What it does
Plants cannot walk to the light, but they can grow towards it. A growth movement towards or away from something is a tropism: towards light is positive phototropism, and growing in response to gravity is geotropism (shoots grow up, roots grow down). This page grows a seedling in fast motion. The tip of the shoot senses light coming from one side, and a plant hormone, auxin, gathers on the shaded side. Auxin makes cells grow longer, so the shaded side grows faster than the lit side and the shoot curves towards the lamp. You can repeat the famous experiments too: cover the tip with foil, or cut it off. Plants bend to light so their leaves catch more of it for the photosynthesis lab; plants have no nerves, unlike the animal control system in the brain map.
Using it
- Pick a Level. Class 10 explains the bending. Class 11 adds how far the tip is from pointing at the lamp, the history of the tip experiments, and a table of the five plant hormones.
- Choose the light: a lamp you can drag anywhere in the picture (or move with the lamp position slider), light from straight above, or dark.
- Choose what happens to the tip: left alone, covered with a foil cap (it cannot see light), or cut off (no auxin is made).
- Stand the pot upright or lay it on its side.
- Press Grow to play hour by hour (up to 44 hours), or +1 hour to step. Move the lamp at any time and the shoot changes course. Start again plants a new seedling.
Worked example
Start with the lamp in its first place, up and to the right. The light reaches the tip 48° from the way it is pointing, so the shaded side gets 72 % of the auxin and the lit side 28 %. After 10 hours the tip leans 36° to the right and the split is down to 59 : 41, because the tip now faces the lamp more. After 20 hours it leans 43°, where the pull of the light balances the shoot’s urge to grow upright. Cover the tip and grow again: after 10 hours the shoot is just as tall but perfectly straight. Cut the tip off and it barely grows at all.
Where this fits in school
Class 10 science (control and coordination) covers coordination in plants: tropisms, auxin and the other plant hormones, and movements like the folding of touch-me-not leaves. Class 11 biology (plant growth and development) adds the discovery of auxin and the jobs of auxins, gibberellins, cytokinins, abscisic acid and ethylene. This page is an independent learning aid. It is not made by or connected with NCERT, and it does not copy any textbook.
Questions
Do roots bend away from light? A little, in many plants, but their main response is to gravity. In this model the root mostly turns down, with a small push away from the lamp.
Is a touch-me-not plant’s folding a tropism? No. It is fast and does not involve growth: cells near the leaf base lose water and change shape.
Is anything saved? No. Nothing you do is stored or sent anywhere.
Where the numbers come from
- Tropisms and the role of auxin in bending, and the tip experiments (Darwin, 1880; Went, 1920s): standard school biology, written by fibub in its own words.
- The growth and bending rates and the auxin split are made-up round values for teaching, labelled as such on the page.
- The model is checked in the tests: a shoot lit from above or kept in the dark grows straight; one lit from the side turns towards the light; a covered or cut tip does not bend; a shoot on its side turns up and its root turns down.
- Drawings are fibub's own simple shapes. Nothing you do is stored or sent anywhere.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.