Which effect becomes weak if the wood grain runs in the same direction on every plane of a cube?

Correct answer: Direction of separate planes and three-dimensionality

The governing concept is that surface texture helps describe planes and three-dimensional form. On a cube, grain should respond to the orientation of each visible plane. If it runs identically across every plane, the boundaries between planes become less convincing and the cube loses some spatial clarity. Local colour, cast shadow, and height may remain unchanged; the weakened effect is the directional distinction of planes and the sense of volume.