Behavioral Psychology and Physiology

Nastic movements are non-directional responses to stimuli and are usually associated with plants. The movement can be due to changes in turgor or changes in growth. Nastic movements differ from tropic movements in that the direction of tropic responses depends on the direction of the stimulus, whereas the direction of nastic movements is independent of the stimulus's position.

Nutation is the bending movements executed by some plant organs such as stems, leaves, roots, etc, by which the part is inclined successively in various directions. Nutations are due to unequal rate of growth of different sides of the organ. Nutational movement includes all the movements in plants caused by growth in contrast to variation movements or movements produced by reversible turgor changes.

The free type of movements induced by some external stimuli like light, temperature, chemicals, etc are called as tactic movements. They are of three types: Phototactic, chemotactic and thermotactic.

A response of a plant, that causes movement independent of the direction of the stimulus is a nastic response. It is not a growth response, is reversible, and can be repeated many times. 

Scotopic in general means under low light conditions or night. For example scotopic stomata are stomata open during night and closed during day as against phototropic stomata, which are open during day but closed during night.

Seismonasty is the nastic movement of plants in response to mechanical stimuli like touch or vibration, which is characteristic of Mimosa pudica.

Mimosa pudica is well known for its rapid plant movement. Like a number of other plant species, it undergoes changes in leaf orientation termed "sleep" or nyctinastic movement. The foliage closes during darkness and reopens in light. The leaves also close under various other stimuli, such as touching, warming, blowing, or shaking. These types of movements have been termed seismonastic movements. The movement occurs when specific regions of cells lose turgor pressure, which is the force that is applied onto the cell wall by water within the cell vacuoles and other cell contents. When the plant is disturbed, specific regions on the stems are stimulated to release chemicals including potassium ions which force water out of the cell vacuoles and the water diffuses out of the cells, producing a loss of cell pressure and cell collapse; this differential turgidity between different regions of cells results in the closing of the leaflets and the collapse of the leaf petiole.

Plants exhibit two types of induced movements of curvature. These are namely tropic and nastic movements. The directional growth movements in which direction of growth is determined by the direction of stimulus are known as tropic movements. Examples: thigmotropism, phototropism, chemotropism, gravitropism etc. This makes option A incorrect. The non-directive growth movements of plants, wherein the direction of stimulus is not the determinant of the direction of movement, are known as nastic movements. These movements always occur in one direction. Examples: Seismonasty, photonasty, thermonasty etc. This makes option B correct. Thus, the correct answer is B.

The leaves of touch-me-not plant show thigmonasty. Its leaves respond to vibrations when they are touched or shaken. The speed of the response depends on the magnitude of the stimulus. Flicking the leaf hard will cause the leaf to close faster whereas a gentle touch at the tip of a leaf will lead to a slower response. Thus the correct answer is option B.

On touching the Mimosa leaves the water concentration inside the plant changes due to which the plant leaves tend to get closed. This movement or response of plant is called seismonastic movement.

Seismonasty or thigmonasty is the nastic movement induced in plants where the stimulus is touch. Touch-me-not plant, which has the scientific name as Mimosa pudica is sensitive to touch and is thus called as seismonastic.

Tropic movement is growth movement shown by plant in response to a unidirectional external stimulus. Leaves and leaflets shows nyctinastic movement, that is movement in night or in dark. This is also called as sleep movement. In this, the leaves become vertical during night. This is growth movement. Leaf shows daily rhythmic movement due to differential growth of upper and lower halves of the petiole. It is seen in papaya and balsam. 

A movement shown by a plant part in response to an external factor is called as paratonic movement. In this, the direction of movement is controlled by the direction of the stimulus. Autonomic movement is the spontaneous movement of the protoplasm seen in aquatic plants due to internal factors. Growth of plant in response to the current of water is called as rheotropism.

Nastic movements are responses of plant parts to stimuli. Seismonastiy is the nastic response of a plant to touch or vibration. Dropping of leaves of Mimosa pudica upon touching is due to seismonasty. Thigmonasty is the nastic response of a plant to touch. The movements of tentacles of Drosera when comes in contact with insect is due to thigmonasty. Nutation is the bending movements shown by some plant parts due to the unequal rate of growth of different parts of plants.