Plants cannot move themselves to areas of preferable conditions. This is why plants have the ability to detect a stimulus and respond to it by growing or bending in its direction or away from it. These responses are called tropisms. For example a plant tends to grow its stem in the direction of sunlight for more photosynthesis, this is a tropism. There are two types of tropism, these are phototropism and geotropism.
Phototropism: the response in which a plant grows towards or away from the direction from which light is coming.
Geotropism: the response in which a plant grows towards or away from gravity.
A tropism can be either positive or negative. If a tropism is in the direction of the stimulus, it is positive. If the tropism is away from the stimulus it is negative.
For example, a plant’s shoot tends to grow in the direction of sunlight, this is positive phototropism. But the plant’s root grows in the opposite direction deeply into the soil, this is negative phototropism. However, positive phototropism can also be described as negative geotropism because it involves the plant growing in the direction opposite to gravity. And negative photo tropism can be described as positive geotropism because it involves the plant growing towards gravity.
Auxins:
Tropisms are controlled by a chemical called Auxin. Auxin is a plant hormone. It is produced by cells at the tip of roots and shoots of plants. At the tip of a shoot, there is an area in which cells are being produced by dividing so that the shoot grows. Old cells do not divide, but they grow longer instead. The growth of these cells longer is controlled by auxins. Auxins is what makes the plant grows this is why a plant doesn’t grow if you cut it’s tip off.
Auxins’ Role in Phototropism:
If the sun shines on the right side of a plant’s shoot, auxins will accumulate on the dark opposite left side. Auxins accumulating there makes the cells on the left side grow much faster than the cells on the right side. When the left side of the shoot starts growing faster than the right side, the shoot will start to bend to the right side towards sunlight. This is phototropism.
Auxins’ Role in Geotropism:
Auxins tend to settle at the bottom end of the root. However, this does not make the sells of the tip of the root grow longer. Instead, auxins prevent the cells at the bottom tip of the root from growing, making the cells at the middle of the root grow faster. When the cells of the middle of the root grow faster, they push the root deeper into the soil and the root gets longer. The root grows in the direction of the gravitational pull. This is geotropism.
Roots show positive geotropism and negative phototropism because they grow towards gravity and away from sunlight at the same time. Shoots show positive phototropism and negative geotropism because they grow towards the sunlight and away from gravity at the same time.
Advantages of Positive Phototropism:
Leaves exposed to more sunlight and are able to do more photosynthesis,
Flowers can be seen by insects for pollination,
The plant gets higher for better seed dispersal.
Advantages of Positive Geotropism:
By growing deeply into the soil, the root fixes the plant into the ground firmly,
Roots are able to reach more water,
Roots have a larger surface area for more diffusion and osmosis.
Selective Weed Killers:
Auxins can be used to kill weeds that grow over grass or cereal crops. If weed grows on crops, auxins are sprayed everywhere. Weeds absorb auxins faster than crops or grass. Auxins accumulate in the weeds making them grow very rapidly. Fast growth of weed kills it leaving the crops or grass alive. Auxins are used ass selective weed killers.
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