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MOVEMENT
TROPISM
A 'tropism' is a growth in response to a stimulus. Plants grow towards sources
of water and light, which they need to survive and grow.
Auxin is a plant hormone produced in the stem tips and roots, which controls
the direction of growth. Plant hormones are used in weedkillers, rooting powder
and to control fruit ripening.
Tropisms
The direction of plant growth
Plants need light and water for photosynthesis. They have developed responses
called tropisms to help make sure they grow towards sources of light and water.
Positive phototropism in plant stems
There are different types of tropisms:

positive tropism – towards the stimulus

negative tropism – away from the stimulus

phototropism – growth in response to the direction of light

geotropism – growth in response to the direction of gravity
Responses of different parts of the plant
response
part of direction
of advantage
plant
growth
positive
stem
growth
to get maximum light
phototropism
tip
towards light
for photosynthesis
negative
root tip growth
phototropism
positive
away less chance of drying out
from light
root tip towards
geotropism
gravity
negative
stem
away
geotropism
tip
gravity
more chance of finding
moisture
from more chance of finding
light
Plant hormone
Controlling the direction of growth
Auxin is a plant hormone responsible for controlling the direction of growth of
root tips and stem tips in response to different stimuli including light and
gravity.
Auxin is made at the tips of stems and roots. It's moved in solution to older
parts of the stem and root where it changes the elasticity of the cells. More
elastic cells absorb more water and grow longer, causing bending in the stem or
root. It's thought that light and gravity can interfere with the transport of
auxin causing it to be unevenly distributed.
You may have noticed that a houseplant grows towards the window and turns
its leaves towards the light. It does this because light coming from the window
side of the plant destroys the auxin in that side of the stem. So growth on
that side slows down.
On the shaded side of the plant there is more auxin. So growth on this side
speeds up. The result is that the shoots and leaves are turned towards the
light for photosynthesis.
Auxin is produced in the tip of growing shoots.
If the tips are removed from growing shoots they cannot produce auxin, so
phototropism cannot occur.
If the tips are covered, light cannot break down the auxin, so phototropism
cannot then occur either.
Auxin is produced in the tip of growing shoots.
If the tips are removed from growing shoots they cannot produce auxin, so
phototropism cannot occur.
If the tips are covered, light cannot break down the auxin, so phototropism
cannot then occur either.