Final answer:
Williams is depicting the plants and landscape transitioning from winter's grip to the cusp of spring, with visual remnants of the last season still lingering.
Step-by-step explanation:
William Carlos Williams, in the passage from Spring and All, is describing the plants that have faded over winter. This particular imagery evokes a scene of foliage that hasn't yet sprung anew, with remnants of the previous year's growth still visible. The reference to dead, brown leaves under the twigs of bushes and small trees, and the mention of leafless vines, clearly points to the aftermath of winter before the full bloom of spring.