ARILLUS, an improper term invented by Linnaeus, and defined to be the proper exterior coat or covering of the feed which falls off spontaneously.
All feeds 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, distamnus; staff tree, celastrus; spindle tree, euonymus; African spirea, diosma; and the coffee tree, coffee; it is very conspicuous.
In the genus hound's tongue, four of these arilli or proper coats, each enfolding a single feed, 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 feeds. The
staff tree has its feeds 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, carilagineus, cartilaginous, or grizzly; as in the African spirea, diosma. Coloratus, coloured; as in the staff tree. Elasticus, endued with elasticity, for dispersing the feeds; 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, feeds 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 gymnosperma of Linnaeus. Seeds are said to be covered (semina testa) 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 feed is, by some former writers, styled calyptra. See CALYPTRA.
The different skins or coverings of the feed, are adapted, say naturalists, for receiving the nutritive juices, and transmitting them within.