Phototropism
Phototropism
Phototropism is the term used to describe a plant's response to light. When we notice that a potted plant on a windowsill has turned its leaves toward the light, we are witnessing phototropism. This is but one form of tropism or plant "behavior." A tropism is a phenomenon in which a plant grows in response to some outside stimulus.
Although plants cannot move in the manner of other organisms, plants are living things and, therefore, are sensitive to external stimuli. Their reactions to the many different outside forces they meet sometimes give the impression that they have indeed moved. Although plants usually appear motionless unless they are moved by the wind, they are in fact growing
much of the time and responding to a variety of environmental stimuli. When a plant's reactions to a stimulus result directly in any type of plant growth, botanists call this phenomenon a tropism. Tropisms can be positive or negative. A positive tropism means that the plant begins to grow toward the outside stimulus. Negative tropism means that it grows away from the source.
When a plant responds to a light source by growing in the direction of that source, it is called positive phototropism. (Photo means light in Latin.) There are many other forms of tropisms, but all must involve plant growth as a response. Chemotropism is a plant's response to chemicals; thigmotropism is its response to being touched; geotropism is its response to the force of gravity. Since all tropisms can be positive or negative, the growth of a seed's roots downward into the soil is positive geotropism (in the direction of the source) but also negative phototropism (away from the light). The upward growth of the new shoot is the reverse (positive phototropism and negative geotropism).
Plants have evolved tropism in order to maximize a particular function and therefore be better able to compete, survive, and reproduce. When a plant grows toward the light (positive phototropism), it can grow more rapidly and if necessary, out-compete its neighbor for scarce resources. An obvious example of tropism is hydrotropism in which a plant during a drought will make its root system grow away from its natural, gravity-pulled downward course and off in a direction containing life-sustaining water. Tropisms are one means that plants have to battle for their survival.
The actual mechanism by which a tropism (stimulus/growth) occurs could be described as uneven growth. Specifically, when a stem or root moves toward or away from an outside stimulus, it must grow in a curve. It achieves this "curved" growth by having the outside of the curve grow faster than the inside. This is caused by the plant hormone called auxin. According to what type of specialized tissue is receiving the stimulus (such as a root or a stem), a larger amount of auxin moves from the growing tip and down one side than moves down the other. Since auxins come from the tip, when a plant wants to move toward the light it sends an unequal amount of auxin down its sides. More auxin goes to the shaded side and less to the sunny side, meaning that the shaded side grows more than the sunny side and the plant therefore grows in a curve toward the light source.