Tropism, taxes and Kinesis

What is a tropism

A growth movement in response to a stimulus

What is IAA

Indoleacetic acid,

a growth factor (like plant hormones)

It is a type of auxin

Where is IAA produced? How does it move through the plant?

It is produced in the tips of shoots and roots and it moves by diffusion

What effect does IAA have on root cells

It inhibits cell growth and elongation

What effect does IAA have on shoot cells

It stimulates cell growth and elongation

Gravitropism in roots

IAA moves downwards - towards gravity

So roots grow towards gravity because the cells in the upper surface of the root will grow and elongate more

whereas the cells in the lower surface will be inhibited

This is known as positive tropism

Gravitropism in shoots

IAA moves towards gravity

shoots grow away from gravity

because the lower side has more IAA so the cells are there grow an elongate more

whereas the cells in the opposite side of the shoe have less IAA so do not grow and elongate as much

this is known as negative gravity tropism

Phototropism in roots

IAA moves away from light

roots grow away from light

because the cells nearer the light will grow and elongate more than the cells nearer the dark

the cells near the dark will be inhibited.

This is an example of negative phototropism

Phototropism in shoots

IAA moves away from light

shoots grow towards light because the shaded side has more IAA so the cells there grow and elongate more

this is an example of positive phototropism

Hydrotropism in roots

IAA moves towards moisture

roots grow towards moisture because the cells in contact with the drier conditions will grow and elongate more

whereas the cells in the side of the root exposed to moisture will be inhibited.

This is an example of positive hydrotropism.

Taxis and kinesis

These are simple responses that maintain a mobile organism in a favourable environment

What is a taxis?

A directional response to a stimulus

How would an animal which uses taxis response to an unfavourable stimulus move?

It would move directly away from the stimulus, probably in a straight line

how would an animal which uses taxis respond to a favourable stimulus?

It would move directly towards the stimulus, probably in a straight line

What is kinesis?

A Non-directional response to a stimulus

How would an animal which uses kinesis respond to experiencing an unfavourable stimulus?

It would have a lower rate of turning but a higher rate of movement this would help get out of an area and move to different (favourable) conditions

How would an animal which uses this kinesis respond to experiencing a favourable stimulus?

It would have a higher rate of turning and a low rate of movement. This would help the animals stay in the same area.