Concept: A revolving klinostat cancels the directional effect of gravity, so geotropic curvature cannot occur.
Explanation:A klinostat is a rotating device that slowly turns the germinating seed and continually changes its orientation.
In normal conditions, a horizontally placed seedling shows geotropism: the plumule curves upward and the radicle curves downward.
However, when the seed is attached to a revolving klinostat, gravity acts uniformly on all sides as the seed rotates.
No side remains downward long enough for the seedling to detect a fixed gravitational direction.
Thus, the plumule does not receive a signal to grow upward, and the radicle does not receive a signal to grow downward.
As a result, both parts continue to grow straight, without any bending or curvature.
Therefore, after three days, neither the plumule nor the radicle will show any curvature.
Answer: Option C — plumule and radicle will not show any curvature.