  In this paper, we report polarized optical microscopy and electrical
transport studies of manganese oxides that reveal that the charge ordering
transition in these compounds exhibits typical signatures of a martensitic
transformation. We demonstrate that specific electronic properties of
charge-ordered manganites stem from a combination of martensitic accommodation
strain and effects of strong electron correlations. This intrinsic strain is
strongly affected by the grain boundaries in ceramic samples. Consistently, our
studies show a remarkable enhancement of low field magnetoresistance and the
grain size effect on the resistivity in polycrystalline samples and suggest
that the transport properties of this class of manganites are governed by the
charge-disordered insulating phase stabilized at low temperature by virtue of
martensitic accommodation strain. High sensitivity of this phase to strains and
magnetic field leads to a variety of striking phenomena, such as unusually high
magnetoresistance (10^10 %) in low magnetic fields.
