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.


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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