Tropisms in Plants
Tropisms in Response to Environmental Cues
- Plants respond to changes in their environment.
- Phototropism: directional growth in response to a light source.
- Geotropism: directional growth in response to gravitational pull.
- Hydrotropism: directional growth in response to a water source.
- Chemotropism: directional growth as a result of chemicals in the environment.
- Thigmotropism: directional growth in response to touch.
- Tropisms are important in plants, as if unable to respond to their environment plants may become malnourished, and ultimately dead.
- Monocotyledonous seedlings are most commonly used in research on tropism, as the stem has no lateral growth and change can be seen and manipulated rapidly.
- Phototropisms are the result of auxins moving across the shoot or root, so that the stem bends towards the direction in which sunlight is generally stronger.
- If grown in normal conditions of gravity, with equal amounts of light all around, then the stem is more or less straight.
- Shoots are positively phototropic, and roots are negatively phototropic.
Practical Explorations into Phototropism
- Explorations into tropisms can be done by...
- Growing plants in omnilateral, unilateral or completely without light then observing rates and directions of growth.
- Growing plants with different filters, to see which wavelength of light trigger phototropism.
- Darwin and Boysen-Jensen are the basis of our understanding of phototropisms.
- Darwin provided evidence for the hypothesis that the growth comes from the apex.
- Boysen-Jensen provided evidence for the hypothesis that the 'messenger' that causes phototropism was a chemical.
- Without light, plants often grow more rapidly. This is because growing upwards is the most likely way for a plant to find light, to enable photosynthesis and survive.
- Gibberellin is responsible for the stem elongation, and scientists have found that gibberellin levels fall once a light source is introduced.
- Etiolation is the process of 'forcing' growth, by growing plants in a dark room, such as rhubarb. This causes the plant to be long, thin and pale.
Geotropisms
- Plants are also sensitive to gravitational pull.
- Shoots are negatively geotropic, and roots are positively geotropic.
- A clinostat is a constantly rotating horizontal drum which can cause a plant grown horizontally to grow straight out, as regular rotation causes equal force of gravity on all sides.
- A petri dish mounted on a wall with germinating seeds in can show positive geotropism in roots, as regular 90 degree rotations cause the roots to spiral as they have to change direction towards gravitational pull each time.
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