Growing towards the light.

Step 1 of 5

A plant that grows at its tips

The Scene shows a week-old sunflower seedling in a clear box of soil. Its shoot grows up into the light and carries the leaves, and its root grows down into the soil, anchors the plant and takes up water and minerals. Below the leaves the stem is the hypocotyl, and the two thick leaves at its top are seed leaves, which held the food stored in the seed.

A plant grows only at a few places. At the tip of each shoot and each root sits a meristem, a small cluster of cells that keeps dividing for the plant’s whole life. Behind it, in the growing zone, the new cells lengthen: in the seedling’s shoot, the top two centimetres or so. A plant cell lengthens by taking water into its vacuole, which is why a seedling can grow a few millimetres an hour.

Step 2 of 5

Towards the lamp

A tropism is a growth movement towards a stimulus coming from one side, or away from it. The seedling grew straight up under the lamp, and now Lamp direction is fixed at 70° to the left. Answer, then move Time since the change to check.

Step 3 of 5

Auxin changes sides

In 1880 Charles Darwin and his son Francis lit grass seedlings from one side, and they bent towards the light. When they covered the tip with a tiny cap, the seedling did not bend; when they covered the part below the tip, it bent as usual. The tip senses the light, they concluded, and something travels from it to the part that bends. That substance is called auxin today. To reason about it, remember: a side that lengthens more becomes the outside of the curve.

Step 4 of 5

Where the shoot stops

Most of the time a shoot feels two pulls at once: light from one side tells it to lean, and gravity tells it to stand up straight. In the Scene the seedling is upright again with the lamp overhead, and now the lamp will move 70° to the right. Predict where the shoot will stop.

Step 5 of 5

A lean of 45 degrees

Each pull grows with the sine of the angle between the shoot and its direction, and in the Scene the pull towards the light weighs three times the pull upwards. So with a lamp at angle $\varphi$⁠ from overhead the shoot settles at the angle $\theta$⁠ where

$\tan\theta = \frac{3\sin\varphi}{1 + 3\cos\varphi}$$\tan\theta =$$\frac{3\sin\varphi}{1 + 3\cos\varphi}$⁠

Use the formula to work out where to put the lamp, and check against the dashed line on the chart.