The pinnate and bipinnate leaves of many leguminous plants open and close as a result of certain changes in temperature, light intensity or humidity of the atmosphere. The process which is however gradual is known as “sleeping movement” and occurs in such leaves as St. Thomas plant, and cassia. Flowers of certain plants are known to open only at certain time of the day. A good example is the “Four O’clock” plant which opens at about 4 pm daily. Sunflower changes its direction to face the sun. The flowers of”Snake tomato” remain closed during hot day and open at night or under conditions of high humidity. These flowers are pollinated at night by nocturnal insects (moth). These various responses of plant parts to external stimulus of the weather are known as nastic responses or simply nastic movements.
Taxis (tactic movement)
Taxis: This is a locomotory movement of an entire organism or cell (e.g. gamete) in response to a directional stimulus. If the locomotory movement is towards the stimulus, it is positive taxis, if it is away it is negative taxis.
Animals respond to stimulus of:
- Light (phototaxis).
- Water (hydrotaxis).
- Chemicals (chemotaxis).
- Current (rheotaxis).
The response of animals to stimulus of light is called phototaxis, the response to stimulus of water is called hydrotaxis while the response to stimulus of chemicals is called chemotaxis. For example Euglena moves towards source of light (positive phototaxis). Earthworm move away from light (negative phototaxis).
Sperm move towards chemical substances produced by eggs (positive chemotaxis). Mosquitoes avoid repellent (negative chemotaxis).
Many freshwater fish (e.g. Tilapia) move against current.
Kinesis (kinetic movement)
Kinesis is the rate of change of a living organism in response to the intensity of stimulus and not the direction of the stimulus, that is, the stimulus does not control the direction of movement. For example, when food substances are added to water, the tentacles of Hydra increase their waving movement.