an improper term invented by Linnaeus, and defined to be the proper exterior coat or covering of the seed which falls off spontaneously.

All seeds are not furnished with an arillus; in many, a dry covering, or leaf skin, supplies its place. In Jeffreys' hound's tongue, cynoglossum; cucumber; fraxinella, dictamnus; staff-tree, celatirus; spindle-tree, euonymus; African spirea, dioptera; and the coffee-tree, coffea; it is very conspicuous.

In the genus hound's tongue, four of these arilli, or proper coats, each enfolding a single seed, are affixed to the stylus; and in this circumstance, says Linnaeus, does the essence of the genus consist. In fraxinella, the arillus is common to two seeds. The staff-tree has its seeds only half involved with this cover.

The arillus is either baccatus, succulent, and of the nature of a berry; as in the spindle-tree, euonymus. Cartilagineus, cartilaginous, or griffly; as in the African spirea, dioptera. Coloratus, coloured; as in the staff-tree. Elafivora, endowed with elasticity, for dispersing the seeds; as is remarkable in the African spirea, dioptera, and fraxinella. Scaber, rough and knotty; as in hound's tongue.

Although covered with an arillus or other dry coat, seeds are said to be naked (semina nuda) when they are not inclosed in any species of pericarpium or fruit-vessel; as in the grapes, and the habiati or lipped flowers of Tournefort, which correspond to the didynamia gymnopermia of Linnaeus. Seeds are said to be covered (semina tecla) when they are contained in a fruit-vessel, whether capsule, pod, or pulpy pericarpium, of the apple, berry, or cherry kind: (See SEMEN). This exterior coat of the seed is, by some former writers, styled calyptra. See CALYPTRA.

The different skins or coverings of the seed, are adapted, say naturalists, for receiving the nutritive juices, and transmitting them within.