Phototropism
Tropism
is a growth response between a plant and an external
stimulus. The stimulus could be weather, touch, time,
gravity or light. A positive response is indicated by
growth toward a stimulus and a negative response is
indicated by growth away from the stimulus.
Light is a stimulus that plants respond to. This is called
phototropism (photo= light). Plants usually display a
positive phototropic response to light, which means they
grow toward a light source. Plant hormones called auxins
play a part in phototropism. Auxin is a plant growth
hormone. When light is shined on one side of a plant the
auxins move to the dark side of the plant. The hormones
stimulate the cells on the dark side of the plant to
elongate, while the cells on the light side of the plant
remain the same. This elongation on one side and staying
the same on the other causes the plant to bend in the
direction of the light. This bending allows more light to
reach more cells on the plant that are responsible for
conducting photosynthesis.
Source:
Biology
of Plants, 6th
ed.
Raven,
Peter H., Everert, Ray F., Eichhorn, Susan E. Worth
Publishing 1999.