  Exclusive measurements of the quasi-free $pn \to pp\pi^-$ and $pp \to
pp\pi^0$ reactions have been performed by means of $pd$ collisions at $T_p$ =
1.2 GeV using the WASA detector setup at COSY. Total and differential cross
sections have been obtained covering the energy region $T_p = 0.95 - 1.3$ GeV
($\sqrt s$ = 2.3 - 2.46 GeV), which includes the regions of $\Delta(1232)$,
$N^*(1440)$ and $d^*(2380)$ resonance excitations. From these measurements the
isoscalar single-pion production has been extracted, for which data existed so
far only below $T_p$ = 1 GeV. We observe a substantial increase of this cross
section above 1 GeV, which can be related to the Roper resonance $N^*(1440)$,
the strength of which shows up isolated from the $\Delta$ resonance in the
isoscalar $(N\pi)_{I=0}$ invariant-mass spectrum. No evidence for a decay of
the dibaryon resonance $d^*(2380)$ into the isoscalar $(NN\pi)_{I=0}$ channel
is found. An upper limit of 90 $\mu$b (90 $\%$ C.L.) corresponding to a
branching ratio of 5 $\%$ has been deduced.
