  The pre-Lie operad can be realized as a space T of labelled rooted trees. A
result of F. Chapoton shows that the pre-Lie operad is a free twisted Lie
algebra. That is, the S-module T is obtained as the plethysm of the S-module
Lie with an S-module F. In the context of species, we construct an explicit
basis of F. This allows us to give a new proof of Chapoton's results. Moreover
it permits us to show that F forms a sub nonsymmetric operad of the pre-Lie
operad T.
