(The). (Latin, populus, 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 Hercule, 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.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894