• The structures involved in detecting the changes may be located far away from the ones that respond.
  • There is need for a communication system within the body.
  • The nervous system and the endocrine system perform this function, i.e. linking the parts of the body that detect changes to those that respond to them.


  • Living organisms are capable of detecting changes in their internal and external environments and responding to these changes in appropriate ways.
  • This characteristic is called irritability, and is of great survival value to the organism.


  • A stimulus is a change in the internal or external environment to which an organism responds.
  • Examples of stimuli include light, heat, sound, chemicals, pH, water, food, oxygen and other organisms
  • A response is any change shown by an organism in reaction to a stimulus.
  • The response involves movements of the whole or part of the body either towards the stimulus or away from it.
  • It also results in secretion of substances e.g. hormones or enzymes by glands.


  • Co-ordination is the working together of all the parts of the body to bring about appropriate responses to change in the environment.


  • Reception is the detection of changes in the environment through receptors.

Irritability in Plants

  • Response in plants is not as pronounced as in animals.
  • This does not in any way diminish the importance of irritability in plants.
  • It is as important to their survival as it is in animals.
  • Plants respond to a variety of stimuli in their environment.
  • These stimuli include light, moisture, gravity and chemicals.
  • Some plants also show response to touch.


  • Plants often respond by growing in a particular direction.
  • Such growth movements are called tropisms.
  • They are the result of unequal growth in the part of the plant that responds.
  • The stimulus cause unequal distribution of growth hormones (auxins) produced in the plant.
  • One side grows more than the other resulting in a bend either towards the stimulus (positive tropism) or away from the stimulus (negative tropism).


  • If seedlings are exposed to light from one direction, their shoots grow towards the light.
  • This response is called phototropism.
  • Shoots are said to be positively phototropic because they grow towards the light.
  • The tip of the shoot receives the light stimulus from one direction (unilateral stimulus) but the response occurs below the tip.
  • The response of the shoot is due to a hormone called auxin produced at the tip.
  • It diffuses down the shoot to this zone of cell elongation where it causes the cells to elongate.
  • Light causes auxin to migrate to the darker side.
  • The auxin is more concentrated in the dark side than on the light side.
  • The cells on the dark side grow faster than the ones on the light side.
  • A growth curvature is therefore produced.

Survival value:

  • Positive phototropism by shoots ensure that sufficient light is absorbed by leaves for photosynthesis.


  • Geotropism is a growth response to gravity.
  • Roots are positively geotropic because they grow down towards the direction of the force of gravity;
  • shoots are negatively geotropic because they grow away from direction of force of gravity.
  • If a seedling is kept in the dark with its plumule and radicle in a horizontal position, the plumule will eventually grow vertically upwards while the radicle will grow vertically downwards.
  • The effect of gravity on roots and shoots can be explained as follows:
  • When the seedling is placed in a horizontal position, more auxin settles on the lower side of the root and shoot due to the effect of gravity.
  • Shoots respond to a higher concentration of auxin than roots.
  • The lower side of the shoot grows faster than the upper side.
  • Resulting in a growth curvature that makes the shoot grow vertically upwards.
  • Root growth is inhibited by high concentrations of auxin.
  • Therefore, the lower side of the root grows at a slower rate than the upper side where there is less auxin concentration.
  • These results in a growth curvature that makes the root grow vertically downwards.
See also  ENZYMES

Survival Value:

  • Roots in response to gravity grow downwards where they absorb water and get anchored in the soil.
  • This results in absorption of nutrients needed for growth.


  • Hydrotropism is the growth of roots towards water (moisture).

Survival Value

  • It ensures that plant roots grow towards moisture to obtain water needed for photosynthesis and transport of mineral salts.


  • Chemotropism is the response of parts of a plant towards chemical substances, e.g. the growth of the pollen tube towards the ovule in flowering plants is a chemotropic response.

Survival Value

  • This ensures that fertilization take place and the perpetuation of the species continues.


  • Thigmotropism is a growth response to touch. e.g. tendrils of climbing plant bend around objects that they come in contact with.

Survival Value

  • This provides support and the leaves stay in a position suitable for absorption of light and gaseous exchange for photosynthesis.

Tactic Movements in Plants and Other Organisms

  • A tactic movement is one made by a whole organism or a motile part of organisms (e.g. a gamete) in response to a stimulus.
  • Tactic movements are named according to the nature of the stimulus that brings about the response.
  • Phototaxis is movement in response to direction and intensity of light.
  • Free-swimming algae such as Chlamydomonas usually tend to concentrate where light intensity is optimum and will respond to light by swimming towards it. This is an example of phototactic response.
  • Osmotaxis is movement in response to changes in osmotic conditions e.g. freshwater amoeba.

Survival Value

  • Ensures favourable conditions for existence.
  • Chemotaxis is movement in response to concentration of chemical substances.

Survival Value

  • In bryophytes, antherozoids move towards archegonia to effect fertilisation Survival Value of taxis:
  • These ensure conditions favourable for life bring maximum benefit to the organism.

Nastic Movements

  • A nastic movement is one made by part of a plant in response to stimulus which is not coming from any particular direction.
  • Nastic movements are also named according to the nature of the stimulus.
  • Seismonasty/haptonasty – response to shock.
  • The ‘sensitive plant’ Mimosa pudica responds to touch by folding up its leaves.
  • This is an example of a seismonastic response.

See also:






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