Scientists call the external factors which direct the movements of plants tropism. The movement of plants toward light is called phototropism. Movement away from or toward the pull of gravity is called geotropism.

Tropism comes from the Greek word trope, meaning, "to turn." When plants are exposed to light, they always turn toward the light and away from the shade. Leaves tilt toward the Sun in order to absorb the greatest amount of light.

Phototropism is the result of a plant hormone that botanists have named auxin. Auxin literally means to increase. Auxin is present in all green plants. It has the peculiar characteristic of always moving away from the lighted side to cells on the shaded side of the plant.

High concentrations of auxin cause stern cells to grow more rapidly and to elongate. Its absence causes cells to slow their growth and to shorten. This causes a stem to grow unevenly. The side away from the light grows more rapidly than the side toward the light. As a result, the stem begins to bend, and as the stem bends, the plant curves toward the light.

Researchers have also found auxin in the roots of plants. Auxin is as sensitive to gravity as it is to the light, and is transported by the root cells toward the pull of gravity, to the lower side of the root. In roots, high concentrations of auxin inhibit the growth of cells while low concentrations stimulate the growth of cells. This causes the root to bend downward as it grows.

When auxin is distributed evenly among the cells of stems and roots, they continue to grow straight without bending one way or the other.

The Sun Regulates the Schedule of Plants

In the Northern Hemisphere, the length of time of the daylight and the darkness are equal only on March 21^st and September 23^rd. On every other day of the year, the lengths of the day and the night are unequal. These lengthening or shortening periods of daylight serve as alarm clocks which signal plants when it is time to flower, drop their leaves, or prepare for winter.

"Dew is formed on leaves when the Sun shines down on them and makes them perspire." - 11-year old, on a science exam

Photoperiodism is the response of plants to changes in the length of the day. Photochrome inside the cells of most plants measures the length of the day by absorbing light. It causes "short-day" plants such as chrysanthemums, ragweed, goldenrod, soybeans, and poinsettias to flower during the spring and fall. "Long-day" plants such as hollyhocks, clover, irises, radishes and beets bloom in the late spring and early summer when the days are longer.

Photochrome also lets a plant know when winter is coming. As the number of hours of daylight decreases in the fall, photochrome stimulates plants to form winter-resistant buds. Photochrome also stimulates leaf stems to seal off the flow of water and nutrients to a plant's leaves. When the length of day begins to shorten, twigs from a cork-like plug called an abscission zone, at the base of each leaf. As the plug thickens, it literally strangles the leaf to death.

When the leaf dies its chlorophyll starts to fade and it loses its green color. This reveals rich colors which have been masked all summer long by the abundance of chlorophyll. Eventually the leaf breaks off at its base and falls to the ground.

The Sun Provides Food

Living things which produce their own food from energy they get from the Sun are called autotrophs. Most plants convert sunlight to food through a process called photosynthesis so they are autotrophs.

Photosynthesis involves only four major ingredients: water, carbon dioxide, chlorophyll, and sunlight. When light strikes a molecule of chlorophyll, its electrons absorb energy. This excess energy excites the chlorophyll's electrons so much that they bounce from one chlorophyll molecule to another.

As the excited electrons bounce around, they split apart molecules of water, producing both hydrogen and oxygen. The oxygen is released into the air, and the hydrogen combines with carbon dioxide to form the basic building blocks which plants use to manufacture food.

While this may seem simple, the actual process is a very complex series of reactions. The first phase of photosynthesis involves only water, light, and chlorophyll. When chlorophyll traps light and releases electrons, energy is stored in the form of ATP. ATP is a chemical known as adenosine triphosphate. ATP contains three phosphate groups; ADP, adenosine diphosphate contains only two phosphate groups. When ADP picks up a third phosphate, it begins to store energy but retains its name. When ATP loses a phosphate, it releases energy and is called ADP. ADP and ATP provide the mechanism through which chlorophyll is able to trap and store the energy of the Sun.

On overcast days or during the late fall when the angle of the Sun decreases, photosynthesis stops because there is not enough energy to split the water molecules apart. The process of photosynthesis also breaks down at temperatures above 90 degrees F because the enzymes of many plants fail to function properly at high temperatures. This means many plants stop producing food on hot summer afternoons.

The second phase of photosynthesis splits water molecules into oxygen and hydrogen. This requires large amounts of energy that comes mainly from the ATP molecules that have been storing up energy from phase one. Using this energy the oxygen is released into the air, and the hydrogen combines with carbon dioxide to form the building blocks of plant food. The leftover hydrogen and carbon dioxide combination then reacts and forms glucose. Glucose is a plant's basic storage unit of energy. Photosynthesis is one of the most efficient energy systems known.