Sunday, 8 January 2017



Coordination refers to the methods of internal communication between organisms cells, tissues, organisms and systems that occur in order for an organism to adjust to the changes in the environment.

Terms used in coordination.

Irritability. Is the ability of an organism to react to changes in environment i.e. power to respond to a stimulus.

Stimulus. It is any change in the environmental conditions which can bring about a change in the activity of an organism e.g. light, temperature, pain etc.

Response. Reaction of the organism, it is the change in the activity of an organism in reaction to a stimulus, e.g. blinking the eye, walking away from too much light.

Internal environment. It is the immediate surroundings of the cells which is mainly blood.

External environment. Is the surrounding of an organism?

Receptors. These are structure or organs that receive stimuli e.g.  Skin, eye, ears, Tongue, and nose. They are also known as sense organs.

Effectors. These are structures that aid animals to react to stimuli i.e. they carry out a response; the most common effectors in the body are glands and muscles.

An impulse. Is a nerve message transmitted a long nerves in the nervous system.

Coordination in plants

There are two main types of response in plants and these are.
Nastic response / movement.


Is a non directional movement of part of a plant in response to an external stimulates. It mainly depends on the intensity of the stimulus. e.g.
Opening of flowers.
Opening and closing of stomata.
Folding of leaves e.g. mimosa pudica.


These are growth movements in plants towards / away stimuli.
The following are types of tropisms named according to stimuli. They may be positive / negative depending on direction taken. If the growth is directly towards the stimuli then it is positive and if the growth is directly away from the stimuli then it is negative.

Photo tropism

Hydro tropism.

Geo tropism

Thigmo tropism

Aero tropism


Is the plants response to light? Shoots are positively photo tropic and roots are negatively photo tropic.
Experiment to demonstrate phototropism

2 sets of seedlings.
2 opaque boxes.
A clinostat

Put one set of seedlings in a box and name it A
Put the 2nd set of seedlings in the 2nd box and place it on a rotating clinostat.
Expose both sets to light from one direction for about 2 weeks.
In A shoots of seedlings will bend towards light and in B shoots will grow up right.

Plant shoots respond positively towards light.
N.B: as the clinostat rotates, it ensures uniform distribution of light
Auxins. (Indole Acetic Acid-IAA)

Auxins are plant hormones which control growth. They influence growth by
Making the cells permeable to useful substance for growth
Increasing the metabolic rate of cells to produce more energy.
Increasing the turgidity of cells.

Auxin theory of phototropic response.
Light affects distribution of auxins i.e. they migrate to the dark side.
High concentration of Auxins promotes growth in shoots.
When a shoot is exposed to light from one side the Auxins migrate to the dark side than the illuminated side. It causes bending of the shoot to the illuminated side hence positive photo tropism.
A coleoptile tip growing towards the light

Study of shoots and Auxins

The tips of shoots have been shown to be the regions where auxins are manufactured. In an experiment the shoot tip was out off after some time, it was observed that growth had stopped. When the tip was replaced by an Agar block which had been in contact with the cut off tip, growth started again. When the shoot tip was chopped and the agar block placed half way the cut off end, the shoot bend towards the side without the agar block. This experiment shows that bending of the shoot is caused by an un equal distribution of auxins.



Its the plant response to gravity. Roots response positively while shoots negatively.

Experiment to demonstrate Geotropism.

Moist cotton wool.
A clinostat
Petri dishes.
2 sets of seedlings.

Put one set of seedlings horizontally on a Petri dish containing moist cotton wool.
Put the 2nd set on a rotating clinostat and leave the set up to stand for about a week.
In the set, the shoot bends upwards (negative Geo tropic) and the roots bends downwards (positive Geo tropic).
In the rotating set both shoot and the root grow horizontally.

Effect of auxin concentration in roots and shoots.
Gravity pulls the auxins down the shoot and the root of the set. This causes a high concentration of auxins on the lower side than the upper side of the seedling.

More cells elongation takes place on the lower side of the shoot because of high auxins concentration. In the root, high concentration of auxins retard growth on the lower side and more cells elongation takes place on the upper side; therefore high concentration of auxins promotes growth in the shoot and retard growth in the root.

The rotating clinostat ensures equals distribution of auxins in all sides of the seedling.


Is the growth movement towards unidirectional stimuli of water. Roots show positive hydro tropism and shoots show negative hydro tropism.
Experiment to demonstrate hydrotropism


Soaked seeds
A chamber which one side containing dry cotton and the 2nd wet wool.
Procedure. (Draw a control but without water and show the root growing straight down)
Seeds are placed in the dry chamber and are allowed to germinate.
The radicals will grow towards the wet cotton wool while the shoots will grow vertically upwards.
Roots are positively hydro tropic and shoots are negatively hydro tropic.


Is the growth movement of part of a plant in response to a unidirectional source of chemicals e.g. pollen tubules grow towards the micropyle in the ovary of a flower where chemicals are produced, therefore the pollen tube is said to be positively chemo tropic.

Thigmotropism (haptotropism)

This is the growth movement of parts of a plant in response to the stimulus of touch from a specific direction. It is shown by climbing plants e.g. pumpkin which have tendrils for support. Root tips grow away from stones and other obstacles, therefore showing negative thigmotropism.
Importance of tropic response

Enables plants to survive because plants are able to move towards or away from stimulus e.g. photo tropism. Which enables plants to move towards light for synthesis of food?

Geo tropism, even if a seed is planted upside down the radicle will grow down wards into the soil because it is positively Geo tropic. Because roots are positively Geo tropic it enables plants to be firm in the soil.
Hydro tropism enables plant to get water from deep soils because roots grow towards water.

Differences between Nastic and tropic response.

Depend on direction of stimulus.
Does not depend on direction of stimulus.

Are growing movements
Can either be growth or turgor pressure

Occur because of hormones (Auxins)
Arent controlled by hormones.

Take part in growing parts of the plant(shoot tip and root tip)
Take place in any part of plant.

Is where an entire cell or organism moves towards/away from the stimulus. The response is directed by an external stimulus. Common in animals e.g. maggots which are positively Geo tropic, earthworms are negatively photo tactic and wood lice are positively hydrotactic.

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