Tropism in Plants

Tropism - a growth in response to a stimulus e.g. Plants grow towards sources of water and light.

Auxin - a plant hormone produced in the stem tips and roots that controls the direction of growth. Plant hormones are used in weedkillers, rooting powder and to control fruit ripening.

Tropisms

Plants require light and water for photosynthesis. Tropisms are responses they have acquired to ensure that they grow towards sources of light and water.

There are two main types of tropisms:

- Positive tropisms – the plant grows towards the stimulus

- Negative tropisms – the plant grows away from the stimulus.

Phototropism - a growth response to light from one direction

Geotropism - a growth response to the direction of gravity ('geo' refers to the Earth)

The roots and shoots of a plant respond differently to the same stimuli:

Responses to stimulus of different parts of the plant

Positive phototropism in plant stems

Tropisms are controlled by plant hormones called auxins. These water-soluble chemicals move through the plant in solution.

There are 2 stimuli acting on the plant in this picture:

- the direction of the light that falls on the plant

- gravity

Both light and gravity are directional stimuli (they act in a particular direction). The growth response of a plant to a directional stimulus is called a tropism. The stem of the plant in the picture above is showing a positive phototropism and a negative geotropism.

The 'shoot' of a plant needs light to carry out photosynthesis; a positive phototropism is the strongest tropic response of the shoot. This ensures that the leaves will receive the maximum amount of sunlight. You can see this response on a plant placed near a window or another source of unidirectional light.

However, the shoot has a negative geotropism. The roots of plants are strongly positively geotropic. This response ensures that the roots grow down into the soil where they can reach water and mineral ions, and to anchor the plant.

Sources:

http://www.bbc.co.uk/schools/gcsebitesize/science/ocr_gateway/understanding_organisms/control_plant_growthrev1.shtml

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