Final answer:
The infinitive phrase in the sentence 'to hurl stones at flowers is silly' is 'to hurl stones at flowers,' and it functions as the subject of the sentence.
Step-by-step explanation:
The infinitive phrase in the sentence "to hurl stones at flowers is silly" is 'to hurl stones at flowers'. This phrase functions as the subject of the sentence, stating an action in its infinitive form (to + base form of the verb) without specifying a doer or a tense. The entire infinitive phrase is treated as a noun in the context of the sentence. An easy identification of infinitive phrases is that they begin with the infinitive verb, usually with the word "to" in front.