Auxin or indoleacetic acid is a well-known plant hormone. Auxin can produce different effects on different plant tissues. High concentrations of auxin stimulate the growth of stem cells and inhibit the growth of root cells. Auxin produced in the apical meristem of a stem causes the stem to grow toward the light and away from the pull of gravity. If the source of light is directly overhead, and if the stem is growing vertically, the concentration of auxin will be the same on all sides of the stem. If the source of light is to one side of the stem, however, auxin will move away from the light to the shaded part of the stem. This will cause the stem to bend toward the light. If a plant is knocked on its side, auxin will accumulate in the lower side of the stem, or the side along the ground, because this side is darker than the upper side. The stem will curve up, even in the absence of light.

Auxin produced in the apical meristem of roots causes the roots to grow away from the light and toward the pull of gravity. If a growing root is exposed to light, auxin will concentrate in the shaded side of the root, just as it did in the shaded side of the stem. But high concentrations of auxin will slow down the growth of root cells. Thus cells in the side of the root where auxin is high will not elongate. Cells in the lighted side of the root will elongate normally, causing the root to bend away from the light.

In addition to controlling cell growth, auxin can also cause cell division in meristematic regions to start or stop. Scientists have learned that high concentrations of auxin inhibit the growth of lateral buds, whereas low concentrations of auxin stimulate growth. Thus the high concentration of auxin produced by the apical meristem inhibits the growth of lateral buds near the tip of the shoot.

These responses of plants to the environmental stimuli are called tropisms from the Greek word that means turning. If a plant grows toward a stimulus, it is said to have a positive tropism. If it grows away from a stimulus, it is said to have a negative tropism. There are several different kinds of tropisms. A plant’s response to light is called phototropism. A response to gravity is called gravitropism. From the very beginning of a plant’s life, these positive and negative tropisms direct the plant’s growth. Hormones, like auxin, control these relatively simple responses of plants.

The purpose of this experiment is too see how roots and stems are affected by gravitropism and phototropism. I expect to see the roots in the petri dish grow towards the pull of gravity and the stems to grow away from the pull of gravity. I expect to see three of the seeds in the cup grow towards the light and the other three seeds with aluminum caps to grow straight up.

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