We propose a formal framework for a modular and hierarchical organization of lexicalized grammars which are expressible with tree descriptions. The typical syntactic constructions of a grammar are expressed with tree descriptions embedded in modules which are organized into a hierarchy by a multiple inheritance relation. The syntactic constructions that constitute the entries of the lexicon are built automatically by crossing the terminal modules of this hierarchy under two kinds of constraints: negative constraints which are expressed by disjunctive inheritance and which forbid crossing some modules and positive constraints which are expressed by co-occurrence relations between modules and which force the crossing of other modules. The lexical modules that result from this crossing process are linked to the words of the language in the lexicon by a mechanism that is based on feature structures. Every module of the grammar is associated with a feature structure or a disjunction of feature structures which describes the morpho-syntactic profile of the words of the language that can anchor the syntactic construction described by the module. Unification is used for generating the profiles of the lexical modules in parallel with the processes of module inheritance and crossing.