These are mostly temporary movements induced by diffused stimuli but not directed by them. These movements are generally confined to bifacial organs like leaves and petals. Changes in the turgor or unequal growth on the both sides of a organ cause these movements.
Different types of nastic movements are:
(i) Nyctinasty – This is induced by diurnal variations in light and temperature. It is seen in leguminous plants. The leaflets fold in the evening and open up in the morning.
(ii) Seismonasty – This is induced by touch or shock. Leaves of touch me not plant (Mimosa pudica) show this movement. The leaf consists of main petiole with 2-4 secondary stalks, each bearing many pairs of leaflets.
ADVERTISEMENTS:
The leaflets are arranged in opposite manner. When the plant is stimulated by touch or shock, the leaflets fold upwards and the main petiole droops. The mechanism of movement is based on changes in the turgor in pulvini. Pulvini are the swollen leaf bases particularly seen among these legume plants. In the pulvinus, it is the cells of the upper side that lose water, allowing the leaflets to move upwards.
(iii) Thermonasty – It is induced by temperature. For example, the opening and closing of the flowers of tulip
(iv) Photonasty – It is induced by light. Leaves of Oxalis take up horizontal position in sunlight and droop down during the night. Many flowers open during the day and close during night or under cloudy sky.
(v) Thigmonasty – In the leaves of certain insectivorous plants, e.g. Drosera, Dionaea the tentacles show variation in movements on coming in contact with an insect. The touch stimulus due to the insect is transmitted to the entire leaf and all the tentacles bend over the insect.