Lusztig's partition and sheets, with an appendix by M. Bulois

We show that, for a connected reductive algebraic group $G$ over an algebraically closed field of zero or good characteristic, the parts, called strata, in the partition of $G$ recently introduced by Lusztig are unions of sheets of conjugacy classes. For $G$ simple and adjoint we refine the parametrization of sheets obtained in previous work with F. Esposito. We give a simple combinatorial description of strata containing spherical conjugacy classes, showing that Lusztig's correspondence induces a bijection between unions of spherical conjugacy classes and unions of classes of involutions in the Weyl group. Using ideas from the Appendix by M. Bulois, we show that the closure of a stratum is not necessarily a union of strata.