Plant movements do exist.
Although unlike animals, plants are incapable of locomotion in that they are unable to move from one place to another, or perform some feat like shaking hands as humans do, they are still capable of some form of movement.
Plant organs move toward scarce resources or otherwise secure food or use movement as an adaptation to escape or minimize injury from harmful external factors or ensure development.
Thus, for example, the primary root moves downward where it can obtain water and mineral nutrients from deep down while the shoot moves upward to be exposed to light from the sun.
Carnivorous plants:
- exhibit movement to trap insects that are to become sources of nutrition;
- certain flowers close at night to prevent chilling injury or when there is intense heat;
- the pollen tube which carries sperms moves toward the ovule which, after fertilization, forms into a seed;
- and the stomata open and close as a mechanism to regulate transpiration and photosynthesis under various environmental conditions.
For centuries the subject of plant movement has in fact occupied the scientific community for want of a deeper understanding of plant growth and development.
Knowing the exact functions of these movements, their environmental requisites, and their control would benefit crop production.
It is possible to achieve increased growth and productivity by manipulating the environment or the internal control mechanisms of these movements.
Terms Used in Describing Specific Types of Plant Movements
Below are some of the terms used in describing specific types of plant movements that occur naturally:
1. Circumnutation or mutation
Rotary or helical, or spiral pattern of movement of plant organs, such as stems, tendrils, and roots, without physical contact with any object. Example: Costus.
Tropic Movements
Tropic Movements are movements of curvature that respond to the direction of the external stimulus.
Movement may be toward the same direction as the stimulus (positive tropism), opposite (negative tropism), or at any intermediate angle (between 0° and 90°).
Tropic responses are due to differential growth in tissues adjacent to and away from the stimulus.
2. Phototropism
The movement of plant organs, such as the stem and coleoptile, in response to illumination by a unilateral (or unidirectional) light.
The phototropic response may be positive, as when the stem tip bends toward a light source, or negative when it bends away from the same light source.
Leaves normally orient at intermediate angles with respect to light direction and are so described as plagiotropic.
3. Geotropism or Gravitropism
The unidirectional response of plants to gravitational pull.
Common terms applied in describing responses of plant organs to the direction of gravitational pull are:
- Positive gravitropism – the organ, e.g. the primary root, grows downward toward the direction of the pull of gravity (center of the earth);
- negative gravitropism – the organ, e.g. the shoot, moves upward in opposite direction to the center of the earth;
- orthogravitropic – the parallel alignment of the primary plant axis (primary root and stem) with the direction of the pull of gravity;
- diagravitropic – the organ, e.g. stolons and rhizomes, grows perpendicular to the pull of gravity;
- plagiogravitropic – the organ, notably the lateral stems and roots, orient at some intermediate angle (between 0° and 90°) with respect to the linear direction of the gravitational pull;
- and agravitropic – the organs do not respond or have little sensitivity to the pull of gravity.
4. Chemotropism
Plant movement in response to a chemical substance.
An example is the growth of the pollen tube toward the substances secreted by the stigma and style and onto the ovule or embryo sac.
Plant roots elongate toward a supply of essential mineral nutrients.
A special type of chemotropism is aerotropism or oxytropism, the bending response to air, particularly oxygen.
5. Electropism or Galvanotropism
The movement of curvature in response to electrical current.
6. Heliotropism
Also called “solar tracking,” is a plant movement in which the organs of plants track the sun across the sky.
The responding organ may be oriented perpendicular, parallel, or obliquely to the sun’s rays.
Examples: sunflower (Helianthus annuus) and compass plant (Silphium laciniatum).
7. Hydrotropism
The growth of plant parts, i.e. the roots, in response to moisture or water. The root exhibits a positive hydrotropic response by moving toward the water source.
8. Rheotropism
The movement of curvature in which a plant organ, i.e. roots, turns away from the water current.
9. Thermotropism
The movement of curvature in response to changes in the external stimulus of temperature.
10. Thigmotropism or haptotropism
The movement of plants in response to touch or physical contact.
It results in curvature and the coiling of tendrils or entire stems on supports but also occurs in other plant organs such as leaves, petioles, and roots.
The curvature is due to differential growth, that is, more cell division and elongation at the outer than on the inner side in contact with the support.
Examples: bitter gourd (tendrils), morning glory (Ipomoea triloba), beans.
11. Traumatropism
Plant movement in response to one-sided injury. Roots tend to turn away from the wounded side.
Nastic Movements
Nastic Movements are plant movements independent of the direction of the external stimulus.
The direction of response is predetermined by internal control mechanisms within the tissues.
Unlike tropisms, there is no pronounced bending toward or against the direction of the stimulus.
These plant responses can be either growth movements that are permanent or turgor movements that are reversible.
12. Epinasty
The bending of an organ, such as petioles, leaves, and peduncles, toward the ground, is not due to gravity.
The bending response is due to a higher rate of longitudinal growth on the upper than on the lower side of the organ.
13. Hyponasty
The upward bending of an organ (reverse of epinasty).
14. Hydronasty
Plant movement (for example the opening and closing of some flowers) in response to atmospheric humidity.
15. Nyctinasty
The sleep movement (opening and closing) of plant organs, such as leaves and flowers, is due to day and night periods of daily rhythm.
The leaves of many nyctinastic plants open during the day or part of the day and close at night.
The nyctinastic movement caused by the change in light intensity is called photonastic (n. photonasty) while that caused by the change in temperature is called thermonastic (n. thermonasty).
16. Seismonasty
Movement in plants in response to touch as well as other forms of physical contact or mechanical disturbance such as shaking, wounding, wind, raindrops, and intense heat or burning.
In the case of the sensitive plant (Mimosa pudica), a leaflet, leaf, or group of leaves rapidly folds and bends in response to the external stimulus.
17. Thigmonasty or haptonasty
Plant movement in response to touch or physical contact without regard to the direction of the stimulus.
Thigmonastic movements are exemplified by the closing of the insect-eating plant Venus’s flytrap (Dionaea muscipula) and the bending of the glandular hairs of sundew (Drosera sp.) as a result of contact with an insect.
REFERENCES
DARWIN C. 1881. The power of movement in plants. Retrieved Feb. 11, 2013 from http://archive.org/stream/powermovementin06darwgoog/powermovementin06darwgoog_djvu.txt.
GANONG WF. 1913. The living plant. Retrieved Feb. 14, 2013 from http://archive.org/stream/livingplant031758mbp/livingplant031758mbp_djvu.txt.
HOPKINS wg. 1999. Introduction to Plant Physiology. 2nd ed. New York, NY: John Wiley & Sons, Inc. p. 391-414.
MOORE R, CLARK WD, VODOPICH DS. 2003. Botany. 2nd ed. New York, NY: McGraw-Hill. p. 443-454.
