Higher plants, being fixed to soil cannot move from place to place. But within the plant body various protoplasmic components are in constant motion, ex., movement of water, minerals, food etc. But certain parts of the plant body in response to external stimuli, exhibit physical displacement called movement. Lower unicellular plants also show movement from place to place. Such movements may be autonomic or induced.
The movement of plant structures in response to stimuli is very interesting. The stimuli might be in the form of light, touch, chemicals, temperature, gravity or water. Thus the agency or factor that causes movement is called stimulus. Not all parts of the plant act as sites of perception to stimuli. Only certain regions or structures are capable of receiving the stimuli and such structures or organs are called perception site. The site of perception need not be the structure that responds to movement. In fact, in many cases the site of response and the site of perception are different. Nonetheless, the stimulus has to cross through the plasma membrane of the cell or cells found in such structures that receives the stimulus. Plasma membrane has all the inbuilt components and the potentiality to receive stimulus and transmit it into intracellular milieu for proper response. Examples; root movement towards water and minerals; carnivorous plants organs show movements in response to Insects, pollen tubes move (growth) towards egg in the ovary along the style and stomata open and close in response to light and moisture.
Types of movements (some): Circumnutation- rotary or helical ex. Tendrils. Tropic movements; curvature movements; Photoptropism; stem terminals move toward light – can be positive or negative; Geotropism or Gravitropism-response to gravity; Chemotropism: response to chemical stimulus; Heliotropism: towards light; Rheotropism; movement of curvature-plant organs; Thermotropism movement in response to touch and Traumatropism: in response to injury.
Nastic movements- independent direction of stimulus, where the direction is predetermined: Epinasty, Hyponasty, Hydronasty, Nyctinasty, Seismonasty, Thigmonasty or hyponasty.
There is a time lag between the time at which the stimulus is applied and the time at which the response begins. This time is called reaction time. It may vary and depends upon the intensity of stimulus and the kind of response. If the stimulus is weak, there may not be any response at all, but if the stimulus is adequate or in right quantity the response is positive. The time required to cause the proper stimulus is called presentation time.
Once the plant body responds to the stimulus say sleeping movement (change in the position direction), structures involved always come back to their original position. This process is called recovery and the time required is referred to as relaxation time, which again varies from species to species. If the stimulus is provided repeatedly the receiver structures do not respond with the same intensity as the first instance, but it slows down. This effect is due to fatigue. Such behavior is probably due to the loss of same material components required for the response. If the stimulation is continued at increased frequency, the plant organs do not respond and behave as if they are dead structures. Such a state is called Tetanus or extreme fatigue. Now it is believed that stimulation (quantity) causes irritation at the site of perception and the products of irritation are then transported to the site of response, where the structures respond and perform movement. All these events initiate with signal, and the receptor that receives the signal becomes active and induces signal transduction pathway in the cell cells. There will be a cascade of events that finally leads to the response.
Plants endowed with different structural adaptation and different potentialities exhibit different types of movement either voluntarily or involuntarily. Based on the behavior as pattern, movement’s stimulus, the plant movements have been grouped into physical movements and vital movements.