  A finite \emph{one-sided tree} of height $h$ is defined as a rooted planar
tree obtained by grafting branches on one side, say the right, of a spine, i.e.
a linear path of length $h$ starting at the root, such that the resulting tree
has no simple path starting at the root of length greater than $h$. We consider
the distribution $\tau_N$ on the set of one-sided trees $T$ of fixed size $N$,
such that the weight of $T$ is proportional to $e^{-\mu h(T)}$, where $\mu$ is
a real constant and $h(T)$ denotes the height of $T$. We show that, for $N$
large, $\tau_N$ has a weak limit as a probability measure supported on infinite
one-sided trees. The dependence of the limit measure $\tau$ on $\mu$ shows a
transition at $\mu_0=-\ln 2$ from a single spine phase for $\mu\leq \mu_0$ to a
multi-spine phase for $\mu> \mu_0$. Correspondingly, there is a transition in
the volume growth rate of balls around the root as a function of radius from
linear growth for $\mu<\mu_0$, to quadratic growth at $\mu=\mu_0$, and to qubic
growth for $\mu> \mu_0$.
