The Ways in Which Coordination is Brought About

The coordination in simple multicellular animals takes place through nervous systemonly. The control and coordination in higher animals called vertebrates (including humanbeings) takes place through nervous system as well as hormonal system called endocrinesystem. Coordination in plants is under the control of hormones.

All the living organisms (plants and animals) respond and react to changes in the environment around them. The changes in the environment to which the organisms respond and react are called stimuli (singular: stimulus).

The living organisms show response to stimuli such as light, heat, cold, sound, smell, taste, touch, pressure, pain,water, and force of gravity, etc. The ability to perceive, interpret and respond to stimuli is called irritability or sensitivity.

There are two types of stimuli:

  • external
  • internal.

External stimuli are associated with the surrounding environment such as wind temperature, light, pressure, touch, water and gravity.

Internal stimuli occur within the organism, for example, a decrease or an increase in the amount of water and glucose in the blood.

When an organism detects a stimulus, it initiates a response. A response is a behavioural,physiological or muscular activity initiated by a stimulus. For example, if a man touches a very hot utensil accidentally, he quickly pulls his hand away from the hot utensil.

Here, heat is the stimulus and the man reacts (responds) by moving his hand away from the hot utensil. Similarly, when the sun is bright, we close our eyes. In this case, light is the stimulus and we react by closing our eyes. Likewise, when the amount of water in the blood drops, the pituitary gland secretes an anti-diuretic hormone (ADH) which stimulates the reabsorption of water in the kidneys.

Multicellular organisms detect stimuli through sense organs called receptors. A receptor is a sense organ (e.g. eye) or sensory nerve ending (e.g. in the skin or internal organ) which receives stimuli and sets nervous impulses. Impulses are electrical transmissions or chemical stimuli that are sent from the receptor to the coordinating system in the organism.

The organs that respond to the stimuli are called effectors. A coordinator is an organ (e.g. the brain and spinal cord) that receives messages from the receptors,translates them and sends the information back to an effector for action. An effector is a muscle or gland which receives impulses from nerves, brain or spinal cord and responds to them.

Response is the end-action, such as a muscle contracting to cause the movement of the arm. The diagram below illustrates the five components of coordination in mammals.

Most plant shoots are positively phototropic because they tend to grow towards light.Most roots are negatively phototropic because they away from light.

The Importance of Tropic and Nastic Responses

Importance of phototropism

Phototropism is important to plants because of it enables the plant leaves to be placed under direct sunlight to absorb maximum light so as to carry out photosynthesis effectively.


Geotropism is the movement of a part of the plant towards gravity. In most plants, roots-grow downward with gravity while shoots grow upward against gravity. Within hours, the shoot of a plant placed on its side will usually bend upward and the roots will bend downward as the plant reorients its direction of growth in response to gravity.

Most plant roots grow towards gravity and are said to be positively geotropic. Most hoots grow away from gravity and are said be negatively geotropic.

Importance of geotropism

Geotropisms are important to plants because of the following reasons:

  1. It enables the plants to send roots into the ground hence anchoring the plant firmlyinto the soil.
  2. It enables plant roots to absorb water and mineral salts from the soil.
  3. Negative geotropism exhibited by the shoot enables the shoot to grow upwards, and in so doing, exposes the plant leaves to maximum sunlight for effective photosynthesis


Hydrotropism is the movement of a plant or part of a plant towards water. Plant roots normally grow towards moisture. They are therefore positively hydrotropic. If you plant a plant near a water source such as porous pot or river, the roots will always grow towardswater.

Importance of hydrotropism

It enables the plants to absorb dissolved minerals and water. Water is necessary forvarious functions such as:

  1. Photosynthesis
  2. Numerous physiological reactions that take place within plant cells.
  3. Turgor pressure, which aids in plant support.
  4. Dissolution of mineral salts.

An example of positive and negative chemo-tropism is shown by a plant‘s roots; the roots grow towards useful minerals displaying positive chemo-tropism, and grow away fromharmful acids displaying negative chemo-tropism.

Chemotropism is the movement or growth of an organism or part of an organism in response to a chemical stimulus. During the process of fertilization the movement of pollen tube towards ovule due to secretion of a sugary chemical in the ovary is an example of chemotropism

Importance of chemotropism

  1. It enables the plant to absorb mineral salts from the soil when the roots grow towards beneficial chemicals such as fertilizers.
  2. Negative geotropism, such as when plant roots grow away from the toxins, enables the plant to survive by avoiding contact with such harmful chemicals.
  3. . It facilitates the fertilization process in flowering plants.

Thigmotropism or haptotropism

Thigmotropsim refers to non-directional movements which take place neither towards noraway from the stimulus. The best example of nastic movement is folding and drooping ofleaves of Mimosa pudica plant when its leaves are touched with fingers or any object.The leaves fold even when swayed about by wind.

Also the specialized touch-sensitivetendrils of many vining plants, such as pea, will bend toward the side receiving a touchstimulus. Continual stimulation can lead to the coiling of the tendril around an object,which enables vining plants to grasp objects on which they can climb

Importance of thigmotropism

1. Thigmotropism enables crawling plants to climb up higher plants and expose their leaves to sunlight for optimum photosynthesis.

2. It enables the insectivorous plants such as the Venus flytrap to trap insects and digestthem to obtain nutrients.


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