Tag: living world of plants - plant physiology

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.

Answer is option A i.e. "Thigmonasty"
Movement in the leaves of mimosa while you touch is called “nastic” movement. Nastic movements are reversible. It means that after some time plant leaf will comes to its original position. Thigmonasty is the nastic response of a plant to touch or vibration.

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.

Movements in nyctinasty plants are not related to auxin levels. 

The leaves of Mimosa pudica show seismonasty. 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 A.

Any nastic movement in response to touch or contact stimulus is known as thigmonasty. The tentacles of Drosera leaves curve on coming with any insects and thus, the movement is categorised as thigmonasty which makes option C correct. Any nastic movements caused in response to sunlight or temperature stimuli are known as photonastic and thermonastic movements respectively. This makes options A and B incorrect. Nastic movements in response to any mechanical shock, such as blow, shaking or pressure, are known as seismonastic movements. This makes option D incorrect. Correct answer is C.

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.

The leaf lamina and apex of pitcher plant form pitcher and lid respectively. The coloured lid and nectar attract the insects which are then drowned into acidic fluid of pitcher present at it bottom. Sitting of insects stimulates the closing of lid of pitcher which makes it an induced/paratonic movement. The vital movements of plant parts which are induced by any external factor (light, temperature, water etc) are known as induced or paratonic movements as opposed to the autonomic/spontaneous movements. Thus, option A is correct. 

Heliotropism, a form of tropism, is the diurnal motion or seasonal motion of plant parts (flowers or leaves) in response to the direction of the sun. The habit of some plants to move in the direction of the sun was already known by the Ancient Greeks.

Thigmonasty is the response of a plant to touch or vibration. The Venus Flytrap an insectivorous plant shows thigmonasty. When an insect lands on its trap, the tentacles present on the two curved lobes of a single leaf, rapidly close. Hence the insect gets trapped and later killed. Thus the correct answer is option C.