Just as humans are sensitive to external stimuli, so are plants:

• their shoots grow towards light
• their shoots grow against gravity
• their roots, however, grow with gravity
• their roots also grow towards moisture

Also like humans, plants produce hormones. These hormones are used to control growth. Plants are affected particularly by light, gravity and moisture.

Hormones are released in unequal measures to different parts of a plant, which allows these areas to grow at different rates and directions. Auxins, for example, are very important hormones in co-ordinating the growth or shoots and roots.

——————————————————

Auxins

Auxins are produced mainly in the tips of both shoots and roots and they’re able to diffuse into other areas of these parts. Their main role is to regulate how fast the cells elongate, thereby controlling the length of a root or shoot. Therefore, the amount, or concentration, of auxins in the shoots and roots determine whether they grow quickly or slowly. Auxins also deal with the direction of growth.

Phototropism

One direction of growth is phototropism. This direction is controlled by where the light source is located. In shoots, the side with the least amount of light produces more auxins. This is so that it grows longer and so is able to bend towards the light.

Roots, on the other hand, are affected differently by auxins. Instead of the lit side containing more auxins it is, in fact, the shaded side. However, this side doesn’t grow more than the lit area: it grows less. This means that the root is able to move in a direction away from the light.

Gravitropism

Gravitropism is to do with growth that is affected by gravity. As with phototropism, shoots and roots react differently. Shoots naturally grow away from the force of gravity, a fact known as negative gravitropism. This is due to the fact that more auxins are produced on the lower side of the stem causing it to grow up vertically.

Roots grow with the force of gravity which is known as positive gravitropism. This is because there is a build-up of auxin on the lower side of the root that stops growth in this area, thereby causing the root to bend downwards.

——————————————————

How humans control plant growth

Just as we can use hormones to control fertility in women so plant hormones can be used as weed killers and rooting hormones in agriculture and horticulture. Weed killers are designed to work on particular plants and not on others. The reason why this is so important is because gardeners or farmers want to be able to kill only the plants they don’t want rather than their flowers or their crops. The main way a weed killer works is by stimulating the weed to grow too quickly by absorbing growth hormone. The other plants still absorb the weed killer but not enough of it to kill them.

On the other hand, rooting hormones can be used to stimulate growth. These are used on stem cuttings to stimulate them into producing roots faster than if left to do so naturally.

NOTE: You don’t need to be able to name any specific weed killers or rooting hormones.