Tag: living world of plants - plant physiology

Plants do not move and have been evolved for stationary mode of life. However plants show several types of movements. For example, opening of a flower bud is an example of increased growth of the upper surface of a plant part (such as a leaf- vegetative or reproductive/ floral), resulting in a downward bending of the part. This type of movement is called epinasty.

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. 

Phototactic movement is movement of organisms influenced by light. Low intensiti of light induces positive phototactic movement in algae.

Plants do not move and have been evolved for stationary mode of life. However, plants show several types of movements. 

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.

Nastic movements are generally observed in leaves, flowers, petals and bud scales. In these structures at some stage of development, growth in one surface is more than the growth on the other surface. There may be two types of such movements: epinasty and hyponasty. If the upper or inner surface has more growth, the movement is called as epinasty. An example for epinasty is the opening up of a flower and the drooping of a bud. In hyponasty, there will be more growth on the lower surface.

Semimonastic movement is the movement caused in response to touch or vibrations. The stimulus from the largely stimulated leaflets travels to other leaflets and their swollen base known as pulvinus. It further activates the leaf rachis and then runs through petiole and finally to the large pulvinus at the end of the petiole, where the leaf attaches to the stem. The potassium ions flows out from the vacuoles of cells in the various pulvini. This allows exit of water from various cells due to loss in turgidity the leaflet closes and the petiole collapses. So, the given statement is true.

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

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.