Sensitivity in plants

From Online Sciences - January 9, 2018

Sensation ( sensitivity ) is one of the functions shown by a living organism to maintain its life , where we find that sensitivity in plants is less obvious ,sensitivity in animals is more obvious ,sensitivity in human reaches its highest efficiency & accuracy ,Sensitivity ( Irritability ) is the suitable responsible of living organism to the internal and external stimuli to maintain its life .

Sensitivity in plants

Sensitivity in plants includes Tropism , response to touch and darkness .

Response to touch and darkness in Mimosa plant

The morphology of Mimosa plant leaves : Leaves are compound and pinnate , Each has a primary rachis which carries at its end four secondary rachises , Every secondary rachis carries two rows of leaflets , At the base of every primary & secondary rachises , there is a swollen structure called a pulvinus .

Response of Mimosa leaflets to touch : If you touch a Mimosa leaflet , its petiole soon droops as if it has wilted , other neighbouring leaflets soon will follow till the effect is seen in all the leaflets .

Response of Mimosa leaflets to darkness : In day time , the leaflets are held in a horizontal position ( undergo a wake movement ) , At night , the leaflets hang downwards and fold their upper surfaces ( undergo a sleep movement ) .

Explanation of the response of Mimosa to the touch and darkness

The swollen structures ( pulvinus ) act as joints in these movements , as the following :

The cell walls of lower half of the pulvinus are more sensitive than those of the upper half and they play the main role in this movement .

When the plant is touched or at darkness , the lower surface of pulvinus shrinks , This leads to water diffusing to the neighbouring tissues , and hence the leaflets droop , but when the stimulus is removed , the cells regain their turgidity and the leaflets open once more .


Tropism and subsequent movement represent the most common type of sensation in plants ,Tropism is the curvature of stem or root of plant , when its sides are subjected to factors ( stimuli ) as light , humidity and gravity in unequal form .

Types of Tropism

Types of tropism are determined according to the affecting factor , as the following :


Phototropism is the response of plant parts to the external stimulus which is the light , causing the curvature of plant parts towards or away from it .

Experiment : to prove the occurrence of phototropism

Steps :

Observation : The stem inclines towards the source of light , The root grows away from it .

Conclusions : The stem is positive phototropic , The root is negative phototropic .

Explanation : Curvature is due to the unequal growth of two sides in both the root and stem , where : The side of stem which is away from the light grows more rapidly , so , the stem curvature is towards the light , The side of root which is near to the light grows more rapidly , so , the root curvature is away from the light .

Experiments to explain phototropism

Experiment 1 : Boysen Jensen experiment

Boysen Jensen explained this phenomenon during his observations and results of experiments on the oat ( Avena ) coleoptile .

Steps :

Observations :

Conclusions :

Explanation : Curvature towards the light ( phototropism ) is the result of unequal concentration of auxins on the two sides of coleoptile which causes the unequal growth of two sides .

Auxins are chemical compounds secreted from the coleoptiles tip of plants and affected by the external factors , The main type of auxin is indole-acetic acid ( I A A ) .

Experiment 2 : Went experiment

Went made his experiment to prove the results of Boysen Jensen , as the following :

Steps :

Observations :

General explanation of phototropism experiments

Explanation of the difference in auxins effect on both root and stem :


Experiment to illustrate the effect of gravity on the root and stem

General explanation of geotropism


Experiment : To prove the hydrotropism phenomenon


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