Tropisms

In plants, a tropism is a reaction any directional stimulus such as gravity or light.

Gravitropism or geotropism is the plant's response to gravity. The roots show a positve reaction to the stimulus, growing toward it, while the stems show a negative reaction because they grow away from the stimulus.

Phototropism is the plants' reaction to light and is gennerally positive. This tropism works through the hormone axim. Axim is produced in the stems and leaves, but is always transfered to the side of the plant that is in shade. There, it loosens the cell wall and induces growth of the cell. This uneven growth causes the curvature plants have when getting light from only a single side.

When the plant is put in full shade, it tends to grow tall and narrow, because all parts of the stem act as if they were shaded, and grow. In bright areas, plants tend to be shorter and thicker, for the same reason, the axim doesn't affect the plant much because it gets a lot of light all around.

Thigmotropism, the movement or growth in response to touch or contact, also has a major effect on plant growth form. Just as trees exposed to a lot of wind and harsh conditions, such as krumholtz, are shot and stubby, all plants with respond to stimuli such as rain, wind, being touched or bumped, or any other vibrational force by having a shorter form and reinforcing their cell wall, to resist bending.

Thigmotropism, like phototropism, is caused by the hormone axim moving to the side of the plant that is not receiving the stimulus. As such, plants will try to bend into the wind, and tendrils of vines will curl around the bars of a trellis.

My brother and I have been attempting to grow basic garden plants, like beans and tomatoes, inside over the winter months. We have been using artificial lighting to help meet lighting requirements, but they still don't compare with the Sun. All of the plants have had issues with stems that are too thin to support their own weight, causing them to fall over in a tangled mess.

I now see that this is caused by phototropism, and that the use of thigmotropism might be able to counteract it. I'm going to work on designing it into a good project, and perhaps I will get a good science fair project before I'm out of high school.

Here's a good page if you want to continue reading: http://www.gardenguides.com/how-to/tipstechniques/seedsbulbs/hardening.asp