Plant Transport and Physiology

Shoot system shows positive phototropism means it will grow or change its direction in the way where light is present. It is known as the directional movement of plant.

Growth independent movement is a directionless movement of the plant caused due to the variation of the outside environment and plants movement is neither toward nor away from a stimulus. The example is touch me not plant or Mimosa Pudica which fold its leaves due to touching. This movement is not involved in growth hence it is growth independent.

Photo means light and tropic means movement so the movement of plants towards light is phototropic movement and is usually shown by the stem of the plant that is why it is known as positively phototropic.

Geotropism is also known as Gravitropism. It is a phenomenon in which growth movement by a plant turns in response to gravity. Roots show positive geotropism as they grow towards the gravity while stems show negative geotropism as they grow upwards. Hence, root shows greater growth on lower side as compared to upper side in positive geotropism.

Auxin is a plant hormone that plays important role in the growth of stem and roots to gravity. Gravitropism is a plant growth response to gravity. Root shows positive gravitropism and shoot shows negative gravitropism. Auxin content is lower on the upper side and more concentration occurs in lower side of the root and stem. This stimulates stem elongation and thus stem grows upwards. In roots, higher growth on upper side occurs due to lower auxin content. This inhibits root elongation and thus root grows downwards showing geotropic response. 

Root cap takes part in the protection of root meristem because it is the layer separating the meristem from the external forces. It protects the tender apex while root makes its way into the soil.

Thigmonasty or seismonasty is the nastic response of a plant or fungus to touch or vibration. Examples are many species in the leguminous subfamily, Mimosoideae, active carnivorous such as Dionaea and a wide range of pollination mechanisms.

Nastic movements are responses of plant parts to stimuli. Seismonasty is the nastic response of a plant to touch or vibration. Dropping down of leaves of Mimosa pudica upon touching is due to seismonasty. Nyctinasty is the rhythmic nastic movement of plant in response to the onset of darkness. Chemonasty is the nastic response of a plant to external chemical. 

Nastic movement is the movement shown by plant part in response to environmental stimuli. This movement is repeatable and reversible. Examples include the folding of leaves of Mimosa pudica when touched. The folding and recoiling of leaves of Mimosa pudica in response to touch is called as thigmonastic movement.

Hygroscopic movement is that, which is caused by changes in moisture content of unevenly thickened cell walls. E.g., elators or the awns of the grains of some grasses. Also hygrometric movement, imbibitional movement.

Rapid rotational movement of cytoplasm is due to increase in water content. Cytoplasmic streaming also called as protoplasmic streaming, the movement of the fluid substance within a plant or animal cell. The motion transports nutrients, proteins and organelles within cells. First discovered in the 1830s, the presence of cytoplasmic streaming helped convince biologists that cells were fundamental units of life.

Paratonic or induced turgor movements are turgor movements induced by stimuli, such as light, temperature and contact. These movements are also called as nastic movements and may be of various types, such as
(i) Siesmonasty
(ii) Nyctinasty
Siesmonasty means a response to shaking. The best example is Mimosa pudica ("Touch - me - not" plant) which is the sensitive plant. Such plants respond to stimuli, such as touch, blow or mechanical shock by folding their leaflets and lowering their leaves. This effect is caused by a change in the turgidity of the leaflets brought about by the movement of water into and out of the parenchymatous cells of the pulvinus or swollen leaf base.
Nyctinasty or sleep movements are caused by relative changes in cell size on the opposite sides of the leaf base called pulvinus. The movements are attributed to the amount of auxin, $K^+$ ions and therefore the movement of water. Entry of water to the lower side of the pulvinus causes the leaves to stand erect and the exit of water causes them to droop.

Seismonastic movement is a response to touch or blow. In Mimosa pudica plant, turgor movement is seen in leaflets in response to touch. When it is touched, it begins to fold up and droop. Nyctinastic movement is the response of a plant in response to darkness. In this, plant shows circadian rhythm that function on a 24 hour clock. It is seen in leaves of many legume plants. Thigmonastic movement is the response of a plant to touch or vibration. In Drosera, tentacles bend towards the insect's direction after coming in contact. Chemonastic movement is the response of a plant to chemical stimulus. An example is growth of pollen tube towards embryo sac due to presence of sugar in the style.

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.

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.