ARILLUS, 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 scurf-skin, supplies its place. In Jessamy; hound's tongue, cynoglossum; cucumber; fraxinella, dictamnus; staff-tree, celastrus; spindle-tree, eunymus; African spiraea, diosma; and the coffee-tree, coffee; it is very conspicuous.
In the geus hound's tongue, four of these arilli, or proper coats, each infolding a single seed, are affixed to the style or pointal; and in this circumstance, says Linnaeus, does the essence of the geus 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, eunymus.
Cartilagineus, cartilaginous, or grilly; as in the African spiraea, diosma.
Coloratus, coloured; as in the staff-tree.
Elasticus, endowed with elasticity, for dispersing the seeds; as is remarkable in the African spiraea, 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 inclosed 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 Gymnospermia 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, (Encycl.)
This exterior coat of the seed is, by some former writers, styled Calyptra. See CALYPTRA, (Encycl.)
The different skins or coverings of the seed, are adapted, say naturalists, for receiving the nutritive juices, and transmitting them within.