arillus

An arillus, or aril, is a specialized, often fleshy outgrowth that partially or wholly covers a seed. It is a botanical structure that commonly serves to attract animals and aid in seed dispersal. The arillus can be derived from different tissues, typically arising from the funiculus (the stalk linking seed to the ovary) or from the outer integument of the seed coat, and its form and color vary widely among plant groups.

Morphology and formation vary by species. Some arilli are bright and visually striking, while others are inconspicuous. The tissue is usually rich in sugars or other attractants and may be succulent, fleshy, or fibrous. In some taxa, the arillus is an integral part of the seed coat, while in others it is a distinct extension that can be easily separated from the seed. The terminology in botany can be fluid, and arillus is sometimes used interchangeably with aril, though some distinctions are made by different authors or in older literature.

Ecological role and examples. The primary function of an arillus is to enhance seed dispersal by animals, including birds and mammals, by providing a rewarding food source. Well-known examples include the aril surrounding the seed of yew (Taxus species), which is red and fleshy, and the aril of nutmeg (Myristica fragrans), which forms the spice mace. Arils occur in diverse families and are a notable case of plant–animal interactions in seed dispersal.

See also: aril, arillode, seed dispersal.