Since plants don’t have muscles and nervous system they cannot move.
Growth and movements in plants occur due to internal and external factors. Plant hormones are internal factors while light, touch, gravity and other stimuli are external factors that affect plant growth and movements.
Stimulus and Response
(i).Any internal or external factor which causes internal or external changes in plants is called as Stimulus.
(ii).Any internal or external change in animals or plants that is caused due to internal or external factors is called as Response.
(iii) The movement of parts of a plant towards an external stimulus is called as Tropism.
(iv).Plant cells change their shapes by absorbing or releasing water.
Positive and Negative Tropism
Tropism is a directional movement of parts of a plant which happens due to its growth. Tropism is also called as tropic movement. Tropism may be towards the stimulus or away from the stimulus. Growth of parts of a plant towards the stimulus is called as Positive Tropism and the growth of parts of a plant away from the stimulus is called as Negative Tropism.
Environmental Factors that affect tropism
Tropism may occur under the influence of different environmental factors like light, gravity, chemicals, touch, and water. Tropism in each of these cases may be Phototropism, Geotropism, Chemotropism, Thigmotropism, and Hydrotropism respectively.