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 scarf skin, supplies its place. In jasmine; hound's tongue, cynoglossum; cucumber; fraxinella; dictamnus; staff-tree, celastrus; spindle-tree, euonymus; African spirea, diosma; 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 reed, 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 succulent, and of the nature of a berry; as in the spindle tree, euonymus—Cortilaginous, cartilaginous, or gristly; as in the African spirea, diosma—Coloratus, coloured; as in the staff-tree—Elasticus; ended with elasticity, for dispersing the seeds; as is remarkable in the African spirea, diosma, 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 enclosed in any species of pericarpium or fruit vessel; as in the grasses, and the labiati or lipped flowers of Tournefort, which correspond to the didynamia gymnosperrima of Linnaeus. Seeds are said to be covered (semina tecta) 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 seeds are adapted, say naturalists, for receiving the nutritive juices, and transmitting them within.