  We study the thermal fluctuations of anisotropic order parameters (OP) in
layered superconductors. In particular, for copper oxides and a d-wave OP, we
present some experimental consequences of fluctuations in the direction normal
to the layers. It is shown that the c-axis penetration depth $\lambda_c$ can
have a "disorder-like" quadratic temperature dependence at low temperature.
  The fluctuations are analyzed in the framework of a Lawrence-Doniach model
with an isotropic Fermi surface. Anisotropies pin the orientation of the OP to
the crystallographic axes of the lattice. Then we study an extended t-J model
that fits Fermi suface data of bilayers $YBCO$ and $BSCCO$. This leads to a
d-wave OP with two possible orientations and, including the thermal
fluctuations, yields the announced temperature dependence of $\lambda_c$.
Furthermore a reservoir layer is introduced. It implies a finite density of
states at the Fermi energy which is successfully compared to conductance and
specific heat measurements.
