A growth or turgor movement of a plant organ in response to an external stimulus, mostly light intensity and temperature, in which the direction of the response is not determined by the direction of the stimulus. The direction of the movement is determined by the structure of the responding organ. The nasties are brought about by growth resembling tropisms and by turgor changes which are usually reversible. Nastic movements are sometimes almost tropic or vice versa, for example haptonasty and chemonasty.
Nastic movements can be classified into:
The nastic movement brought about by changes in light intensity are called photonasty.
For example the opening of a dandelion (Taraxacum officinale) flower head in the light and its closure at dusk or in low light. The movement is because of growth differences on two sides of the organs concerned.
The nastic movements depending on growth difference on two sides of the organ concerned because of temperature changes.
For example, flowers of crocus and tulip open during day when the temperature is high and close at night when the temperature is low. Both photonasty and thermonasty are called nyctinastic movements or sleep movements.
The nastic movement due to stimulus of touch, for example initial movements of tentacles in insectivorous plant, the Drosera spp. It is a growth movement.
The nastic movements because of stimulus of chemical is termed chemonasty, for example later movements of tentacles in Drosera spp. It is a growth movement.
These are nastic movements displayed by certain fully developed parts of plants which are caused not by growth, but by more or less turgor changes in cells located in definite regions of the tissue. These movements are usually very much rapid than those caused by growth.
Photonasty & Thermonasty – Nyctinasty or Sleep Movements
Photonastic or thermonastic changes are brought by diurnal cycle of day and night that causes flowers of certain plants to open and close. The cycle is generally termed nyctinastic, therefore the movements are called nyctinastic (Grn. Nux=night) or sleep movements. Opening and closure are due to localized growth in a particular part of the flower or inflorescence. Opening is caused by more rapid growth of the upper sides (epinasty) and closing by more rapid growth o the lower sides (hyponasty) of petals at night.
A dandelion (Taraxacum officinale) flower head opens up in the light and closes at dusk or in low light. This referred to as photonasty. The flower head consists of a flattened, disc-shaped stem called the capitulum, which carries a large number of flowers called florets. The growth of the lower surface of the capitulum closes the flower head, and the growth of the upper surface of the capitulum opens the flower head. Flowers of crocus and tulip make similar responses stimulated by changes in temperature (thermonasty) and as well as changes in light. They open during day when the temperature is high and close at night when temperature is low. The night flowering species behave differently.
The developing leaves of certain plant species, for example young leaves of Impatiens species also exhibit nuctinastic movements. They sink when they are darkened due to accelerated growth on upper side, however the growth is compensated later by the growth of lower side even in darkness. The fully developed leaves show nastic movements only when a pulvinus is present.
Leaves or leaflets of leguminous plants such as Albizza, Mimosa, etc. They exhibit sleep movements. In Albizza, at night the tips of opposite leaflets press together, rise upward and become pointed towards the upper end of the rachis; and during day they return to their normal position. The leaves have swellings called pulvini (singular – pulvinus) at the bases of petioles or leaflets which possess large parenchyma cells called motor cells. The water moves in and out of these cells because of turgor changes in response to stimulus of light intensity. At night the water moves out of cells present on the lower side and become flaccid. The turgid cells of upper side press the cells of lower side and the leaf droops. During daytime reverse turgor changes in these cells result in return to normal position.
Haptonasty & Chemonasty
The glandular head of the marginal tentacles of the leaf of Drosera (an insectivorous plant) react to stimulus of touch. The stimulus is rapidly transmitted to the base of the tentacle and the growth of the tentacle is stimulated particulary on abaxial side so that the tentacle curves towards the middle of the adaxial surface of the leaf. The movement is a growth movement and called haptonasty.
The tentacle respond not only to the stimulus of touch but even more strongly to chemical stimulation. The stimulation arises when the insect is called. The stalks of the tentacles exhibit marked growth curves, sometimes up to 180°. Like haptonasty only the glandular head of the tentacle is sensitive to chemonastic stimulus. The movement is called chemonasty and a growth movement.
Seismonasty – A Turgor Movement
Seismonasty is a nastic movement in response to the stimulus of shock. The movement is caused by more or less reversible turgor changes in cells located in definite regions of the tissue. Such movement is usually referred to as turgor movement and much more rapid than those caused by growth. Many petals, stamens, stigmas and stomata (movement of guard cell) show seismonastic movements. They also occur in leaves of Mimosa pudica (sensitive plant), and in those of some other species and genera, indicluding many insectivorous plants.
Mechanism of Seismonasty
Mimosa pudica (sensitive plant) has bipinnate compound leaves. A slight swollen structure called pulvinus is present at the base of the petiole. Similarly, structures known as pulvinules are also present at the bases of leaflets. If the terminal leaflet of the plant is shocked they fold upward. If the stimulus is applied constantly successive pairs of leaflets fold up and the stimulus passes through whole leaf causing leaf to droop. The stimulus will pass in reverse direction when the stem is stimulated.
The movements are caused by a change in the turgor of the upper and lower half of the pulvinus part of the petiole. The lower half of the pulvinus is formed of thin-walled parenchyma cells with large intercellular spaces, whereas the upper half comprises of relatively thick-walled cells with a intercellular spaces. Upon receiving the stimulus, the cells of the lower half lose water that passes into the intercellular spaces. As a result the cells of lower half lose turgor and become flaccid or compressed. The cells of the upper half absorb water present among the intercellular spaces, swell and develop considerable turgor. They exert pressure on the flaccid cells of lower half causing the petiole to droop. When the stem is stimulated reverse of these changes take place that help the leaf to regain its normal position.