Halliday & Matthiessen (1999: 198-9):
… we can differentiate both processes and participants into finer and finer subcategories, until we reach a degree of differentiation that is associated with the choice of words (lexical items). Note that it is not (usually) the lexical items themselves that figure as terms of the systems in the network. Rather, the systems are systems of features, and the lexical items come in as the synthetic realisation of particular feature combinations. Thus lexis (vocabulary) is part of a unified lexicogrammar; there is no need to postulate a separate “lexicon” as a pre-existing entity on which the grammar is made to operate.