A poem lovely as a tree
Joyce Kilmer
Phrase structure trees may not be as lovely to look at as the trees Kilmer wa thinking of, but if a poem is written in grammatical English, its phrase and sentences can be represented by trees, and those trees can be specified bu phrase structure rules.
The rules that we have discussed, repeated here, define some of the phrase structure trees of English.
S --- NP VP
NP --- Det N
VP --- V (NP) (PP)
PP --- P NP
There are several possible ways of viewing phrase structure rules. They can be regarded as tests that trees must pass to be grammatical. Each syntactic category mentioned in the tree is examined to see if the syntactic categories immediately beneath it agree with the phrase structure rules. If we were examining an NP in atree, it would pass the test if the categories beneath it were Det and N, in that order, and fail the test otherwise, insofar as our (incompete) set of phrase structure rules is concerned. (Obviously, in a more adequate grammar of English, NPs would be specified to include many more structures.)
The rules may also be viewed as a way to construct phrase structure trees that conform to the syntactic structures of the language. This is by no means suggestive of how speakers actually produce sentences. It is just another way of representing their knowledge, and it applies equally to speakers and listeners.
Conventions on generating trees have been developed. Generating as used here simply means 'specifying'. One convention is that the root of the tree, the S, occurs at the top instead of the bottom. Another convention specifies how the rules are to be applied. First, find a rule with an S on the left side of the arrow, and put the categories on the right side below the S, as shown here :
Once you have started, continue by matching any syntactic category at the bottom of the partially constructed tree to a category on the left side of a rule, and expanding the tree with the categories on the right sde. Proceed in this manner until only categories remain that never appear on the left-hand side of any rule, such as Det, N, and so on.
We may expand the tree started above by applying the NP rule to produce :
The categories at the bottom are Det, N, and VP, but only VP occurs to the left of an arrow in the set of rules. The VP rule is actually four rules abbreviated by parentheses. They are :
VP --- V
VP --- V NP
VP --- V PP
VP --- V NP PP
Any one of them may be chosen to apply next ; the order in which the rules appear in the grammar is irrelevant. ( Indeed, we might equally have begun by expanding the VP rather that the NP. ) Suppose VP --- V PP is chosen to apply. Then the tree has grown to look like this :
Convention dictates that we continue in this way until none of the categories. at the bottom of the tree appear on the left-hand side of any rule. Thus the PP must be expanded into a P and an NP, and that NP expanded into a Det and an N. We can use a rule as many times ats it can apply. In this tree, the NP rule was used twice. Sfter we have applied all the rules that can apply, the tree looks like this :
By following these conventions, only trees specified by the phrase structure rules can be generated. By implication, any tree not so specifies will be ungrammatical. Whether we choose to use the rules to generate only well- formed trees, or use the rules to test the grammaticality of all possible trees, is immaterial. Both methods achieve the goal of revealing syntactic knowledge. Most book on language use the rules to generate trees.
The categories that occur to the left of the arrow in a phrase structure rule are called phrasal categories; categories that never occur on the left side of any rule are lexical categories. Phrase structure trees always have lexical categories ta the bottom since the rules must apply until no phrasal categories remain. The lexical categories are traditionally called 'the part of speech', and include determiners, nouns, verbs, prepositions, and so on.
The previous tree structure corresponds to a very large number of sentences because each lexical category may contain many words, all listed and marked as to categoty in the lexicon. This tree structure corresponds to the following sentences and mllions more.
The boat sailed up the river.
A girl laughed at the monkey.
The sheepdog rolled in the mud.
The lions roared in the jungle.
At any point during the growth of a tree, any rule may be used providing its left-side category occur somewhere at the bottom of the tree. At the point where we chose the rule VP --- V NP PP. This would have resulted in a different tree that, when the lexical categories were filled, would have been the structure for such sentences as :
The boys left. ( VP --- V)
The wind swept the kita into the sky. ( VP --- V NP PP)
Since there are an infinite number of possible sentences in every language, there are limitless numbers of trees, but only a finite set of phrase structure rules that specify the trees allowed by the grammar of the language.
