 Trees  is a poem of twelve lines in strict iambic tetrameter . All but one of the lines have the full eight syllables of iambic tetrameter . The eleventh , or penultimate , line begin on the stressed syllable of the iambic foot and drop the unstressed syllable — an acephalous ( or " headless " ) catalectic line — that result in a truncated seven @-@ syllable iambic tetrameter line . Making the meter of a line catalectic can change the feeling of the poem , and is often used to achieve a certain effect as a way of changing tone or announcing a conclusion . The poem 's rhyme scheme is rhyming couplets rendered aa bb cc dd ee aa .