A seedling laid on its side lifts its shoot and turns its root down within hours: heavy grains of starch in the root cap show it where down is, and the same auxin speeds up the shoot and slows the root. The bend stays in the stem, even once the tip is straight.
Step 1 of 5 · On its side
Step 1 of 5
On its side
Lay a potted seedling on its side, and within a few hours something happens to its shoot and its root. Here Lamp direction is fixed overhead, so the light pulls neither way, and Box tilt is fixed at 90°, on its side: the box’s own up points right. Predict, then move Time since the change to see.
Step 2 of 5
How to tell where down is
A plant has no inner ear, yet its root turned down within hours, and the shoot rose at the same time. Answer first, then move Zoom to the root tip to Root tip: the root magnified about thirty times, with the root cap, the zone where the cells divide, the growing zone and the root hairs.
Step 3 of 5
One hormone, two responses
In the shoot too, auxin flows to the low side. Zoom to the root tip now shows the Shoot cut lengthwise: the cells on one side are longer, and the amber specks are auxin. Move Time since the change and see which side they drift to. But if auxin gathers on the low side of the root as well, why does the root not curve up like the shoot?
Step 4 of 5
Light against gravity
The box lies on its side, and the shoot points right. Now the lamp will move to 70° to the right, almost where the shoot already points. The light hardly needs to move it, and gravity pulls it upright with all its strength. Predict where it will stop, in degrees from upright, and then compare it with the upright seedling of the last Lesson.
Step 5 of 5
The bend stays
A shoot that grows on does not straighten back. As its cells leave the growing zone they harden in the shape they were given, and the new growth above the bend runs straight. That is why the roots of tipped seedlings, and trees on a slope, show sharp elbows.
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Each step sets up the Scene for you, and a control marked “fixed” stays put for that step.
Practice · Predict, then look
The lamp is overhead, and the box has been laid on its side, so the shoot points right and the root points left. What happens over the next three hours?
Practice · Predict, then look
The lamp is overhead, and the box has been laid on its side, so the shoot points right and the root points left. What happens over the next three hours?
Practice · Choose
The root of the tipped seedling turned down within hours. How does the root tip know where down is?
Practice · Choose
The root of the tipped seedling turned down within hours. How does the root tip know where down is?
Practice · Choose
In the tipped seedling auxin gathers on the low side of both the shoot and the root. Yet the shoot curves up and the root curves down. Why?
Practice · Choose
In the tipped seedling auxin gathers on the low side of both the shoot and the root. Yet the shoot curves up and the root curves down. Why?
Practice · Predict, then look
The box lies on its side, so the shoot points right, 90° from upright. Now the lamp moves to 70° to the right, almost where the shoot already points. At what angle from upright does the shoot settle after a day?
Practice · Predict, then look
The box lies on its side, so the shoot points right, 90° from upright. Now the lamp moves to 70° to the right, almost where the shoot already points. At what angle from upright does the shoot settle after a day?
Practice · Calculate
A day after the box was laid on its side, the shoot’s tip points straight up again. By how many degrees has the shoot bent relative to the box?
Practice · Calculate
A day after the box was laid on its side, the shoot’s tip points straight up again. By how many degrees has the shoot bent relative to the box?
Lesson complete. Every practice question here is done.