If you've ever planted pinto beans, chances are you just dropped them into a trench or a hole and didn't pay much attention to which side of the bean was up. Yet your plants probably all germinated just fine and grew as expected: with the roots growing down into the soil, while the leaves pushed up toward the air. A phenomenon called gravitropism explains how plants "know" up from down and why you never check your garden and find pinto bean roots protruding from the earth and the plant forming leaves and beans underground.
If you carefully cut a pinto bean in half, you can observe the structures that play a role in orienting the plant so that it grows correctly. The hard outer covering, called the testa or seed coat, protects the embryo from harm. Seeds belonging to the legume family--which includes the pinto bean--include a pair of primitive leaves that contain nutrition to sustain the plant and a tiny root structure called the radicle.
Although gardeners and botanists alike have always observed gravitropism, the mechanism behind it was subject to speculation and is not fully understood today. Early hypotheses proposed that roots grew downward because they sensed higher humidity in the soil or because their weight pulled them in that direction. Eventually, British physiologist T. A. Knight discovered that gravity caused the growth habits of shoots and roots.
Gravitropism provides several benefits that help plants to survive. Plants draw water from the soil with their roots and synthesize sugar from sunlight, water and carbon dioxide in their leaves, meaning that roots must grow underground and leaves must have access to sunlight. Gravitropism ensures the correct orientation. Furthermore, as pointed out on Indiana University's Plants in Motion website, if you knock over a bean plant, gravitropism helps the leaves to grow upward one again, restoring access to sunlight.
Gravitropism exerts dramatic effects on plants. As Dr. von Sengbusch of the University of Hamburg points out, if you dig up a germinating bean seed and replant it with the roots pointing upward and the leaves downward, it will reorient itself. As the direction of the roots changes, small cellular structures called amyloplasts fall to one side of the cell. These inhibit growth on the undersides of the root cells. The region of the cell with fewer amyloplasts continues to grow, pushing the roots deeper into the earth.
The opposite holds true with respect to stem growth, explains retired Harvard biology professor John W. Kimball. In stems, the presence of amyloplasts stimulates growth, pushing the bean's small leaves upward, toward the surface and the sun.