In this paper we give
relations between the Radon-Nikodym-Property (RNP), in sequentially complete
locally convex spaces, mean convergence of martingales, and σ-dentability.
(RNP) is equivalent with the property that a certain class of martingales is
mean convergent, while σ-dentability is equivalent with the property that
the same class of martingales is mean Cauchy. We give an example of a
σ-dentable space not having the (RNP). It is also an example of a sequentially
incomplete space of integrable functions, the range space being sequentially
complete.