Chapter 9: Reproduction in Humans

Plant hormones and geotropism, clinostats

1) Explain how geotropism affects a plants growth and how a clinostat overcomes the effects of geotropism.

If a broad bean seedling is placed in the dark in a horizontal position, its shoot will bend upwards and its root downwards. The root tip is showing a positive geotropism because it is growing towards gravity so that it has more chance of finding moisture. The shoot tip on the other hand is showing a negative geotropism because it is growing away from gravity to have more chance of finding light.

Because the motor turns the disc and germinating seeds around very slowly, any directional stimulus that may be acting on the seeds is eliminated by the movement; every side of the seedling is acted on by gravity, so the roots will grow horizontally.

2) Describe and give examples of why a clinostat is used. What sort of investigations would use one?

A clinostat is a piece of apparatus consisting of an electric motor turning a cork disc which can be set to rotate in a vertical or horizontal plane by a motor. Germinating seeds can be attached to the disc. Clinostats are used for control experiments, and make factors uniform (evenly distributed).

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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