  A class $\mathcal{G}$ of graphs is said to be {\em $\chi$-bounded} if there
is a function $f:\mathbb{N} \rightarrow \mathbb{R}$ such that for all $G \in
\mathcal{G}$ and all induced subgraphs $H$ of $G$, $\chi(H) \leq f(\omega(H))$.
In this paper, we show that if $\mathcal{G}$ is a $\chi$-bounded class, then so
is the closure of $\mathcal{G}$ under any one of the following three
operations: substitution, gluing along a clique, and gluing along a bounded
number of vertices. Furthermore, if $\mathcal{G}$ is $\chi$-bounded by a
polynomial (respectively: exponential) function, then the closure of
$\mathcal{G}$ under substitution is also $\chi$-bounded by some polynomial
(respectively: exponential) function. In addition, we show that if
$\mathcal{G}$ is a $\chi$-bounded class, then the closure of $\mathcal{G}$
under the operations of gluing along a clique and gluing along a bounded number
of vertices together is also $\chi$-bounded, as is the closure of $\mathcal{G}$
under the operations of substitution and gluing along a clique together.
