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Sunday, 2 December 2018

External Stimuli




Most external stimuli are due to non living or abiotic factor in the environment. Thus, organisms generally respond to

• a change in light intensity
• a change in Temperature,
• a change in humidity,
• gravity,
• touch, and 
• chemicals in their environment including food and  water.

    Organisms have the means of detecting stimuli. In animals, stimuli are detected by special cells in structures called receptors. These may be found all over the body such as the human skin or they may be in special sense organs such ad the eyes and ears.

   In plants,  there are no special receptors for  detecting external stimuli. The protoplasm of young cells in the shoot and root apices usually act as a receptor of external stimuli. In the flamboyant trees, certain cells in their leaves can detect changes in light intensity, while cells in the mimosa leaves can detect touch

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