"Follower" contains a great deal of assonance. In general, the vowel sounds that echo throughout the poem add texture and feeling to the atmosphere, while also drawing attention to important words and the ideas they represent.
For example, in line 2, the long /oh/ sound in "shoulders globed" visually mimics a circle, via the "o" that appears on the page and the shape the reader's mouth takes when sounding it out. This literal roundness plays off of the image it describes—the hunched shoulders of the speaker's father that resemble a globe—making the image stick in the reader's memory. Throughout the poem, the speaker continues to use maritime and navigational language to dignify his father. Thus, the assonance in line 2 helps the speaker establish a positive perception right off the bat and lay the groundwork for the imagery to come.
In lines 15-16, which contain a concentrated group of short /i/ sounds, the speaker feels the ripples of his father's movements, riding on his back while he works:
Sometimes he rode me on his back
Dipping and rising to his plod.
The short, snappy /i/ sounds come and go in quick succession and occur in both stressed and unstressed syllables. The meter here is already slightly irregular, as both lines begin with a trochee (DUM-da) rather than an iamb (da-DUM). The addition of assonance creates an increasingly turbulent and unpredictable rhythm and mood, resembling the dips and rises that the speaker feels as the cadence of his father's tread ripples up to him.
Lastly, the poem’s final lines contain three long /e/ sounds:
It is my father who keeps stumbling
Behind me ...
Here, the assonance of long sounds slows the reader down, increasing their awareness of the poem’s final events, which represent an abrupt break from the established setting and dynamic—the speaker’s father, who is otherwise described as strong and skillful, has aged and now staggers behind his son. The additional emphasis on “keeps stumbling / Behind me” drives that reality home suddenly and adamantly, before leaving the reader with one final note—this is the indefinite state of their relationship—making for a memorable ending.