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iGCSE Edexcel Biology
Plants respond to stimuli.
Tropisms are directional responses to their environment.
Phototropisms are responses to light.
Geotropisms are responses to gravity.
Roots and stems have geotropic responses, roots positive
(towards gravity), stems negative (away).
Stems have positive phototrophic responses (towards light).
Their responses are much slower than animal responses as
they don't have a nervous system. However some plants
(Venus fly traps) do react quickly.
The shoot tip grows towards light.
When unidirectional light (light from one direction) hits the
stem, auxins (plant growth hormones) build up on the dark
side which stimulate growth. This makes the dark side grow
more, making it grow towards the light.
This can be shown experimentally by covering the tip of a
plant and not covering another plant, then pointing
unidirectional light towards them:
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GCSE Edexcel Biology
Gravity acts downwards on the plant.
Even if a seedling is placed on its side the root tips will always
Auxin levels may be a part of this response however as of yet
not much is known about it.…read more