How plants respond

Plants respond too

A dog sees a ball and runs after it. It uses its nerves and its muscles. A plant has neither. Still, a plant on a windowsill turns its leaves to the light, and its roots find water deep in the soil. So plants also sense changes around them and respond.

A plant responds in two ways:

• by a quick movement, which happens in a few seconds and does not involve growth;
• by a growth movement, which is slow and takes hours or days.

A quick movement: the touch-me-not

Touch a leaf of the touch-me-not plant, Mimosa pudica, and its leaflets fold up within a second or two. Soon the whole leaf droops. This is not growth, because nothing new is made.

How can a plant move so fast without muscles?

1. The touch is sensed at one point of the leaf.
2. The message passes from cell to cell. There is no special tissue such as a nerve to carry it.
3. Certain cells at the base of the leaflets lose water quickly. They become soft and change shape.
4. The leaflets fold, because the cells that held them up have gone limp.

Animal muscle cells contract with special proteins. The plant cells instead change shape by changing the amount of water in them.

Growth movements: tropisms

Most plant movements are slow. They come from growth. When one side of a part grows faster than the other side, the part bends. A growth movement towards or away from a stimulus is called a tropism.

A movement towards the stimulus is called positive, and a movement away from it is negative.

• A shoot shows positive phototropism and negative geotropism: it grows towards light and up, against gravity.
• A root shows positive geotropism and positive hydrotropism: it grows down, and towards water. It usually grows away from light.

The tendrils of a pea plant also respond to touch. The side of the tendril that touches a support grows more slowly than the other side, so the tendril coils around the support.

How a bend is made

Picture a young stem lit from the left. The cells on its shaded, right side grow longer than the cells on the lit side. A strip that is longer on one side must curve, with the longer side on the outside. So the shoot curves to the left, towards the light.

Try it below. The more the shaded side outgrows the lit side, the more the tip swings over.

Try it in Python

A stem bends by an angle, in radians, equal to the extra length of one side divided by the thickness of the stem. The program uses this rule for one day of growth.

Practise with new numbers

Common mistakes

• "The touch-me-not folds its leaves by growing." The folding comes from cells losing water. Nothing grows.
• "Plants send messages along nerves." Plants have no nerves. A message passes from cell to cell.
• "A shoot bends because the lit side grows more." The shaded side grows more, so the shoot curves towards the light.
• "Roots grow down because they look for water." Roots grow down because of gravity. They also grow towards water, which is a second response.
• Mixing up positive and negative. Positive means towards the stimulus. Negative means away from it.

Remember these

• Plants respond without nerves or muscles, by quick movements or by growth movements.
• The touch-me-not folds its leaflets because some cells lose water and change shape.
• A tropism is a growth movement towards or away from a stimulus: light, gravity, water or a chemical.
• Shoots are positively phototropic and negatively geotropic. Roots are positively geotropic and positively hydrotropic.
• A part bends when one side grows faster than the other.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Is the folding of a touch-me-not leaf a tropism? Give a reason.

2. A pea tendril wraps around a stick. Which side of the tendril grows faster: the side touching the stick, or the other side?

3. Name the stimulus in each case: a root bending down; a pollen tube growing into the ovule.

4. Why does a shoot lit from the right bend to the right?

5. A shoot grows up and a root grows down. Both respond to the same stimulus. What is it, and how do their responses differ?