How does a corn seed know which way to grow?
A plant response that involves a specific movement is called tropism from the Greek word “to turn”. And any factor that brings forth such a response is called a stimulus.
Tests done way back in 1806 confirmed that gravity was the primary cause of plants growing in the correct direction. The tests showed that moisture was not the cause. Plant shoots kept in the dark still grew up and roots grew down, so light was not the primary reason.
How did they prove that gravity was the culprit? A Dr. Knight put seedlings in a rotating wheel, so they had an artificial gravity-like pull of centrifugal force. The plant roots grew downward at a 45 degree angle. The 45 degree angle was the result of both centrifugal force and gravity.
So the tropism responsible for plants growing in the correct direction is geotropism, a response to gravity. NASA uses the term gravitropism.
Thigmotropism is a response to touch. It accounts for the twining or wrapping of a vine around an object and the climbing plants and ivy you see up the sides of buildings and old windmills out in the country.
Cells that are touched produce auxin, a plant hormone, and transport it to untouched cells. The untouched cells on the outside of a bend grow faster than the touched or contact cells. This causes the tendril or vine to curve toward the side of contact. It’s almost a miracle how that happens!
The Scheckel boys had to go through certain areas of the corn fields on that Seneca farm and pull the Morning Glories that wrapped around a corn stalk. We didn’t know at the time that we were at war with thigmotropism.
Another tropism is phototropism, where light is the stimulus. Some sunflowers exhibit this phenomena. The sunflower plant head bends toward a light source, the sun, allowing more light to reach more cells to produce photosynthesis. That same plant growth hormone, auxin, moves to the dark side of the stem. The dark side grows longer causing the plant head to bend toward the light.