aril

An aril is a specialized outgrowth that partially or completely covers a seed. It is typically derived from the funiculus, the stalk that attaches the seed to the ovary, though in some species it may originate from the outer integument. Arils are often fleshy and brightly colored, but they can also be dry, leathery, or hairy. They are distinct from the seed coat (the testa) and, in some cases, may be only loosely associated with the seed.

The principal function of an aril is to aid seed dispersal by attracting animals, especially birds and mammals, that consume the aril and disperse the seed in the process. The aril can provide a nutritious reward that encourages consumption and subsequent seed handling or excretion at a distance from the parent plant. Aril development varies among species: the aril can be an extension of the funicle, an outgrowth of the outer integument, or a distinct tissue layer.

Notable examples include the mace, the dried aril surrounding the nutmeg seed; the red aril around seeds in yew trees; and fleshy arils around seeds in lychee, longan, and rambutan. In these and other plants, the aril is a key adaptation for attracting dispersers, contributing to the plant’s reproductive success while adding notable diversity to seed morphology.