Plants need sunlight, so the stems (or shoots) respond to sunlight by growing towards it. The plants also turn their leaves to face the sun. This makes sure that the leaves get as much sunlight as possible. When a plant is grown in the open ground with the sunlight coming from above, then the stem of plant grows straight up.
If, however, the plant is grown with sunlight coming from one side, then the stem of plant bends towards the direction from which the sunlight comes. The root of plant, however, bends away from the direction from which the sunlight comes. We will now describe an experiment to show the response of plant parts to light.
We take a potted plant growing in a transparent glass jar. When this potted plant is kept in the open space, the sunlight falls from above due to which the stem of plant grows straight up towards the source of light ‘sun’. The root of plant also grows straight but in the downward direction.
Let us now keep the potted plant having straight stem and straight root near the window in a dark room so that sunlight falls on it from the right side (through the window) only. After some days we will see that the stem of the plant bends towards the right side from where the light is coming.
This observation shows that the stem of plant responds to light and bends towards it. Even the leaves of the plant turn towards the sun so as to obtain the maximum sunlight. Thus, the stem (and leaves) of a plant are positively phototropic. Now, if we look at the root of the plant, we find that the root bends to the left side away from the light. This observation shows that the root of plant responds to light by growing away from it. Thus, the root of plant is negatively phototropic.
We will now explain the bending of a plant stem towards sunlight. The plant stem responds to light and bends towards it due to the action of ‘auxin hormone’. This happens as follows:
(i) When sunlight comes from above, then the auxin hormone present in the tip of the stem spreads uniformly down the stem. Due to the equal presence of auxin, both the sides of the stem (A and B) grow equally rapidly. And the stem grows straight up.
(ii) When the light falls only on the right side of the stem then the auxin hormone collects in the left side (shady side A) of the stem, away from light. This is because auxin hormone prefers to stay in shade.
(iii) Now, more auxin hormone is present in the left side of stem but not on its right side. Due to more auxin hormone, the left side (A) of stem grows faster than its right side (B) where there is no auxin. Since the left side of stem grows faster and becomes longer than its right side, therefore, the stem bends towards the right side (in the direction of light).
We can also explain the bending of a plant root away from light by the action of auxin hormone. For this we have to remember that the effect of auxin on the growth of a root is exactly opposite to that on stem.
Thus, though auxin hormone increases the rate of growth in a stem but it decreases the rate of growth in a root. Now, the side of a root away from light will have all the auxin concentrated in it. Due this, the side of root which is away from light will grow slower than the other side and make the root bend away from light. Please draw the diagram to show the bending of plant root away from light yourself.
The force, with which the earth pulls all the things towards it, is called gravity. The force of gravity always acts in the downward direction. The response of plants to gravity is called geotropism. Geotropism is also known as gravitropism.
(i) The roots of plants always grow downward in response to gravity. This makes sure that they will find soil and water.
(ii) The stems (or shoots) of plants always grow up, away from the pull of gravity. This makes sure that they will get light.
The movements of plant roots towards the earth and that of stem away from earth, both are cases of geotropism. Since the roots grow down towards the pull of gravity, so the downward growth (or downward movement) of roots is called positive geotropism.
The stem (or shoot) grows upwards, away from the pull of gravity, so the upward growth (or upward movement) of stem or shoot is called negative geotropism. The response of plants to gravity (or geotropism) will become clearer from the following experiment.
(i) We take a potted plant growing in a transparent glass jar. When this potted plant is kept in the normal position, we can see that its roots are growing downwards and its stem is growing upwards.
(ii) Let us now tilt the potted plant and keep the pot horizontally on its side. In this position, the roots and stem both are parallel to the ground (or earth). Allow the plant to remain in this position for a few days.
(iii) After a few days we will find that the roots of the potted plant bend downwards towards the earth and the stem of plant bends upwards, away from the earth. The roots of plant grow downwards in response to the pull of gravity. The stem of plant responds to gravity in the opposite way, by growing upwards (away from the pull of gravity).
The growth (or movement) of a plant part due to chemical stimulus is known as chemotropism. The growth (or movement) of a pollen tube towards the ovule induced by a sugary substance as stimulus, is an example of chemotropism. This can be explained as follows: The ripe stigma in the carpel of a flower secretes a chemical substance (which is a sugary substance) into the style towards the ovary.
This sugary substance acts as a stimulus for the pollen grains which fall on the stigma of the carpel. The pollen grain responds to this stimulus by growing a pollen tube in the downward direction into the style of the carpel and reaches the ovule in the ovary of the flower for carrying out fertilisation. This growth of the pollen tube in response to a chemical substance secreted by the stigma of a flower is an example of chemotropism.
The roots of plants always go towards water, even if it means going against the pull of gravity. Though roots normally grow downwards but in order to reach water, they can grow sideways or even upwards. The roots grow in the direction of source of water so as to obtain water for the developing plant.
Since roots always grow (or move) towards water, therefore, roots are positively hydrotropic. When the roots bend by growing towards water, it appears that they move towards water. We will now describe an experiment to demonstrate hydrotropism. This will show us the response of roots to water.
We take two glass troughs A and B and fill each one of them two-thirds with soil. In trough A we plant a tiny seedling. In trough B we plant a similar seedling and also place a small ‘clay pot’ inside the soil. Water the soil in trough A daily and uniformly. Do not water the soil in trough B but put some water in the clay pot buried in the soil. Leave both the troughs for a few days.
Now, dig up the seedlings carefully from both the troughs without damaging their roots. We will find that the root of seedling in trough A is straight. On the other hand, the root of seedling in trough B is found to be bent to the right side (towards the clay pot containing water). This can be explained as follows.
In trough A, the root of seedling gets water from both sides (because the soil is watered uniformly). But in trough B, the root gets water oozing out from the clay pot which is kept on the right side. So, the root o seedling in trough B grows and bends towards the source of water to the right side. This experiment shows that the root of a plant grows towards water. In other words, the root of a plant is positively hydrotropic.