For Summer has o’er-brimm’d their clammy cells.
Summer has filled the cells of their honeycombs with sweet, sweet honey. (Actually, the bees themselves have done this, but the poem is giving seasons a lot of credit.)
Count the number of lines in this stanza. What do you have? 11. Important? Very. The ode is an ancient form of celebratory verse designed to praise a subject, in this case the season of Autumn. Ten-line stanzas are much more common for odes (see Keats’s own “Ode on a Grecian Urn” and “Ode on Melancholy” for examples), but here Keats adds a line. This deliberate tinkering with form is no accident. The ‘extra’ line almost subconsciously alludes to the excessive nature of Autumn that he debates in he poem itself. The poem itself is as ‘o'er brimmed’ as the subject it praises. And as readers, we feel the discomfort of this in the forced, slightly awkward rhyme scheme. Keats, in a display of true poetic mastery, echoes Subject in Form. Whoa.
Also, the ‘extra’ line in each stanza is slightly sinister and foreboding, suggesting the onset of dark things ahead. A subtle addition, but deeply arresting in the context of a poem dealing with the expiry of life (by a poet who was aware of his own impending death). Genius.
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