rachis

Rachis is a term used in biology to denote the central axis or shaft of a structure that bears lateral parts. It is most often encountered in botany and in the anatomy of feathers, where it refers to the main support axis.

In botany, the rachis commonly refers to the central axis of a compound leaf or a fern frond. In a pinnate leaf, leaflets attach to the rachis along its length, and the rachis itself extends from the petiole to the leaf tip. In ferns, the frond is produced by the stipe and the rachis forms the main axis from which the pinnae arise. The term also applies to the axis of a compound inflorescence; in grasses and similar plants, the rachis is the central stem to which spikelets attach, with a smaller axis called the rachilla serving each spikelet.

In feather anatomy, the rachis is the central shaft of a feather, extending from the calamus at the base to the tip, with vanes and barbs branching off along its length. The rachis provides structural support and rigidity to the feather.

Etymology and usage note: the word derives from Greek rhachis meaning spine or backbone. In writing, the plural can be rachises or rachides, and the term appears across plant morphology, inflorescence architecture, and avian anatomy to describe a central supporting axis.