Seal a seedling in a dark box. Put a few walls between it and the only gap. Come back a week later and it has threaded the maze — bent around every corner, leaves tilted at the opening. No eyes. No muscles. No plan. It still gets there.
A tropism is growth aimed by a stimulus. Phototropism is growth aimed by the direction of light — the response your cress is using to climb out of the box.
A plant can't walk to the window. Instead it builds new cells faster on one side than the other, and the whole shoot tilts. The bend is permanent. It is made of growth, not motion.
The tip of the shoot senses where the light comes from. The bending happens lower down, where cells stretch — and a hormone called auxin carries the message between them.
The shoot will lean toward it — everyone knows that much. The interesting part is how a stem with no muscles makes itself lean.
Which side of the stem grows faster — the side facing the light, or the side in shadow?
The shaded side. Light drives auxin sideways, away from the bright face, so it builds up on the darker side of the shoot — and auxin makes cells stretch. The shaded side ends up with longer cells, and the longer side pushes the tip over until it points at the light.
If you've ever seen a houseplant leaning at a window, the side that did the growing is the one facing the room — the dark side.
Light shifts auxin sideways across the tip, away from the bright face. Within hours it has collected along the shaded side of the shoot.
Auxin's instruction to a shoot cell is simple: loosen your wall, take in water, stretch. More auxin means more stretch. The cells on the shaded side elongate more than their neighbours in the light.
One side of the stem is now built from longer cells than the other. Unequal growth on two sides can do only one thing: tip the whole shoot over — toward the light. Nothing pulls it. Nothing decides. The chemistry just adds up.
Wherever the light is, auxin gathers on the far side — and the far side grows longer. That single rule is enough to steer the whole plant.
Drag to move the light around the seedling. Watch where the auxin gathers, and which way the shoot leans. Drag the slider and watch.
Send that same auxin into a root and it does the reverse. In root cells, a high dose of auxin slows growth down instead of speeding it up.
So in a root the crowded side grows less, and the root curves the other way — bending away from light and downward, with gravity. That downward response has its own name: gravitropism.
One hormone, two organs, opposite outcomes. Shoots climb toward the light to feed; roots dive into the dark to anchor and drink. Each tropism earns the plant something it can't move to fetch.
A shoot caught mid-bend, with light arriving from the left. Match each label to the spot it describes.
Young seedlings were lit from one side for 24 hours. Group A was left intact. Group B had its tip covered with an opaque cap. Group C had its tip cut off. Group D wore a clear, see-through cap over the tip. Afterwards, only groups A and D had bent toward the light.
State = a specific factual answer, no explanation needed.
Accept: "growth toward light". Do not accept: "heliotropism" / "the plant moves to the light".
Explain = give the reason and the mechanism — both what happens and why. Award 1 mark per point, max 3.
Accept: "hormone" for auxin. Do not accept: "the plant couldn't see".
Deduce = reach a conclusion that the data support. Award 1 mark per point, max 2.
Accept: "elongation / growth zone" for the bending region.
Fill in the blanks. Stuck? Tap Reveal answers.
Phototropism is the directional of a shoot toward . The tip the light, and the hormone collects on the side, where cells more — tipping the shoot toward the source.
The tip senses the light, auxin shifts to the shaded side, and uneven elongation tips the whole shoot. No intention, no pulling — just chemistry that adds up.
A plant can't travel, but it can grow in a chosen direction. Phototropism is a permanent bend made of new cells.
The tip detects the light; auxin gathers on the shaded side; those cells elongate more, and the longer side tips the shoot over.
The same auxin sends shoots toward light and roots away from it. Same signal — opposite response, because root cells read it in reverse.