Movement in plants – The behavior of an organism is greatly affected by the responses it makes to both external (outside the body) and internal (within the body) stimuli. This ability of response to the stimulus is present in the protoplasm and is the characteristics of all the living organisms both plants and animals.
The direction of the stimulus has relationship to the direction of the movement of a from the stimulus is known as negative response.
The movements in plants are of following three types:
- Tactic
- Nastic
- Tropic movement
Tactic Movement
In this type of movement the plant body as a whole moves from one place to another in response to external stimuli such as light, water, chemicals etc, e. g chlamdomonas always move towards light of low intensity and away from light of high intensity.
This type of response is known as positive phototactic response and negative phototactic response respectively.
Fig 14.7 tactic movement in chlamydomonas
Nastic Movement
This is a movement made by a part of a stationary plant in response to external stimulus. Stimulus in this case influences the organ with equal intensity from all the directions.
Nastic movements are exhibited by bud scales and flower petals. These bend upwards due to fast growth on their lower surface. When the bud unfolds itself, the growth of its upper surface is faster than that of the lower surface, hence the petals bend downwards. In the case of ipomea (fig. 14.8) fading of the flower is due to the turgor changes in its rib cells. The cells in the inner side of the rib lose solutes and water while the outer rib cells expand, causing curling.
Fig 14.8 nastic movement in lpomea
Similarly changes in the water contents of the tissues at the base of the leaf of mimosa plant results in the shrinkage and folding of its leaflets. Such response may be due to different stimuli, such as light, touch, heat, or electric shock. After a suitable recovery period the leaflets open again. Normally they remained expanded in the day time while at night they are closed (fig. 14.9).
Fig. 14.9 nastic movement in the leaflets of mimosa plant
Tropic movement
When the fixed part of the stationary plant moves in response to a stimulus the reaction is known. As tropic movement or tropism. Stimulus in this case influences from some specific direction. These are growth movements as response against the stimulus is produced by a plant’s growing points, the root and shoot apex. The growth movement is caused by an increased and decreased rate of growth on the side of the organ which is under the influence of the stimulus with respect to the opposite side.
The negative tropic movement occurs when the plant organ grows more rapidly on the side nearer to the stimulus so that it bends away from the stimulus. In case of a positive tropic movement, the side of the organ away from the stimulus grows more rapidly so the organ bends towards the stimulus.
Movement in plants – Tropism or tropic movements are of following three types.
- Geotropism
- Phototropism
- Hydrotropism
Geotropism
The response shown by the tips of the shoots and roots of a plant to the stimulus of gravity is known as geotropism.
The growing shoot apex and the leaves response negatively to the gravity exhibiting negative geotropism [fig. 14. 10(i)].
(ii) negative geotropism. Fig. 14.10 (i) positive geotropism
Negative geotropic response of shoots
- Shoots which are negatively geotropic grow vertically and it helps the plant to compete for light and carbon dioxide.
- The flowers are brought into an advantageous position for pollination through insects or wind.
- Seed dispersal may be more effective on long and vertical stem.
Positive geotropic response in roots
- It causes roots to grow downwards into the earth where they can anchor the plant firmly in the soil.
- It causes them to obtain water and mineral salts.
- Lateral roots, not positively geotropic grow at right angles or slightly downwards from the main root. This enables a large volume of soil to be exploited and helps to anchor the plants securely.
Phototropism
The response shown by the tips of shoots and roots of the plant to the stimulus of light is known as phototropism.
The growing shoots and leaves show a positive response exhibiting positive phototropism (fig. 14.11) while the growing roots generally show a negative response exhibiting negative phototropism.
- By growing towards the source of light, a shoot brings its leaves and stems to absorb the maximum amount of sunlight for photosynthesis and synthesis of chloroplast.
- Flowers are brought into an exposed position where they are most likely to be seen and pollinated by flying insects.
Fig. 14. 11 positive phototropism
Hydrotropism
The response of growing tissues of plant to the stimulus of water is known as hydrotropism.
Root apices i.e. tips respond positively towards water while shoot apices respond negatively.
In such cases as shown as shown in (fig. 14.12), water is an even stronger stimulus than gravity.
Fig. 14.12 positive hydrotropism
Many plants are capable of solar tracking, in which flat or cup shaped lamina of the leaf remains nearly at right angles to the sun throughout the day, maximizing the light harvested by the leaf. Solar tracking in some members of family malvaceae e.g. malva or lavatera is shown below. The lamina in this case receive directional signals from the sun and tilt to face it as the tissues at its junction with the petiole, gain or lose water (a and b).
The leaves track the sun during the day, much as a radio telescope tracks a satellite. © an hour or two after sunset, the laminae are in the “relaxed” position, which they maintain during most of the night. (d) About an hour before sunrise, laminae have moved to face on the horizon where the sun will rise.
Isn’t it an interesting phenomenon? Would you like to investigate it in leaves of other species like cotton, beans, alfalfa etc?
Impotence of tropic response
Positive phototropic response of shoots
Practical work: to investigate geotropism.
- Place four soaked corn grains on the bottom of an empty Petri dish.
- Arrange them as shown in figure 14.12 with their pointed ends directed towards the center of the dish.
- Cut a piece of filter paper to fit tightly inside the bottom of a Petri dish.
- Place the paper over the corn grains.
- Pack the rest of the dish tightly with the wet cotton.
- Cover it with other Petri dish bigger than this and seal the edge with transparent tape.
- Using a water proof marker, label the location of the seeds A, B, C, and D.
- Stand the Petri dish up on its side in a kark place and fix it down with any supporting martial.
- Make a sketch of the set up in your note book. Using different colors for the roots and shoots, sketch the daily additional growth. Specially direction of the seedling.
- Rotate the dish on one-quarter turn so that A is at B, B is at C, C is at D, and D is at A.
- Allow the seedlings to grow for two more days. Continue recording your observations with sketches.
- What is the general direction of growth of root tips and shoot tips?
Geotropism of root and phototropism of shoots in terms of auxins
Geotropism and phototropism are caused by the unequal growth of plant parts which is controlled by chemical substances, known as auxins. Auxins are plant hormones. They are produced by the root and shoot apices from where they pass down to the growing region, specially the region of cell elongation and exert their growth stimulation effects there.
The increased concentration of auxins on the lower side of the root apex results in the decreased rate of growth on the underside. This results in movement of root towards force of gravity showing positive geotropic response.
14.13 geotropism of root and phototropism of shoot in terms of auxins.