Welcome to Tropism & Plant Hormones.
- Click on the highlighted links for videos presenting Tropisms.
- Read the facts below.
- Review the slides show at the bottom of the blog.
- Add the pink colored terms & definitions on your 3 x5 cards.
- Due: 12/11
- Quiz: 12/13
TROPISM #2
'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.
There are different types of tropisms:
- Tropism – growth in response to a stimulus
- 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
- Thigmotropism - the directional bending or turning response of a plant upon contact with a solid surface or object; it is basically a sense of touch in plants. For example, the tendrils of vines are thigmotropic.
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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.
Auxin experiment
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
Rooting powder - Rooting powder makes stem cuttings quickly develop roots. Rooting powder contains growth hormones.
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
Uses of plant hormones
Weedkillers
Selective weedkillers kill some plants but not others. This can be useful for getting rid of dandelions in a lawn without killing the grass, or getting rid of thistles in a field without killing the wheat plants. The selective weedkiller contains growth hormone that causes the weeds to grow too quickly. The weedkiller is absorbed in larger quantities by the weeds than the beneficial plants.
