  A class of groups C is root in a sense of K. W. Gruenberg if it is closed
under taking subgroups and satisfies the Gruenberg condition: for any group X
and for any subnormal sequence Z \leqslant Y \leqslant X with factors in C,
there exists a normal subgroup T of X such that T \leqslant Z and X/T \in C. We
prove that a class of groups is root if, and only if, it is closed under
subgroups and Cartesian wreath products. Using this result we prove also that,
if C is a nontrivial root class of groups closed under taking quotient groups
and G = <A*B; H=K, \varphi> is the generalized free product of two nilpotent
C-groups A and B possessing \varphi-compartible central series, then G is
residually a solvable C-group.
