Auxin and plant growth

Last time

Before we start: last time, in Excretion, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Plants that bend

Put a plant on a windowsill and in a few days it bends towards the window. Plants have no nerves or muscles, yet they respond to light and gravity. They do it with a plant hormone called auxin. Today: phototropism, gravitropism, and how auxin causes them.

Tropisms

A tropism is a growth response of a plant to a stimulus that comes from one direction. Phototropism is a response to light. Shoots grow towards light: positive phototropism. Gravitropism is a response to gravity. Roots grow down, with gravity; shoots grow up, against it.

Why it helps

Shoots growing towards light get more light for photosynthesis. Roots growing down anchor the plant and reach water and mineral ions.

Auxin

Auxin is made in the shoot tip. It diffuses down through the plant from the tip. In shoots, auxin stimulates cell elongation: the cells grow longer.

Phototropism

Here is phototropism. When light comes from one side, auxin moves to the shaded side of the shoot. The cells on the shaded side elongate more, so that side grows faster and the shoot bends towards the light.

Phototropism in steps

Light shines on one side of the shoot. Auxin collects on the shaded side. Cells on the shaded side elongate more, so the shoot bends towards the light.

Gravitropism in a shoot

Lay a plant on its side. Gravity makes auxin collect on the lower side of the shoot. The lower side elongates more, so the shoot curves upwards. This is negative gravitropism: the shoot grows away from gravity.

Investigating tropisms

Cover a shoot tip with foil: it no longer bends, because the tip detects the light and makes auxin. Use a clinostat, which slowly turns the plant, to cancel the effect of gravity or one-sided light. Germinating seeds on a clinostat grow straight, while seeds left still bend.