  It is argued that the expected turn-down in $x- Q^2$ of the cross sections
(structure functions $F_2(x,Q^2)$), assumed to result from the saturation of
parton densities in the nucleon, is related to a phase transition from the
(almost) ideal partonic gas, obeying Bjorken scaling, to a partonic "liquid".
This can be quantified in the framework of statistical models, percolation and
other approaches to collective phenomena of the strongly interacting matter.
Similarities and differences between the case of lepton-hadron, hadron-hadron
and nuclear collisions are discussed.
