  Semi-transitive graphs, defined in \cite{hps98} as examples where ``uniform
percolation" holds whenever $p>p_c$, are a large class of graphs more general
than quasi-transitive graphs. Let $G$ be a semi-transitive graph with one end
which can be properly embedded into the plane with uniformly bounded face
degree for finite faces and minimal vertex degree at least 7. We show that
$p_u^{site}(G) +p_c^{site}(G_*)=1$, where $G_*$ denotes the matching graph of
$G$. This fulfils and extends an observation of Sykes and Essam in 1964
(\cite{SE64}) to semi-transitive graphs.
