rachises

Rachis (plural rachises, and occasionally rachides) is a biological term referring to the central axis of a branching structure in plants and animals. The word comes from the Greek rhachis, meaning spine or axis. In general usage, rachises are the main stems or shafts that bear secondary units such as leaflets, feathers, or inflorescence branches, and they are distinct from the supporting base or petiole.

In botany, the rachis is the main axis of a compound leaf or a leaflike structure. In pinnate or palmate leaves, the rachis runs from the petiole toward the tip, with leaflets or lobes arranged along its length. The rachis may be slender or robust, and in some species it can be winged or extended beyond the point where the leaf attaches to the stem. In grasses and other inflorescences, the rachis describes the central axis from which spikelets or branches arise.

In ferns, the frond’s rachis forms the central axis to which pinnae are attached, and in some groups a further subdivision called the rachilla bears the ultimate leaflets. The rachis in ferns may be green or brown and can show various degrees of branching and articulation.

In birds and other feathered animals, the rachis is the central shaft of a feather, extending from the calamus (the basal hollow stalk embedded in the skin) to support the vane, which is formed by barbs and barbules.

Rachises play a structural role by providing support and organization to complex, branching anatomical arrangements across different taxa.