  Suppose that $G$ is a simple adjoint reductive group over $\mathbf{Q}$, with
an exceptional Dynkin type, and with $G(\mathbf{R})$ quaternionic (in the sense
of Gross-Wallach). Then there is a notion of modular forms for $G$, anchored on
the so-called quaternionic discrete series representations of $G(\mathbf{R})$.
The purpose of this paper is to give an explicit form of the Fourier expansion
of modular forms on $G$, along the unipotent radical $N$ of the Heisenberg
parabolic $P = MN$ of $G$.
