To bend with apples the moss’d cottage-trees,
The tree is so stacked with delicious apples, it is made to buckle and bend under the intense weight. ‘Bend’ is a key verb here, alluding to the excessive growth and maturity that typifies the season. This is developed in subsequent lines as Keats loads his verse with images of excess (fill, swell, plump, o'er-brimmed).
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