Geotropism and hydrotropism

1How does a buried seed know which way is down?

When a farmer in Bugesera plants maize at the start of the rainy season, she drops the grains into the holes without looking which way up they land. Some lie on their side and some upside down. Yet a week later every seedling has its root growing down into the soil and its shoot growing up into the air. Later in the season, when the rains stop, the roots keep growing towards the damp soil deep down. Two stimuli guide them: gravity and water.

2Experiment 1: seedlings on a clinostat

Aim: to show that roots respond to gravity.

1. Soak bean seeds overnight and let them germinate on damp paper until the roots are about 2 cm long and straight.
2. Pin three seedlings to the cork disc of each of two clinostats, with their roots pointing sideways (horizontal). Cover each disc with a damp, dark box so that the roots do not dry out and no light reaches them.
3. Switch on clinostat B so that it turns slowly. Leave clinostat A switched off.
4. After two days, draw the roots.

Figure 1: The clinostat experiment. Only the root that felt gravity from one side (A) bent. The shoots, not shown, do the opposite: on A they curve upwards.

Clinostat B is the control. It shows that the root in A did not bend just because it was pinned sideways or damp: it bent because gravity always pulled on the same (lower) side. The clinostat does not switch gravity off; it spreads its effect evenly round the root.

3Explanation: auxin in roots and shoots

When a seedling lies on its side, heavy starch grains in special cells settle to the lower side, and auxin moves to the lower side of both the shoot and the root. The surprise is that the same hormone has opposite effects in the two organs:

• In the shoot, extra auxin makes cells elongate more. The lower side grows faster, so the shoot curves upwards.
• In the root, which is far more sensitive to auxin, the same extra auxin makes cells elongate less. The upper side grows faster, so the root curves downwards.

Figure 2: A seedling laid on its side. Auxin collects on the lower side of both organs, but speeds up growth in the shoot and slows it in the root.

4Experiment 2: roots bending towards damp cotton wool

Aim: to show that roots respond to water.

1. Line the left wall of a clear plastic box with thick damp cotton wool and the right wall with dry cotton wool.
2. Pin three germinated bean seeds to a cork strip across the top, with their roots hanging straight down between the two walls. Close the lid so the air inside stays moist near the damp side.
3. Set up a control box with damp cotton wool on both walls. Keep both boxes in the dark.
4. Measure each root and note its direction after three days.

Figure 3: The hydrotropism experiment. Roots still grow downwards, but curve towards the damp side when water is on one side only.

The results of one class are shown below. These are illustrative figures, made up for this lesson; your own results will differ.

5Using geotropism and hydrotropism on the farm

• Planting seed any way up. Because roots are positively geotropic and shoots negatively geotropic, maize, beans and sorghum can be sown quickly without turning each seed the right way up.
• Watering deeply. A young fruit tree watered with a little water every day keeps its roots near the surface, where the soil dries first. Watered deeply and less often, the moisture soaks down and the roots follow it, so the tree survives the dry season (icyi) better. This matters in the drier Eastern Province, in districts such as Bugesera and Kayonza.
• Trees and pipes. Tree roots grow towards the damp soil around a leaking water pipe, drain or septic tank and can crack it. Plant large trees well away from them.