  We model the process of dust coagulation in protoplanetary disks and
calculate how it affects their observational appearance. Our model involves the
detailed solution of the coagulation equation at every location in the disk. At
regular time intervals we feed the resulting 3-D dust distribution functions
into a continuum radiative transfer code to obtain spectral energy
distributions. We find that, even if only the very basic -- and well understood
-- coagulation mechanisms are included, the process of grain growth is much too
quick to be consistent with infrared observations of T Tauri disks. Small
grains are removed so efficiently that, long before the disk reaches an age of
1E6 years typical of T Tauri stars, the SED shows only very weak infrared
excess. This is inconsistent with observed SEDs of most classical T Tauri
stars. Small grains somehow need to be replenished, for instance by aggregate
fragmentation through high-speed collisions. A very simplified calculation
shows that when aggregate fragmentation is included, a quasi-stationary grain
size distribution is obtained in which growth and fragmentation are in
equilibrium. This quasi-stationary state may last 1E6 years or even longer,
dependent on the circumstances in the disk, and may bring the time scales into
the right regime. We conclude that a simple evolutionary scenario in which
grains slowly grow from pristine 0.1 micron grains to larger grains over a
period of a few Myr is most likely incorrect.
