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In botany a 'capsule' is a type of simple, dry fruit produced by many species of flowering plants. A capsule is a dehiscent structure composed of two or more carpels, that, at maturity, split apart (dehisce) to release the seeds within. In some capsules, the split occurs between carpels, and in others each carpel splits open. In yet others, seeds are released through openings or pores that form in the capsule. In the Brazil nut, a lid on the capsule opens, but is too small to release the dozen or so seeds (the actual "Brazil nut" of commerce) within. These germinate inside the capsule after it falls to the ground.
Capsules are sometimes mislabeled as nuts, as in the example of the Brazil nut or the Horse-chestnut. A capsule is not a nut because it releases its seeds and it splits apart. Nuts on the other hand do not release seeds as they are a compound ovary containing both a single seed and the fruit. Nuts also do not split.
Examples of plants that produce capsules are poppy, lily, orchid, willow, and cotton.