Controlling plant growth
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.
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.
There are different types of tropisms:
|Response||Part of plant||Direction of growth||Advantage|
|Positive phototropism||Stem tip||Growth towards light||To get maximum light for photosynthesis|
|Negative phototropism||Root tip||Growth away from light||Less chance of drying out|
|Positive geotropism||Root tip||Towards gravity||More chance of finding moisture|
|Negative geotropism||Stem tip||Away from gravity||More chance of finding light|
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.
3 groups of seeds are grown in a cardboard box.
A - when the tips are removed, no auxin is made so the stems do not grow
B – when the tips are covered, auxin moves to all parts of the stem causing all parts to grow
C - when the tips are lit from one side only auxin accumulates on the shaded side causing it to grow more than the illuminated side
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