Mature and immature arils of Taxus baccata, a European yew. A fleshy aril surrounds each seed.
An aril is a fleshy covering of certain seeds formed from the funiculus (attachment point of the seed). The aril creates a fruit-like structure (called a false-fruit) and is produced by a few species of gymnosperms, notably the yews of the Family Taxaceae. Instead of having a cone-like structure 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 interpreted as a highly modified scale.
In the photographs of a European yew (Taxus baccata) at right and 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 birds and non-toxic (all other parts of the yew are toxic), serving therefore to promote dispersal of the yew seed by birds.
The term aril is not limited to yews. It means any specialized outgrowth from the funiculus (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, an example being the edible parts of the pomegranate fruit.