The aspen leaf is said to tremble, from shame and horror, because our Lord's cross was made of this wood. The fact is this: the leaf is broad, and placed on a long leaf-stalk so flexible as scarcely to be able to support it in an upright position. The upper part of the stalk, on which the play mainly depends, is flattened; and, being at right angles with the leaf, is peculiarly liable to be acted on by the least breath of air.
“Those aspen leaves of theirs never leave wagging.” —
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894