In botany a capsule is a type of simple, dry fruit produced by many species of flowering plants. A capsule is a structure composed of two or more carpels. In Angiosperms (flowering plants), the term locule is used to refer to a chamber within the fruit. Depending on the number of locules in the ovary, fruit can be classified as uni-locular (unilocular), bi-locular, tri-locular or multi-locular. The number of locules present in a gynoecium may be equal to or less than the number of carpels. The locules contain the ovules or seeds.
In most cases the capsule is dehiscent, i.e. at maturity, it splits apart (dehisces) to release the seeds within. A few capsules are indehiscent, for example those of Adansonia digitata, Alphitonia, and Merciera. In some capsules, the split occurs between carpels, and in others each carpel splits open. Where a capsule splits into distinguishable parts, these are called valves; the borders of the valves may or may not coincide with the borders of carpels.
If the split occurs along junctions between carpels (i.e. along the septa) with the valves remaining in place, the capsule is described as septicidal. By contrast when it splits into cells (i.e. not at the septa) along the midrib or dorsal suture of the loculess, it is described as loculicidal. The latter is common among many members of the Liliaceae.
In yet other forms of capsule, such as those of poppies, seeds are released through openings or pores that form in the capsule. If it is the upper part of the capsule that dehisces, the capsule is also called a pyxis. For example, 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.