In this article we will discuss about Plant Movement. After reading this article you will learn about: 1. Subject-Matter on Plant Movement 2. Classifications of Plant Movement.
- Subject-Matter on Plant Movement
- Classifications of Plant Movement
1. Subject-Matter on Plant Movement:
By plant movement it is meant the change in position of an organism or a part of its body. In a strict sense of the term, the movement is not usually associated with plants particularly with trees or shrubs. If movement is thought in the narrow sense of an entire organism moving, then very few instances are found in plant world.
Perhaps the best known examples are the unicellular alga Chlamydomonus which moves towards light or when spermatozoid moves towards the egg cell by means of flagella.
Again if the movement is thought in the sense of a part of an organism moving due to unequal growth rates of the opposite side, then numerous examples can be cited, viz., parts of the plants, their roots, their leaves, their stems all are capable of movement.
Plant movement may be classified into two broad divisions on the basis of convenience rather than on principle. A. Autonomic or spontaneous movement this is purely a mechanical movement unrelated to any irritability of the protoplasm.
Movements associated with dehiscence of sporangia and with explosive bursting of many fruits, dispersal of spores and seeds, movement of elaters are some of the examples. B. Induced or paratonic movement—these movements are induced when stimuli act on irritable or sensitive protoplasm.
2. Classifications of Plant Movement:
A. Demonstration of Autonomic Movement:
(a) Bursting of fern sporangium:
Some mature brown and some immature fern sporangia from son are mounted on a slide with a drop of water and observed under the microscope after carefully covering them with cover glass.
It is observed that mature sporangia dehisce at the region of annulus and spores are liberated. Whereas immature sporangia do not burst at all.
The autonomous movement of the sporangium causes bursting of it for dispersal of spores.
(b) Bursting of fruits:
Some mature (black) fruits Ruelia tuberosa are collected and put in a petridish containing water.
It is observed that the fruits burst open to liberate seeds.
The autonomous movement of fruit caused dispersal of seeds. It is a purely mechanical movement unrelated to any irritability of protoplasm, rather this is purely a hygroscopic movement largely determined by presence or absence of water.
(c) Movement of Oscillatoria:
Some freshly collected Oscillatoria filaments are mounted on a slide and observed under the microscope.
It is observed that the ends of the filament of the alga move to and fro.
Due to the autonomous movement the filament moves.
B. Demonstration of Induced Movement:
(I) Movement of locomotion:
(a) Movement by internal stimulus:
A thin section of Vallisneria leaf is mounted on a slide and observed under the microscope.
Protoplasmic streaming movement (shown by the movement of cytoplasmic granular plastid? which are actually taking along the stream) is observed.
This movement is due to internal stimulus. But the actual mechanism of this movement is not known. However, expenditure of respiratory energy is thought to be involved.
(b) Movement by external stimulus:
A fern prothallus is washed and slightly dried and mounted on a slide in a drop of water under cover glass. Now a drop of sodium malate solution (1%) is put on the edge of the cover glass and allowed to come in contact with the prothallus.
When observed under the microscope it is found that the antherozoids show movement being induced by sodium malate.
This type of movement is induced by a substance which is chemical in nature. So it is called chemotactic movement.
(II) Movement of curvature:
(a) Movement by internal stimulus:
This movement is exhibited by leaflets of telegraph plant, Desmodium gyrans (Figure 39) or by the leaflets of Oxalis.
It is observed that two small lateral leaflets of Desmodium or Oxalis display periodic movement.
The movement shown by the two small lateral Desmodium leaflets is effected by periodic changes in turgidity of the pulvinus of the leaf stalk. The movement in the leaflets of Oxalis is also due to periodic changes in turgidity of cells of pulvinus.
The best example of nutation movement is exhibited by twining plant like Cuscuta, Pisum, etc. This movement is also exhibited by stem tips in course of their elongation, showing an irregular spiral path. The following experiment is performed to demonstrate this movement.
A narrow glass thread is prepared from a glass rod and about 5 cm portion of this thread is attached vertically to the stem tip of a vigorously growing potted plant by means of sealing wax. A white square paper (2 cm × 2 cm) is perforated in the middle and passed through the glass thread and kept in a position near the base of the thread.
Now a rectangular glass plate is horizontally placed on a ring stand and its height is so adjusted that the tip of the glass thread just remains below the glass plate. The position of the tip of the glass thread is observed from time to time from top of the glass plate and its position is marked on the plate with India ink (Figure 40).
By plotting’ the dots at different time intervals the direction of movement can easily be observed.
This movement is called nutation or spiral movement it is due to some internal factors that affect the growth rate of plant segments.
N.B. The twinning of tendrils along the support may be explained on the basis of the fact that the tissues of the tendril which are touching the support grow at a faster rate than those which are away from the support. Because of this uneven growth on two sides of the tendrils, a curvature results and the tendrils twine around the support. This movement is under the control of hormones and stress-ethylene production.
(b) Movement by external stimulus:
Some Phaseolus seedlings are grown in a pot which is placed at the bottom of a heliotrope chamber (wooden cabinet leaving holes. At first the holes of the chamber are closed to cut off light source. Some control plants may be grown in a dark room.
When the seedlings have reached a height of 2 to 3 cm in the heliotrope chamber, one hole of suitable diameter is opened so that the diffused light comes from one direction only (Figure 41).
