(professor)

Today I'll be talking about the concept of tropism as it relates to plants. Tropism, for those of you who don't know, refers to a bending of a plant or a part of a plant in response to an outside stimulus.
There are three important kinds of tropism. They are phototropism, geotropism, and hydrotropism. In each of these kinds of tropism, a plant, or a part of a plant, bends in response to a different kind of outside stimulus.
First, we'll discuss phototropism. The outside stimulus in phototropism is light. When a plant is affected by phototropism, it grows in the direction of a light source such as the Sun.
The second kind of tropism is geotropism. In geotropism, the outside stimulus is gravity. In a plant affected by geotropism, the affected part of the plant grows directly downward because of the pull of gravity. When a plant's affected by geotropism, it's often the root structure that's affected.
OK. The final kind of tropism I'll discuss today is hydrotropism. When hydrotropism affects a plant, this means that the plant is drawn toward water. A plant under the effect of hydrotropism will grow in the direction of its water source.