How Does Your Garden Grow?

The following article is from the book Uncle John’s Bathroom Reader Nature Calls.
The way in which plants move and grow has intrigued everyone from the poets of ancient Greece to Charles Darwin, but modern science still hasn’t cracked all the secrets.
WHY WOULD THEY DO THAT?
Since ancient times, people have been scratching their heads and wondering why plants grow the way they do. In particular, why do shoots grow up out of the ground, while roots head down into the earth? Particularly intriguing to the ancient Greeks was the question of why plant stems bend toward light, a phenomenon that scientists now call phototropism.
The Greeks offered one explanation in a myth about a beautiful water nymph named Clytie, who fell in love with the sun god Apollo as he rode a fiery chariot across the sky. As is usual in myths, things didn’t end well: Apollo loved someone else, and when Clytie realized that her infatuation was hopeless, she stood weeping in the same spot without eating or drinking. All she did was watch for Apollo and his chariot. Eventually the grieving nymph transformed into the first sunflower, which turned on its stalk as it followed the sun. Through the centuries, scientists offered more… well, scientific explanations, but it wasn’t until the 19th century that people really began to understand phototropism.
TIPSY

In 1809 Swiss botanist Augustin de Candolle discovered that plants turned toward light because was there was more growth on the shady side of the plant (making it longer) than on the sunny side. Then in 1880 Charles Darwin (already famous for his theory of evolution) wrote a book called The Power of Movement in Plants, which explained how that uneven growth occurred. Darwin and his son Francis had experimented with the seedlings of canary grass that they grew to feed their birds. They covered the tips of the seedlings with a material that blocked light. This stopped the plants from turning toward the light even though light was still shining on the stems. When the tips of the seedlings were uncovered, the plants went back to bending toward the light again.
Darwin wrote that these and other experiments demonstrated that the grass seedlings had “some matter in the upper part which is acted on by light, and which transmits its effects to the lower part.” It wasn’t until the 1920s that the “matter” was discovered. Botanist Fritz Went found that it a hormone caused plants to turn toward the sun. He named the hormone auxin.

MAKING THEIR MOVE
Today scientists know that not only phototropism, but all “tropisms” (turnings in plants) are affected by auxin. The hormone is produced at the top of the main stem of a plant (just as Darwin thought) and moves downward to the root tips, where it’s stored. Auxin stimulates growth because it causes plant cells to get longer. The larger the concentration of auxin in a plant cell, the more the cell lengthens. So when auxin accumulates in certain places in the plant, it causes unequal growth that makes plants bend and turn in certain directions, creating a tropism.
Tropisms are especially important because they help plants reach out and get the elements they need for survival. For example, all green plants need light to produce oxygen and make food, a process called photosynthesis. Since plants can’t get up and walk over to the light source, they use auxin to create a tropism (phototropism) that bends toward it. But there are other tropisms out there. Here are the rest:
• Geotropism is turning in response to gravity. When a seed is in the ground and a root emerges, auxin collects and works to make the upper side of the root grow longer, turning the root tip down. Meanwhile, auxin collects on the underside of the stem, causing the bottom to grow faster than the top, which pushes the stem upward.
• Thigmotropism is turning in response to touch. It causes plants to “feel” their way up walls, poles, or trellises. An example is a vine that coils its tendrils around a trellis for support. The coiling of this tendril is believed to be the result of more auxin in the cells of the outer side of the tendril than on the inner side.
• Hydrotropism is turning in response to moisture. Roots grow down toward gravity, but auxin will also collect in root cells and direct their growth toward moisture.
• Positive and negative tropisms. Some of the tropisms that plants use are called positive because they turn a plant toward a stimulus like light, moisture, or gravity. Negative tropisms use auxin to bend plants away from that stimulus. These are usually mentioned when talking about gravity: Positive tropisms cause the plant roots to grow down toward gravity. Negative tropisms cause the plant stem to grow upward, away from gravity.
A GARDEN VARIETY MYSTERY
Scientists know a lot about tropisms and auxin, but they have many unanswered questions too. In particular, they still don’t understand the mechanisms at work within the plant and how those are able to send out auxin in response to light, moisture, gravity, and touch. But they keep looking… and hoping that the plants will reveal more about their methods.