Aril

Aril (arillus), a peculiar covering of the seed in some plants, formed by an expansion of the funiculus (the cord which attaches the ovule to the placenta). This expansion takes place after fertilisation, and sometimes invests the seed entirely, sometimes only partially. The succulent red cup round the seed of the yew is a familiar example, and the white water-lily, the passion flower, and the willow also form characteristic arils. Another accessory covering, often called the arilode, is developed from the edges of the micropyle (the aperture by which the pollen tube enters the ovule). This in the nutmeg forms what is called mace. In the spindle-tree (Euonymus europæus), it forms the remarkable orange-coloured covering of the seed. See OVULE.