Why did Whitman name his poems after Leaves of Grass?

Whitman's poetry collection Leaves of Grass embodies the poet's democratic thought, because it endows the most common trampled little people with lofty status and dignity. Leaves of Grass is also an American "newcomer" image with strong vitality, including poets, which symbolizes the unique American spirit and character. One of the famous poems is "Captain, my captain!" My own song and so on.