  The growth factor of linear fluctuations is probably one of the least known
quantity in observational cosmology. Here we discuss the constraints that
baryon oscillations in galaxy power spectra from future surveys can put on a
conveniently parametrized growth factor. We find that spectroscopic surveys of
$5000 deg^2$ extending to $z \approx 3$ could estimate the growth index
$\gamma$ within 0.06; a similar photometric survey would give
$\Delta\gamma\approx 0.15$. This test provides an important consistency check
for standard cosmological model and could constrain modified gravity models. We
discuss the errors and the figure of merit for various combinations of redshift
errors and survey size.
