  We have observed 123 pulsars with periods longer than 0.1 seconds in the
Meterwavelength Single-pulse Polarimetric Emission Survey. In this work a
detailed study of polarization behaviour of these pulsars have been carried
out. We were able to fit the rotating vector model to the polarization position
angle sweeps in 68 pulsars, and in 34 pulsars the emission heights could be
measured. In all cases the radio emission was constrained to arise below 10\%
of the light cylinder radius. In pulsars with low spindown energy loss,
$\dot{E}<10^{34}$ ergs s$^{-1}$, we found the mean fractional linear
polarization of the individual times samples in single pulses to be around 0.57
(57\%) which is significantly larger than the fractional linear polarization of
0.29 (29\%) obtained from the average profiles. On the other hand the mean
fractional circular polarization of the individual time samples in single
pulses is around 0.08 (8\%), similar to the measurements from the average
profiles. To explain the observed polarization features, we invoke the
partially screened vacuum gap model of pulsars, where dense spark associated
plasma clouds exist with high pair plasma multiplicity, with significant
decrease of density in the regions between the clouds, that are dominated by
iron ions. The coherent radio emission is excited by curvature radiation from
charge bunches in these dense plasma clouds and escape as linearly polarized
waves near cloud boundaries. We suggest that the circular polarization arises
due to propagation of waves in the low pair multiplicity, ion dominated
inter-cloud regions.
