Barb Steen, age 10, of Le Roy, New York, for her question:

How do petals open and close?

A plant is rooted to the spot and we do not expect it to move. Nevertheless it does. As it grows, it lifts its top higher and spreads its twigs wider. All this is done in slow motion, too slow to watch. But many flowers can open their petals wide or close them shut in just a few minutes.

You use lots of little muscles to open your hand and you have lots of little bones to hold your fingers out stiff and straight. Those same muscles and bones can close your hand into a fist. But there are no muscles and no bones in the silky smooth petals of a pretty flower. A rose bud is a young flower with its petals tightly furled and neatly folded together. As it grows older, its petals spread apart and the rose opens its full face to be seen and admired. Many daisy type flowers open their starry petals at sunrise and close them together soon after the sun sets. The flowers of the morning glory open their bright blue eyes at dawn and close them at noon.

To understand how all this is done we must know how petals are put together. They are made of miniature building blocks called cells. We need a microscope to see them because it takes several thousand of them to measure one inch. The tiny cells in the petals of a flower are living things and very busy carrying on all sorts of complicated chemical processes. Each cell has a special skin called a cell wall that touches the walls of other cells. The inside of the cell is stuffed with a watery mixture of busy chemicals. The cell wall keeps the cell together but it also acts like a miraculous sieve. It lets in the chemicals the cell needs and keeps out those it does not need.

Water oozes through the thin wall and when the cell is filled, it stretches to the limit like a firm, fat balloon. So do zillions of cells around it. Together they puff up a petal and make it spread out stiff and firm. When all the petal cells are filled, the flower opens wide. But when all the cells lose some of their water, they sag and the petals droop. In order to open and close, a flower needs a number of soft, saggy cells and a number of stiff ones. The hinges are at the base of the petals. In these places, at certain times, some of the water seeps away from the cells and here the petals become soft and bendable. The rest of the petal cells remain stiff and filled with water. As a rule the changes that create these hinges happen at dawn and sunset.

Different plants have their own schedules for opening and closing their blossoms. And certain leaves also open and close each calendar day. Clover leaves and bean leaves usually fold themselves together when the sun sets and open up again in the morning. They also close up when you put them into a darkened room.

Plants, of course, have no sense to know what they are doing. Their changes are triggered by outside factors such as gravity, temperature, water and chemicals. We do not know what triggers the changes that cause a flower to open and close. One factor may be sunlight, another may be temperature. Many plants go through mysterious changes every 24 hours, almost like clockwork. Certain flowers will close up at night, even when they are placed under a brilliant sunlamp.