  Young stars are formed within dusty discs. The grains in the disc are
originally of the same size as interstellar dust. Models predict that these
grains will grow in size through coagulation. Observations of the silicate
features at micron wavelengths are consistent with growth to micron sizes
whereas the slope of the SED at longer wavelengths traces growth up to mm
sizes. We here look for a correlation between these two grain growth
indicators. A large sample of T-Tauri and Herbig-Ae/Be stars was observed with
the Spitzer Space Telescope at 5-13 micron; a subsample was observed at mm
wavelengths. We complement this subsample with data from the literature to
maximise the overlap between micron and mm observations and search for
correlations. Synthetic spectra are produced to determine which processes may
produce the dust evolution. Dust disc masses in the range <1 to 7 x 10^-4 MSun
are obtained. Most sources have a mm spectral slope consistent with grain
growth. There is a tentative correlation between the 10-micron silicate feature
and the mm slope of the SED. The observed sources seem to be grouped per
star-forming region in the micron-vs-mm diagram. The modelling results show
that the 10-micron feature becomes flatter and subsequently the mm slope
becomes shallower. Grain size distributions shallower than that of the ISM
and/or bright central stars are required to explain specific features. Settling
of larger grains towards the disc midplane affects the 10-micron feature, but
hardly the mm slope. The tentative correlation between the strength of the
10-micron feature and the mm slope suggests that the inner and outer disc
evolve simultaneously. Dust with a mass dominated by mm-sized grains is
required to explain the shallowest mm slopes. Other processes besides grain
growth may also be responsible for the removal of small grains.
