The non-directional movements of the plant in response to external stimuli are called nastic movement. The variation in the intensity of some external- factor acts as stimulus. The direction of the movement is determined by the structure of the plant organ itself. It is not determined by an external stimulus. Nastic movements occur only in bifacial organs like the leaves and petals. These organs can bend only in one direction. Nastic movements occur due to growth changes or movements of variation. There are following types of nastic movements:
(a) Nyctinasty or nyctinastic (sleep movements). The diurnal movements are called nyctinastic or sleep Movements.
The flowers and leaves of Tirtants take up different positions during day and night. These movements are caused by alteration in light or temperature. Thus it has two types:
- Photonasty: The principle stimulus of Photonasty is the
photoperiod. The flower opens and closes due to light intensity. The flowers of Oxalis and the capitula open in the morning and close at night. But the flowers of some plants like evening primrose and Cestrum (night jasmine) open in the evening but closed in the morning.
- Thermonasty: It takes place due to temperature changes. The flower
opens when temperature is raised and closed when temperature is lowered. There is a rapid growth in the lower side of petal in the flower tulip at night. Thus it causes upward and inward bending of the petals. Thus the flower of tulip closes at night.
Mechanism of Nyctinastic movements
- GrowthGrowth curvatures cause nyctinastic movements.There is a increased growth on the inner or morphologically upper side (epinasty) of the petals. It opens the flower. There is increased growth on the outer or lower side (hyponasty). It closes the flower. The advantage of these movements to the plants is that their flowers open at a time when their pollinating insects are active.
- Change in turgor pressure of pulvinusSome leguminous plants like bean lower their leaves in the evening and raise them in the morning. These are known as sleep movements. These sleep movements take place due to daily change in the turgor pressure in the pulvinus. The pulvinus is the swollen portion of petiole. It is composed of parenchymatous cells and central strand of vascular tissues. These parenchymatous cells have large intercellular spaces. When turgor pressure on the lower side of the pulvinus increases, the leave rise and become horizontal. When turgor pressure decreases on The lower side of the pulvinus, the leaves are lowered and it goes to sleeping position.(b) Selsmonastic movements
The shock movements are called Seismonastic. It occurs inmimosa. When the leaves of Mimosa (touch me not) is touched, the leaflet fold together. This response takes one or two seconds.
These movements are caused by differential loss of turgor on the two sides of the pulvinus. The lower half the pulvinus is made up of thin walled cells. It has large intercellular spaces. The upper half the cells are comparatively thick walled. They have few intercellular spaces. There is following mechanism of these movements.
- The cells of both sides of the pulvinus are fully turgid during normal conditions. Therefore, the leaf is fully erect.
- The cells of the lower half lose water into the intercellular spaces on stimulation. Therefore, their turgor falls considerably. The cells of the upper half retain their turgidity. They even become more turgid by absorbing water from the intercellular spaces. Therefore, the upper turgid half of the pulvinus presses down the lower flaccid half. Thus the leaf droops.
- The cells of the flaccid half gradually reabsorb water from the intercellular spaces. Therefore, they regain their turgor and the leaf returns to its normal position after some time.
(c) Haptonastic or thigmonastic movements
The nastic movement occurs due to stimu:us of touch are called Haptonastic movements. Haptonastic moven-salts occur in the leaves of Drosera and Dionaea.
- Haptonastic movement in Drosera: An insect flies on the marginal tentacles of Drosera. It stimulates the tentacles. Growth curvature occurs in the basal part of its stalk of the tentacles. Therefore, the tentacle bends towards the centre of the leaf. The direction of movements is determined entirely by the properties of the tentacle. Therefore, it is a nastic movement. It brings the insect in contact with the smaller central tentacles. The stimulus is transmitted to the other marginal tentacles. They bend towards the insect from all directions.
- Chemonastic movements: The tentacles of Drosera are also stimulated by ammonium salts, phosphates or a drop of water containing proteins. Therefore. tentacle of Drosera also shows chemonastic movements.
- The one of the six sensitive hairs on the upper surface of the leaf is stimulated by contact. The two halves of the leaf close rapidly on the midrib. The midrib acts like a hinge.