With the diagram serving as a visual metaphor, we can say that the features of the head word project upward in the diagram. In the case of the phrase this diligent worker, the noun worker is the ultimate head of the whole phrase, as well as immediate head of the nominal diligent worker. But in the larger sentence, worker is not the head of any higher unit. There is no arrow from the direct object to the VP because the direct object doesn't head the VP, the verb does. This observation gives us a way of conceptualizing the difference between nominals and NPs. Looking at the diagram, we can see that each phrase is the maximal projection of a head word. In other words, the head word's features project up to the phrase level and no further. The nominal is a constituent that has a noun as its head, but it is not the maximal projection. A unit higher up in the tree also has the same word as its head.
 For present purposes, we treat the clause as non-headed, which is the traditional assumption.