Maze Plants

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 Response of Plants to Gravity: Geotropism

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.

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