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A hesperidium (plural hesperidia) is a modified berry with a tough, leathery rind.
The outer ovary wall becomes the thick spongy layer of the rind, while the inner ovary wall becomes very juicy with several seeds.
The outermost, pigmented layer of rind contains essential oils and is known as the flavedo. When scraped off and used as a culinary ingredient it is called zest. A confection called succade can also be produced by candying the inner rind (known as pith or albedo) of the citron or lemon.