Movement in plants – Tropism or tropic movements are of following three types.
- Geotropism 2. Phototropism 3. 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?
14.3.1 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 colours 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?
14.3.2 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.