Lesson

The Plant That Leaned

Poppy turned her bean plant around. Within days it was leaning toward the window again.

Poppy Fenn with her magnifying glass

1Notice

Poppy kept a bean plant on the kitchen windowsill. It grew fast. Then one morning she saw something odd.

The plant was not standing straight. It was bending. The whole top leaned toward the window.

She turned the pot around. The plant looked wrong for a day. Then it slowly bent again, toward the window.

Flicker tried to lean the same way. He fell over.

Poppy. It is following the window. A plant has no eyes. How does it know where the light is?
Poppy Fenn with her magnifying glassPoppy noticed this one
Toby Fenn with his notebookToby wrote it all down

2Try it

Toby wanted proof. You can test it too. You need two small plants, a shoebox and a window.

  1. Put one plant by a sunny window. Put the other in a closet or a box with a lid.
  2. Give both the same amount of water.
  3. Look at them every day for a week. Draw what you see.
  4. Then turn the window plant around and watch for three more days.

No plants? Dried beans in a wet paper towel in a cup will sprout in a few days.

3Oops

Toby's first test went wrong.

He put one plant in the sun and one in the closet. But he gave the sunny one water every day. He forgot the closet plant until Friday.

The closet plant was droopy and yellow. He was sure darkness had done it.

Poppy. It could be the missing water. You changed two things.

He started over and gave both plants the same water. This time the closet plant did not droop. It grew tall and thin and pale instead. Now he knew the light did that.

4Figure it out

Here is what the Fenns worked out.

A plant stem has a growth helper inside it. When light shines on one side, the helper moves to the shady side. The shady side grows a little longer than the sunny side.

A stem that is longer on one side bends the other way. So it bends toward the light.

A plant in the dark has no light to follow. It stretches as fast as it can, searching. That makes it tall, thin and pale. It has not made its green color, because that needs light.

The plant does not see the light. It feels it with its growing.

Grandpa Fenn in his lab coat at the workshop doorGrandpa Fenn

5Grandpa's notebook

Plants grow toward the light.

The shady side of the stem grows longer. That bends the plant toward the sun.

No light makes a plant tall, thin and pale.

Big word for the day. Phototropism. It means a plant bending toward light.

6Your turn

Two bean plants share a window box. Move the lamp. Cover a plant. Then let the days go by.

Lamp
Cover plant B with a box
Day0
Plant A leansStraight
Plant BStraight
  • ✓Make a plant lean toward the lamp.
  • ✓Move the lamp to the other side and watch the plant bend back.
  • ✓Grow a plant in the dark until it is tall and pale.

The plant felt the light with its growing and followed it.

Turn a plant at home and wait three days. See which way it bends.

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