Plants do not show noticeable movements. The movements of higher plants are chiefly in the form of bending, twisting, elongation of certain parts etc. Plant movements occur in response to certain stimuli.
Types of Movements
There are two types of movement in plants.
1. Automatic movement
2. Paratonic Movement
1. Automatic Movement: Movements which occur due to factors inherent inside the plant body itself are known as automatic or spontaneous movements.
Automatic movements are further of two types.
a. Locomotion Movement
b. Growth Movement
a. Locomotion Movement: It is the movement of whole plant body or an organ or material within the plant cells which are in response to inherent factors. These movements include streaming movement of protoplasm, chromosome movement during cell division, ciliary movement and flagellary movement etc.
b. Growth Movement: The changes in the form and shape of plant organs are called growth movement. It is due to unequal growth on two sides of plant organs like stem, root, tendril, bud etc. These movements are of three types.
i. Epinasty: The upper surface of lead in bud condition shows more growth as compared with lower surface, as a result the buds open. Other examples are leaves, petals etc.
ii. Hyponasty: If the growth in the lower surface of the leaf in bud condition is more than that of the upper surface then the bud will remain close.
iii. Nutation: The growth in apex takes place in zigzag manner due to alternate change in growth rate on opposite side of the apex. These movements are called nutation. A nutation movement may be called circumnutation when an apex makes a rotational growth around the axis.
2. Paratonic Movements: Movements which occur due to external stimuli are known as paratonic or induced movements. This movement occurs due to external stimuli. There are three types of paratonic movement.
i. Tactic Movement
ii. Tropic Movement
iii. Nastic Movement
i. Tactic Movement
These movements occur in response to certain external stimuli. The tactic movement may be positive or negative. Tactic movements are the movement of locomotion. These are classified on the basis of nature of stimulus.
a. Phototactic Movement
It is the movement in response to light e.g. the passive movement of chloroplast due to cyclosis.
b. Chemotactic Movement
It is the movement in response to chemicals e.g. movement of sperms of liverworts, mosses, ferns toward archegonia in response to stimulus of nucleic acid released by the ovum.
ii. Tropic Movement
(Greek Tropos means Turn) These are curvature movement caused by stimulus coming from one side. These movements are controlled by the direction of the stimulus and occur in one direction due to differences in the rate of growth of the two sides of the moving organ, which results in curvature e.g. a. Phototropism b. Thigmotropism c. Chemotropism d. Hydrotropism e. Geotropism
a. Phototropism: This movement is in response to light and is caused due to differential growth of root or stem.
b. Thigmotropism: It is in response to touch e.g. when vines come in contact with some solid object the growth on the opposite side of contact increases and the tendril coils around the support.
c. Chemotropism: Such movement is in response to chemicals e.g. Hyphae of Fungi.
d. Hydrotropism: It is in response to water e.g. growth of roots towards water.
e. Geotropism: It is also known as gravitropism which is in response to gravity. Roots show positive geotropism and shoots show negative geotropism.
iii. Nastic Movement
In this type of movement the direction of the stimulus is not fixed. It is of the following types.
a. Nyctinasty: It is in response to external stimuli leading to differential growth. These are due to turgor and growth changes. It is of two types.
i. Photonasty: It is due to photoperiod. The flowers open and close due to light intensity.
ii. Thermonasty: It is due to temperature e.g. the flowers of tulip close at night because of rapid growth in the lower side by upward and inward bending of the petals.
b. Haptonasty: This movement is in response to contact e.g. closing action of Venus flytrap.
c. Seismonastic or Turgor Movement: Turgor movement is due to loss or gain of water due to shaking, pressure etc. e.g. Sleep movement and rapid movement of leaflets.
Sleep Movement: Some plants raise their leaves in the morning and lower them in the evening e.g. bean, Sylvia, some members of legume family. These are known as sleep movements. Pulvinus is the swollen portion of the petiole. It is composed of parenchymatous cells having large intercellular spaces and central stand of vascular tissues. The sleeping movements are due to daily changes in turgor pressure in the pulvinus. When turgor pressure on the lower side of the pulvinus increases the leaves rise and become horizontal. When turgor pressure decreases on the lower side of the pulvinus, the leaves become lower in position.
|Sleep movement in Prayer Plant|
Image: Rapid movement of leaflets
Rapid Movement of Leaflets
The leaves of the plant Mimosa Pudica show sensitivity so it is known as “touch me not”. It is herbaceous plant with bipinnately compound leaves. The base of the petiole is swollen which is called pulvinus and similarly smaller pulvinules are present at the base of leaflets. On stimulation, the main petiole falls, the secondary petiole close together and leaflets fold towards one another in pairs. The movement is caused by differential loss of turgor on two sides of pulvinus. It consists of thin walled loosely arranged parenchymatous cells, with large intercellular spaces and upper half consists of a bit compactly arranged parenchymatous cells. When stimulus reaches pulvinus, the osmotically active water enters cytoplasm of cells present on the lower half of the pulvinus, thus, lowering the osmotic pressure of the cells effectively and forcing out water into the intercellular spaces. These flaccid cells now fail to hold pinnules erect and cause drooping down of pulvinules and ultimately pulvinus. The whole leaf bends downward. After about ten minutes the plant regains turgor and the internal turgidity of the leaf is restored.