Tree-systems of morphisms

Starting from the idea of determinism in membrane systems, we introduce a l anguage generating device consisting of morphisms placed in the nodes of a tree. Initial strings are given in the leaves: by iteratively applying the morphisms to them, we produce new strings, which are collected in the root of the tree. Such a device is called a tree-system of morphisms (in short, a T system). We investigate here the power of T systems, both in the extend ed (a terminal alphabet is considered and only strings over it are accepted ) and non-extended case, mainly in comparison with classes of languages in Lindenmayer hierarchy.