The behavior of an organism is greatly affected by the responses it makes to both external (outside the body) and internal (within the body) stimuli. This ability of response to the stimulus is present in the protoplasm and is the characteristics of all the living organisms both plants and animals.
The direction of the stimulus has relationship to the direction of the movement of a from the stimulus is known as negative response.
The movements in plants are of following three types:
- Tactic 2. Nastic 3. Tropic movement
- Tactic Movement
In this type of movement the plant body as a whole moves from one place to another in response to external stimuli such as light, water, chemicals etc, e. g chlamdomonas always move towards light of low intensity and away from light of high intensity.
This type of response is known as positive phototactic response and negative phototactic response respectively.
Fig 14.7 tactic movement in chlamydomonas
- Nastic Movement
This is a movement made by a part of a stationary plant in response to external stimulus. Stimulus in this case influences the organ with equal intensity from all the directions.
Nastic movements are exhibited by bud scales and flower petals. These bend upwards due to fast growth on their lower surface. When the bud unfolds itself, the growth of its upper surface is faster than that of the lower surface, hence the petals bend downwards. In the case of ipomea (fig. 14.8) fading of the flower is due to the turgor changes in its rib cells. The cells in the inner side of the rib lose solutes and water while the outer rib cells expand, causing curling.
Fig 14.8 nastic movement in lpomea
Similarly changes in the water contents of the tissues at the base of the leaf of mimosa plant results in the shrinkage and folding of its leaflets. Such response may be due to different stimuli, such as light, touch, heat, or electric shock. After a suitable recovery period the leaflets open again. Normally they remained expanded in the day time while at night they are closed (fig. 14.9).
Fig. 14.9 nastic movement in the leaflets of mimosa plant
- Tropic movement
When the fixed part of the stationary plant moves in response to a stimulus the reaction is known. As tropic movement or tropism. Stimulus in this case influences from some specific direction. These are growth movements as response against the stimulus is produced by a plant’s growing points, the root and shoot apex. The growth movement is caused by an increased and decreased rate of growth on the side of the organ which is under the influence of the stimulus with respect to the opposite side.
The negative tropic movement occurs when the plant organ grows more rapidly on the side nearer to the stimulus so that it bends away from the stimulus. In case of a positive tropic movement, the side of the organ away from the stimulus grows more rapidly so the organ bends towards the stimulus.