Plant Responses to Changes in the Environment (A-level Biology)
Plant Responses to Changes in the Environment
Tropism
A tropism is a directional growth in plants in response to a stimuli. Plants must respond to changes in their environment to increase their chances of survival.
A growth towards a stimulus is a positive tropism.
A growth away from a stimulus is a negative tropism.
Types of Tropisms
- Hydrotropism is growth in response to water.
- Geotropism is growth in response to gravity.
- Phototropism is growth is response to light.
For example, the shoots of a plant show positive phototropism (grow towards light) and negative geotropism (grow away from gravity).
The roots of a plant show positive hydrotropism (grow towards water) and positive geotropism (grow towards gravity).
Some plants can respond to herbivory, which means being eaten by animals. To prevent herbivory, they can do several things:
- Chemical defence such as tannins, alkaloids and pheromones. Tannins taste bitter and animals can have difficulty digesting them. Alkaloids create bitter tastes and bad smells, which put animals off. Pheromones can be released by a plant into the air to warn other plants of potential threats.
- Folding in response to touch such as Mimosa pudica. This plant responds to touch by folding up, in the process knocking off insects and protecting itself from being eaten.
Growth Factors
Auxins
- To enable a response to any stimuli, plants must produce growth factors. These are chemicals that alter the rate of plant growth, and are likened to human hormones.
- Auxins are a type of these chemicals that control the growth of plants by promoting cell division and cell elongation.
- Auxins are produced in the tips of the growing stems and roots, but can diffuse to other areas of the plants as necessary.
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