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A drupe is a fruit with a differentiated pericarp whose innermost region, the endocarp, forms a hard stone around the seed. The outer exocarp and middle mesocarp remain distinguishable from this stone. A peach supplies a familiar example: skin, flesh, stone and enclosed seed occupy different anatomical positions.^[ref]

The stone is therefore part of the fruit wall, rather than the seed coat. Cracking the stone exposes the seed within. This distinction matters in plant development: the pericarp derives from the ovary wall, whereas the seed develops from an ovule. A structure's hardness alone does not identify its origin.

The endocarp is the innermost region of the pericarp, the wall of a fruit. It lies inward of the mesocarp and surrounds the space containing the seeds. In a drupe such as a peach, the endocarp hardens into the stone.^[ref]

The prefix specifies position, not one universal material. A hard endocarp is characteristic of a drupe, but naming an inner fruit-wall layer does not require every fruit to possess a stone. Following the tissue from the ovary into the mature fruit distinguishes endocarp from the seed coat inside it.

The exocarp is the outermost region of the pericarp. It forms the external covering of the fruit wall, outside the mesocarp and endocarp. The skin of a peach illustrates the term.^[ref]

The exocarp belongs to the fruit, whereas the seed coat belongs to the enclosed seed. They may both act as protective coverings, but function alone does not make them the same structure. In a caryopsis their close attachment can conceal the distinction; tracing their origins to ovary and ovule respectively preserves it.

The mesocarp is the middle region of the pericarp, between the outer exocarp and inner endocarp. In a peach, much of the juicy edible flesh is mesocarp, while the stone is hardened endocarp.^[ref]

Its name describes anatomical position rather than edibility or a fixed degree of softness. A useful dissection follows the layers from the surface inward before assigning names. This prevents the everyday category “flesh” from replacing the developmental distinction between fruit tissue derived from the ovary and the seed derived from an ovule.

The pericarp is the wall of a fruit, derived from the wall of the ovary. Its regions can be distinguished as outer exocarp, middle mesocarp and inner endocarp.^[ref]

These regions need not have the same texture: in a drupe, such as a peach, the fleshy middle surrounds a hard inner stone. The stone is hardened pericarp; the seed lies inside it. In a caryopsis, by contrast, the pericarp is closely fused to the seed coat. These examples explain why a fruit's protective covering should be identified by its developmental origin as well as its appearance.