We show that the absolute Galois group acts faithfully on the set of Hubbard
trees. Hubbard trees are finite planar trees, equipped with self-maps, which
classify postcritically finite polynomials as holomorphic dynamical systems on
the complex plane. We establish an explicit relationship between certain
Hubbard trees and the trees known as ``dessins d'enfant'' introduced by
Grothendieck.Comment: 27 pages, 8 PostScript figure
