Untitled Flashcards Set

Tropisms

Specification Point 2.83

Understand that plants respond to stimuli.

Plants must respond to their environment in order to survive.

These responses are much slower than in animals.

In a plant, a movement towards or away from a stimulus is called a tropism:

Phototropism is a movement towards or away from light.

Geotropism is a movement towards or away from gravity.

Hydrotropism is a movement towards or away from water.

Tropisms can be either positive or negative.

Positive phototropism for example is growth towards a source of light.

Daisies exhibiting positive phototropism in response to light. Positive phototropism allows plants to orientate themselves to where there is most sunlight, maximising their rate of photosynthesis.

Shoots will grow upwards to maximise their ability to photosynthesise.The movement of shoots is an example of a positive phototropism. Shoots grow upwards towards sunlight. The movement of shoots is also an example of a negative geotropism. Shoots grow upwards, against gravity. Roots will grow downwards into the soil for nutrients. The movement of roots is an example of a negative phototropism. Roots grow downwards away from sunlight.The movement of roots is also an example of a positive geotropism.Roots grow downwards towards gravity.

The roots of plants are both positively geotropic (growing towards gravity) and negatively phototropic (growing away from light). The shoots of plants are both positively phototropic (growing towards light) and negatively geotropic (growing against gravity).

Auxins are a type of plant hormone. Auxins control the growth rate of plant cells. Most auxins speed up the rate of growth in plants. Auxins are made in the tip of growing stems. One type of auxin is called indoleacetic acid (IAA). IAA causes cells to elongate. IAA changes the behaviour of the plant cell wall, causing cells to stretch out.

When the plant is illuminated on all sides, IAA causes the cells in the shoot to grow at the same rate. The shoot will grow directly upwards. When light shines on one side of the shoot, IAA will accumulate on the shady side of the shoot: Cells on the shady side of the shoot will stretch. This will cause the shoot to bend towards the light. The mechanism is an example of positive phototropism.