  Large $N$ two-dimensional QCD on a cylinder and on a vertex manifold (a
sphere with three holes) is investigated. The relation between the saddle-point
description and the collective field theory of QCD$_2$ is established. Using
this relation, it is shown that the Douglas--Kazakov phase transition on a
cylinder is associated with the presence of a gap in the eigenvalue
distributions for Wilson loops. An exact formula for the phase transition on
disc with an arbitrary boundary holonomy is found. The role of instantons in
inducing such transitions is discussed. The zero-area limit of the partition
function on a vertex manifold is studied. It is found that this partition
function vanishes unless the boundary conditions satisfy a certain selection
rule which is an analogue of momentum conservation in field theory.
