If a germinating seed is attached horizontally to a revolving klinostat, what would be the effect on the seedling after three days? The

Analyze the question A klinostat is a device that rotates a seedling slowly and continuously about a horizontal axis. We need to know what happens to the plumule (shoot) and radicle (root) when it is attached to one.

Concept: Geotropism

• Normally, gravity acts on a seedling from one direction. The radicle shows positive geotropism (grows downward), and the plumule shows negative geotropism (grows upward).
• This happens because gravity causes auxin to accumulate unevenly on the lower side of the organ. The uneven distribution leads to uneven growth, and that causes curvature.
• A klinostat rotates the seedling continuously, so every side of the seedling is exposed to gravity equally over time. Auxin is therefore distributed evenly, and the effect of gravity is cancelled out.

Solution

1. The seed is fixed horizontally on the revolving klinostat.
2. Rotation means gravity no longer acts on one side only. Its effect is spread evenly around the organ.
3. Auxin is distributed evenly in both the plumule and the radicle.
4. Growth is equal on all sides, so both organs grow straight, with no curvature.

Compare the options:

• A, B and D all suggest that gravity still produces a one-sided effect on at least one organ. That would only happen on a stationary seedling.
• C correctly says that neither organ curves.

Answer: C. The plumule and radicle will not show any curvature.

Exam tip: Remember that a klinostat eliminates the one-sided effect of gravity (or light, in phototropism experiments). If the question says the klinostat is revolving, expect straight growth. If it says the klinostat is stationary, expect normal curvature: the plumule grows up and the radicle grows down.