The understory is the layer of plants and small trees that grow under the main canopy of a forest. It is usually darker and more humid, with its own special plants and animals.
From "under" plus "story," using "story" in the sense of a level of a building. It imagines the forest as a multi‑level structure, with the understory as a lower floor.
A forest is like a living skyscraper: canopy on top, understory in the middle, forest floor at the bottom. Many animals you never see live their whole lives in that shaded middle layer.
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