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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, the mace of the nutmeg seed, or the hairs of a cotton plant. The aril is an edible enticement, encouraging transport by animals and thereby assisting in seed dispersal. Pseudarils are aril-like structures commonly found on the pyrenes of Burseraceae species - the fleshy, edible pericarp splits neatly in two halves, then falling away or being eaten to reveal a brightly coloured pseudaril around the black 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, ackee and lleuque fruits are highly developed arils 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 the woody cone 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, 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.
The kahikatea tree, Dacrycarpus dacrydioides, is native to New Zealand. In pre-European times the aril of the kahikatea was a food source for Māori. The washed arils were called koroi and were eaten raw.
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