  The effective interaction between a quark and an anti-quark as obtained
previously with by the method of iterated resolvents is replaced by the so
called up-down-model and applied to flavor off-diagonal mesons including the
pion. The only free parameters are those of canonical quantum chromo-dynamics
(QCD), particularly the coupling constant and the masses of the quarks. The so
obtained light-cone wave function can be used to calculate the pion's form
factor, particularly its mean-square radius can be computed analytically. The
results allow for the exciting conclusion that the pion is built by highly
relativistic constituents, in strong contrast to composite systems like atoms
or nuclei with non-relativistic constituents.
