Plant Responses Tropisms

Explained

Tropisms: how a plant responds without being able to move

A plant cannot walk towards the light, so it does the only thing available to it. It grows in a particular direction. A growth response to a directional stimulus is called a tropism, and the whole topic follows from that one idea.

Positive and negative

A positive tropism is growth towards the stimulus. A negative tropism is growth away from it. The stimulus is named in the first part of the word, so phototropism is a response to light and gravitropism, also called geotropism, is a response to gravity.

Put those together and each part of the plant has a description. A shoot is positively phototropic and negatively gravitropic: it grows towards light and away from gravity. A root is the reverse, negatively phototropic and positively gravitropic.

Auxin, and why one side grows more

Auxin is a plant hormone that makes cells elongate. When light comes from one side, auxin accumulates on the shaded side of the shoot. The cells there elongate more than those on the lit side, so that side of the shoot becomes longer and the shoot bends towards the light.

The bending is caused by uneven growth, not by movement. Nothing pushes or pulls the shoot. One side simply gets longer than the other, and a longer side on the outside of a curve is what a bend is.

Why roots behave in the opposite way

Auxin accumulates on the lower side of a root as well, but in roots a higher auxin concentration slows elongation rather than speeding it up. The lower side grows less, the upper side grows more, and the root bends downwards.

Same hormone, same accumulation, opposite outcome. That difference in response between shoot and root is the part most worth remembering, because a question about roots is usually testing whether you have simply memorised the shoot answer.

Answering a tropism question

Name the stimulus and its direction. Say where the auxin accumulates. Say what that does to cell elongation on that side. Then say which way the plant bends and whether that makes the response positive or negative.

Answers that jump straight from "light on one side" to "it bends towards the light" describe what happens and miss the marks for why. The auxin and the elongation are the explanation.