Hormones are chemicals which are used by living things to send messages between cells. We have hormones such as testosterone, adrenaline and insulin which tell our body to do certain things. Plants also have hormones to coordinate different processes.

The plant hormone auxin coordinates primary growth. It allows the shoot to grow upwards and towards the light and the roots to grow downwards. It acts in different ways on different parts of the plant (i.e. shoots and roots): it accelerates growth in the shoot but slows growth in root cells.

Auxin moves downwards through plant cells. In a shoot, this makes the cells at the bottom grow longer, which turns the shoot upwards.

However, in roots auxin makes the cells on the bottom edge elongate more slowly, which makes the root grow downwards.

So no matter which way up a seed is planted, its shoots will always grow up away from gravity and the roots will always grow down towards gravity.

In shoots (leafy part of the plant) auxin also stimulates growth towards the light. Auxin is produced at the tip of the plant and filters downwards through the stem. It will move away from light, stimulating more growth on the darker side and resulting in the tip of the shoot bending towards the light.

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Plant Sensitivity

Sensitivity means being able to sense changes in the environment. A stimulus is a change in the environment which produces a response (e.g. the bell is a stimulus which causes your response – to pack up!).

One way plants respond to their environment is with tropisms. This is a growth response to a stimulus (e.g. light, gravity, touch).

Tropisms can be:

positive: growth towards the stimulus
negative: growth away from the stimulus

We have names for tropisms caused by different stimuli:

light: photo
gravity: gravi
chemicals: chemo
touch: thigmo
water: hydro

For example, growth towards light is called positive phototropism. Growth away from gravity is called negative gravitropism.

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Growth in Flowering Plants