Special covering of certain seeds that commonly develops from the seed stalk. It is often a bright-coloured fleshy envelope, as in such woody plants as the yews and nutmeg and in members of the arrowroot family, oxalis, and the castor-oil plant. Animals are attracted to arils and eat the seeds, dispersing them in their wastes. The aril of nutmeg is the source of the spice known as mace.
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An aril (or arillus) is any specialized outgrowth from the funiculus (attachment point of the seed) (or hilum) that covers or is attached to the seed. It is sometimes applied to any appendage or thickening of the seed coat in flowering plants, such as the edible parts of the mangosteen and pomegranate fruit, or the mace of the nutmeg seed.
The aril may create a fruit-like structure (called a false-fruit). False fruit are found in numerous Angiosperm taxa. The edible flesh of the longan, lychee, and ackee fruits is a highly developed aril surrounding the seed rather than a pericarp layer. Such arils are also found in a few species of gymnosperms, notably the yews and related conifers.Instead of having a woody cone as is typical of most gymnosperms, the reproductive structure of the yew consists of a single seed that becomes surrounded by a fleshy, cup-like covering. This covering is derived from a highly modified cone scale.
In the photographs of a European yew (Taxus baccata) below, note that the aril starts out as a small, green band at the base of the seed, then turns brown to red as it enlarges and surrounds the seed, eventually becoming fleshy and scarlet in color at maturity. The aril is attractive to fruit-eating birds and is non-toxic (all other parts of the yew are toxic), serving therefore to promote dispersal of the yew seed by birds, which digest the fleshy aril as a food source, and pass the seed out in their droppings.