Science Puzzle
Why Does a Plant Lean Towards the Window?
Put a young plant on a windowsill and, within a few days, its stem curves over to point at the glass. Turn the pot round and, a few days later, the stem has bent back towards the window again. A plant has no muscles and no eyes.
How does it manage to lean towards the light?
The Answer
The shaded side of the stem grows faster than the sunny side, so the stem curves towards the light.
The tip of a growing shoot can sense where light is coming from. In response, a growth hormone called auxin builds up on the shady side of the stem. Auxin makes plant cells stretch and grow longer. With the cells on the shady side lengthening more than those facing the window, the stem bends, the same way a strip of two metals curls when one side expands more than the other.
Charles Darwin and his son Francis studied this in 1880 using grass seedlings. When they covered just the very tip of each seedling with a tiny cap, the shoots stopped bending towards the light, even though the bend itself happens lower down. Something made in the tip, they realised, travels down the stem and controls the growth. That something turned out to be auxin.
It's worth it for the plant. Leaves make food from sunlight by photosynthesis, so turning them towards the brightest light helps the plant grow. Roots do something similar with gravity, growing downwards whichever way up you plant a seed.
The principle: Phototropism. Plants bend towards light because a hormone, auxin, gathers on the shaded side of the stem and makes those cells grow longer than the ones facing the light.