Date: Fall 2011
Do plants always grow facing the light? How do they do this?
Many plants grow towards the light:
Anthony R. Brach, Ph.D.
Harvard University Herbaria c/o Missouri Botanical Garden
I may be wrong on this; I'm not a botanist. I understand that some
growth hormone in a plant's stem is inhibited by sunlight, so that the
shady side will grow faster. This makes the plant lean into the light
until its exposure more even.
Of course that isn't the only factor controlling plant growth. They
need to grow against gravity, too. A tree anywhere but at the equator
would fall over if it only grew toward the light!
Richard E. Barrans Jr., Ph.D., M.Ed.
Not ALL plants always grow facing the light. Some, sunflowers is a
classical example, do. On there is another flowering plant, called a moon
flower, remains closed during the day, and only opens after the sun has set.
Other plants, like evergreens, don't show a particular preference, as far as
I know. Plants need sunlight to activate photosynthesis, but some require
more than others, and others actually require shade. If they are exposed to
too much light, they don't do well and may even die from "sunburn". Examples
are tropical plants that thrive under a canopy of trees. And of course
certain specialized vegetation such as mushrooms don't need sunlight at all.
With a couple of exceptions, all plants grow toward the light. This is called phototropism. If you break down the word, “photo” means light and “tropism” means growth or turning.
Phototropism basically helps to position a plant so that it can harvest the maximum amount of light for photosynthesis. How does it do this? Plants have a number of chemical receptors that can detect light. These receptors, in turn, signal plant cells to grow. What makes this interesting is that only the cells on the shaded side are signaled to grow longer. Since one side of the plant has longer cells than the other, the plant literally bends toward the light!
If shaded (or kept in total darkness) for an extended period of time, plants will actually go searching for light. They’ll grow upright, tall and spindly. This is called etiolation. If they are exposed to light, then phototropism kicks in.
Dr. Tim Durham
Instructor, Office of Curriculum and Instruction
Department of Biological Sciences
Florida Gulf Coast University
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Update: June 2012