Frist, in all languages, heads select dependents of a particular WORD CLASS: only dependents of certain class can occur with each kind of head. For example, in English, a head noun can be modified by an adjective such as bright as in (1a), but not an adverb such as brightly: * very brightly sunflowers. And head verb is modified by an adverb rather than an adjective, so we get spoke sincerely, but not *spoke sincere.