After 2 or 3 days it is observed that the seedlings bend towards the incident light. In the control, the seedlings grow straight.
If a seedling is exposed to unilateral light it results in an unequal distribution of hormones on the two sides of the tip. The darker side always has more hormone content than the illuminated side.
Light causes the destruction of hormone on the illuminated side. Other possibilities of this unequal distribution of hormone in different amount on the two sides are that there may be a light induced transverse migration of hormone from the illuminated side to dark side or that the sensitivity of the tissue to hormone on the lighted side may be decreased.
The hormone diffuses downwards and reaches the zone of response in unequal amount causing different rates of cell elongation which in turn causes curvature to take place.
N.B. Effect of different amounts of light may be studied by adjusting the holes of different diameters.
Some gram seeds are allowed to germinate until the plumules are 1 to 2 cm long. The inner side of a glass jar is covered with moist cotton. The seedlings are arranged in different positions on the moist cotton so that some have the plumular end downward, others have the plumular end upward while the others have the plumular end in horizontal position.
It is observed that in all the cases the radicle grows downwards and plumule upwards.
The primary root is positively geotropic, i.e., bends towards the centre of the earth and stem is negatively geotropic, i.e., grows vertically upwards. The stimulus of gravity must be unilateral since the force is exerted from one direction only.
When such organs are forced out of their upright position, they may assume it again if they are still capable of growth. The geotropic movement is due to unequal growth which is caused by an unequal distribution of hormones.
The reaction of roots to hormones differs from that of stem. In the stem the site containing more hormones grows fast, the opposite is the case in the root. A particular concentration of hormone induces acceleration of growth in the short end while the same concentration may produce an inhibitory effect in the growth and enlargement of the root cells.
The hormone in this case concentrates on the lower side of the plant and possibly growth inhibitors accumulate on the upper part of the horizontally placed mesocotyl resulting in more rapid growth of the cells of the lower side of the stem tissue causing them to bend upwards (negatively geotropic) and faster growth on the upper side of the root tissue, causing roots to bend downwards.
When vertically erect shoots of many plants are kept horizontally, their shoot apices bend upward to grow vertically. That the force of gravity acts as a stimulus in bringing about geotropic curvature can be proved by means of a clinostat (Figure 42).
This instrument is of many types, but the main thing is that they have a clock to the axis of which is attached a disc (perpendicularly attached at the centre) that can be rotated horizontally either slowly or rapidly with the clock.
A pot containing a seedling is fixed on the disc and the whole apparatus is placed in a horizontal position and the clock is started so that the pot rotates one to four revolutions per hour. Observation/ after two or three days it is observed that the shoots and roots keep on growing in horizontal directions without bending upward or downward.
This is because the gravitational stimulus is successively perceived equally on all the sides of the stem or the root as they rotate; keeping all the sides to grow at the same rate and thus the organ grows in horizontal direction.
In this condition the gravitational stimulus does not fall unilaterally on the same side of the organ. However, if the clock is stopped, the gravitational stimulus becomes unilateral and causes unequal growth so that the shoot moves upward and the root downwards.
The positive hydrotropism of roots may be demonstrated by growing gram seeds in moist saw dust in a shallow perforated funnel placed vertically on a conical flask (Figure 43).
The roots grow vertically downwards. As the growth proceeds they bend back by curvature from the dry air outside towards the moist saw dust.
Primary roots tend to grow towards the wetter regions due to positive hydrotropism. At first roots grow vertically downwards due to stimulus of gravity until they penetrate the sieve. But due to hydrotropism the tips of the roots bend again towards sawdust in search of water. Apparently the hydrotropic stimulus is more powerful than-the gravitational impulse.
The classical example is afforded by growth of the pollen tube through stigma and style towards the encibryosacs. Some pollen grains are germinated in nutrient jelly in which some carpels have been sown.
Germinating pollen tubes grow away from the air and move towards the pieces of carpel.
The pollen tubes show positive chemotropism. The first orientation of germinated pollen tube into the stigma takes place by hydrotropism, followed by a mechanical conduction all along the slimy way. For the final part of the movement of pollen tube through the style the chemotropic orientation is responsible.
Opening of some flowers during day time is due to photonasty. For example, flowers of Pentaptes Phoenicia fully open during noon but remain closed during night. The opening and closing of leaves of many leguminous plants may be studied.
The opening of stomata in light and closing in dark is also due to photonastic movement. In photo- nastic movement light from all directions acts as a stimulus which causes opening and closing of flower petals, leaflets and stomata. Light and darkness cause variation in turgidity of the cells hence the movement.
The opening and closing of certain flowers, e.g., Cactus, Cestrum, Nyctanthes, Mirabilis, etc., are due to nyctinasty. Here changes in temperature and light during .the day or night induce visible response which may be termed as nyctinasty. Changes in turgour pressure are also involved here.
This phenomenon is observed in the movements of leaflets of Mimosa pudica in response to touch, shock, etc. When a leaf of M. pudica is suddenly touched the leaf and its parts droop down rapidly due to shock stimulus.
If the shock is strong the entire leaf droops down. The whole thing consists of three movement’s movement of main petiole, movement of secondary petiole and closing and folding up of respective leaflets.
The bases of the main and secondary petiole are provided with pulvini consisting of loose parenchymatous cells with intercellular spaces inside and the movements are brought about by changes in the turgor of the cells of pulvinus.