pressure on the eastern side of the stem and drives back the vigorous auxins. The sunflower bends its flabby chin to face the rising sun.
As the sun moves around the sky, its beams strike from different directions. At noon, the stiffening around the stem is fairly evenly distributed and the sunflower lifts its face up toward the sun ovezliead: In late afternoon, the sunbeams strike from the west and the big beauteous flower bends to face the setting sun.
Scientists learned long ago how moisture is absorbed and evaporated to create turgor pressure in living plant cells. But this only partially explained the daily motions of the sunflower. A few years ago, auxins and their light sensitive activities were discovered. They helped to explain more of the sunflower story: