Definition
Phototropism is a plant's ability to sense which direction light is coming from and bend its stem to grow toward it. It's a survival strategy that evolved to help plants catch more sunlight and photosynthesize better.
Why a Windowsill Plant Leans Toward the Window
Leave a potted plant by a sunny window for a few days, and you'll see its stems and leaves all tilted toward the glass. Turn the pot 180 degrees, and a few days later it has turned around and is reaching for the window again.
Plants don't have legs, so they can't walk to a sunny spot. But sunlight is their food and their energy source. The more light that hits the leaves, the more actively they can make nutrients through photosynthesis.
So plants chose a different strategy: changing the shape of their own bodies to catch even a little more light. This change in the direction of growth in response to light is called phototropism. The tip of the stem notices where the light is coming from and bends to face the sun.
Auxin's Trick: Stretching the Shady Side
The secret to how a plant bends is uneven growth inside the stem. At the tip of a stem, the plant makes a growth hormone called auxin, which makes cells grow longer.
Auxin itself doesn't sense light. When light hits just one side of the stem, light-sensing proteins (called phototropins) send a signal, and auxin gets moved to the side away from the light, the shady side. As a result, auxin piles up in the back of the stem, where there's no light.
The cells on that shady side soak up water and stretch out very fast. The cells on the lit side grow slowly or almost stop. Think of runners on a track: the runner on the outside lane has to cover more distance to make the curve. In the same way, when only the back side gets longer, the whole stem naturally curves toward the front, toward the light.
A Closer Look: Roots Follow Gravity, Not Light
Stems usually grow toward light, but roots do the opposite. If you shine light on a root, it bends and grows away from the light. This is called negative phototropism.
Auxin is thought to play a part in this too, but scientists are still working out exactly how. Studies even disagree on which side the auxin builds up on. Roots are also much more sensitive to auxin than stems are. At a level that helps a stem grow, auxin can actually slow a root down.
Still, light isn't the main reason roots grow downward. Hardly any light reaches underground in the first place. Roots grow down because they sense gravity and grow in that direction, which is called gravitropism. So stems reach up toward the light, roots reach down with gravity, and the plant gets sunlight, water, and nutrients all at once.
Common misconceptions
A plant bends toward light because the cells on the lit side love sunlight and grow faster.
It's actually the opposite. A light signal moves the hormone auxin to the shady side, so the cells in the back, away from the light, get longer. That pushes the stem to curve forward toward the light.
Where you meet it
A monstera on your windowsill, whose stems bend toward the window within a few days.
If you grow bean sprouts or pea seedlings in a dark box with a small hole, they bend and grow toward the light coming in through the hole.
In one sentence
The plant hormone auxin stretches the cells on the shady side, which makes the plant bend and grow toward the sunlight.