  Riemann surfaces with nodes can be described by introducing simple composite
operators in matrix models. In the case of the Kontsevich model, it is
sufficient to add the quadratic, but ``non-propagating'', term (tr[X])^2 to the
Lagrangian. The corresponding Jenkins-Strebel differentials have pairwise
identified simple poles. The result is in agreement with a conjecture
formulated by Kontsevich and recently investigated by Arbarello and Cornalba
that the set ${\cal M}_{m*,s}$ of ribbon graphs with s faces and
$m*=(m_0,m_1,\ldots,m_j,\ldots)$ vertices of valencies
$(1,3,\ldots,2j+1,\ldots)$ ``can be expressed in terms of Mumford-Morita
classes'': one gets an interpretation for univalent vertices. I also address
the possible relationship with a recently formulated theory of constrained
topological gravity.
