Résumé : A tree without leaves is uniquely determined by a cross ratio
function on its set of ends, this is a result of Tits. The cross ratio
on the boundary of a proper CAT(-1) space, defined and studied by
Bourdon, generalizes the projective cross ratio and its relationship to
hyperbolic geometry. In this talk I will discuss Bourdon's cross ratio
in the context of hyperbolic spaces and thick metric trees, bringing
together the ideas of Tits and Bourdon.