Phototropism

How It Works — Step by Step

1. 1

   Plants Grow by Stretching Cells

   Start with the smallest part: one cell. The close-up on the right shows the growing zone of a seedling shoot as two columns of green cells, one for each flank. A shoot lengthens mainly because cells just below the tip stretch, not because they divide. Each cell loosens its wall, takes up water and gets longer. Taller boxes in the close-up mean faster stretching.

2. 2

   The Tip Makes the Growth Signal

   Cells only stretch when told to. The signal is auxin, a plant hormone made in the shoot tip and carried down into the growing zone: the orange dots and the orange lines along the shoot. Choose 'Tip cut off' in the 'Shoot tip' menu. The orange disappears and growth falls from 1 mm to 0.05 mm per hour. No tip, no auxin, almost no growth.

3. 3

   Light Pushes Auxin Into the Shade

   Choose 'Intact tip' again. With the lamp to one side, the tip senses the light and carrier proteins move auxin across to the shaded flank. The 'Auxin split' readout shows 70 : 30. Now move the 'Light direction' slider to 0, straight overhead. Both flanks are lit equally, the split returns to 50 : 50 and the two cell columns match.

4. 4

   Uneven Growth Turns the Shoot

   Put the light back to the side. The shaded flank, with more auxin, stretches faster than the lit flank. When one side of a stem gets longer than the other, the stem has to curve toward the shorter side, like a train track that is longer on the outside of a bend. Press 'Grow 1 hour' and watch the tip tilt toward the lamp.

5. 5

   Worked Example: 23 Degrees an Hour

   Light comes straight from the side at full brightness and the tip is upright. The shaded flank gets 70 % of the auxin and the lit flank 30 %. With an average growth of 1 mm per hour, the shaded flank grows 2 × 1 × 0.70 = 1.4 mm per hour and the lit flank 0.6 mm. The 0.8 mm difference across a 2 mm wide shoot bends it 0.8 ÷ 2 = 0.4 radians, about 23 degrees, in an hour.

6. 6

   Bending Stops When the Tip Faces the Light

   Let the shoot grow (press Play if it is paused) and watch the 'Bend rate' readout fall as it turns. Once the tip points at the lamp, both flanks are lit the same, auxin evens out to 50 : 50 and the shoot grows straight toward the light. Move the 'Light direction' slider to the other side mid-growth: the shoot starts curving back, leaving an S-shaped stem that records where the light used to be.

7. 7

   Testing Which Part Does What

   Press Reset, then choose 'Foil cap on tip'. The shoot grows at full speed but straight up: the tip is where light is sensed. Reset and choose 'Mica barrier through tip'. Again no bend: the tip sees the light but auxin cannot cross to the shaded side. Classic experiments with caps, cuts and barriers pinned down each step this way.