E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
(Latin, poplus, from populus, the people.) Being symbolical of the people, both because its leaves are dark on one side and white on the other, and also because they are never still, but blown about by the least gust of wind. In France, to the present day, the poplar is an emblem of democracy. There are black and white poplars, and the aspen-tree is one of the species.
The white poplar was consecrated to Herculs, because he destroyed Kakos in a cavern of Mount Aventine, which was covered with poplars. In the moment of triumph the hero plucked a branch from one of the trees and bound it round his head. When he descended to the infernal regions, the heat caused a profuse perspiration which blanched the under surface of the leaves, while the smoke of the eternal flames blackened the upper surface. Hence the Herculean poplar has its leaves black on one side and white on the other.