  Let \q be a simple algebraic group of type A or C over a field of good
positive characteristic. We show for any x \in \q =\Lie(Q) that the invariant
algebra S(\q_x)^{\q_x} is generated by the p^{th} power subalgebra and the mod
p reduction of the characteristic zero invariant algebra. The latter algebra is
known to be polynomial \cite{PPY} and we show that it remains so after
reduction. Using a theory of symmetrisation in positive characteristic we prove
the analogue of this result in the enveloping algebra, where the p-centre plays
the role of the p^{th} power subalgebra. In Zassenhaus' foundational work
\cite{Zas}, the invariant theory and representation theory of modular Lie
algebras were shown to be explicitly intertwined. We exploit his theory to give
a precise upper bound for the dimensions of simple \q_x-modules.
  When \g is of type A and \g = \k \oplus \p is a symmetric decomposition of
orthogonal type we use similar methods to show that for every nilpotent e \in
\k the invariant algebra S(\p_e)^{\k_e} is generated by the p^{th} power
subalgebra and S(\p_e)^{K_e} which is also shown to be polynomial.
