5.4.4 Tree diagrams

　 A tree diagram is a formal representation of sentence structure with syntactic categories provided ( It is also referred to as a phrase maker or a constituent structure tree). In 5.3, we analyze the ambiguous phrase green grapes and pears by way of tree diagrams. But in those tree diagrams there are no syntactic categories. A tree diagram without syntactic categories fails to reveal ambiguous structures in some cases. For example, the sentence They can fish is ambiguous. The ambiguity can be shown clearly if the syntactic category of each constituent is marked. Consider the two tree diagrams:

These up-side-down “trees” disclose three aspects of speakers' syntactic knowledge:

　　1. The linear structure of the sentence (i.e. the order of words in the sentences);
　　2. The hierarchical structure of the sentence (what category is above what other category and what are its immediate constituents);
　　3. The lexical category of each word (what class of words each lexical item at the bottom belongs to).

　 In transformational generative grammar, tree diagrams and phrase structure rules are closely related. Phrase structure trees are believed to be generated by phrase structure rules. There should be no discrepancies between the two. In 5.4.3 we generalized four tentative phrase structure rules:

(i) S → NP VP

(iii) VP → (Aux) V (NP) (PP)
(iv) PP → P NP

These rules can generate a great number of sentences. Rule (i) generates:

Rules (ii) can generate five different NP structures, as the rule can be expanded into:

　　NP → N
　　NP → Adj N
　　NP → Det N
　　NP → Det Adj N
　　NP → Pro

Rule (iii) can generate eight different VP structures, as its constituents are:

　　VP → V
　　VP → Aux V
　　VP → V NP
　　VP → V PP
　　VP → V NP PP
　　VP → Aux V NP
　　VP → Aux V PP
　　VP → Aux V NP PP

As rule (iv) specifies that PP is composed of P and NP, the structure of PP can be as various as NP structures. Now we can see that phrase structure rules can generate an enormous number of tree diagrams even without lexical items.

　 Tree diagrams correspond to phrase structure rules in specifying syntactic categories. The categories that occur to the left of the arrow in a phrase structure rule are phrasal categories. They do not appear at the bottom of the tree. They label the node point or branching of the tree. The categories that never occur on the left side of the arrow are lexical categories, which are labels of class of words right above the words at the bottom of the tree.