CELL REACTION TO ITS ENVIRONMENT

All living things are capable of responding to internal and external stimuli. The ability to do this is called irritability or sensitivity. By this means, plants and animals are able to detect and respond to changes in their environment. Irritability is one of the basic characteristics of living organism.

Types of Responses

Generally, animals respond very quickly to external stimuli while plants respond slowly. Again, only certain organs respond to external stimuli in plants but in animals, often, the whole organism respond to the stimulus. Organs of higher plants that respond to stimuli are:

1. Flowers.
2. Leaf.
3. Root.
4. Stem.
5. Tendril.

There are three major types of responses. These are tactic, nastic and tropic movements.

Taxis or Tactic Movements

Taxis or Tactic response is a directional type of response or movement in which a whole organism moves from one place to another in response to external stimuli such as light, temperature, water or certain chemicals.

Locomotion or movement could be positive or negative.

Examples of tactic movements are

• Thermotaxis.
• Phototaxis.
• Hydrotaxis.
• Chemotaxis.

1. Thermotaxis: This is the movement or organism in response to temperature or heat. e.g. Motile bacteria or snail moving away from hot region to a cold region (negative thermotaxis).
2. Phototaxis: This is the movement in response to light stimulus. It is the source of light by organisms. E.g. Euglena moving away from light.
3. Hydrotaxis: This is the response of a whole organism to the stimulus of water or humidity. E.g. Woodlouse moves towards areas of high humidity.
4. Chemotaxis: This is the movement of an organism in response to a chemical stimulus; e.g. Amoeba , volvox. chlamydo- monas, Euglena, tilapia and paramecium moving away from chemicals.

Nastism or Nastic Movements

Nastism is a type of response in which a part of a plant moves in response to non-directional stimuli such as changes in light intensity, temperature or humidity.

The response movements are also non-directional. Examples of nastic movements are:

1. closing of the Morning glory flower when the light intensity is low,
2. the petals of Sunflower which open in the light and close in the dark,
3. the folding of the leaflets of the Mimiosa plant when it is touched,
4. the closing of the leaflets of the flamboyant tree i.e. sleeping movements due to low light intensity examples are Pride of Barbados, four-o-clock plant, morning flower Desmodium, Glover, etc.

Tropism or Tropic Movements

Tropism is a type of response in which a part of a plant moves in response to a directional stimulus. The direction of the response is related to that of the stimulus and is positive if the plant part grows towards the stimulus, and negative if the part grows away from it.

Tropisms are very slow growth movements. They are named according to the stimuli, e.g. phototropism is a response to light while hydrotropism is a response to water.