Rachis

This article is about a term used in biology; for the genus of land snails, see Rhachis.

Rachis is also the alternate spelling of Ratchis, king of the Lombards, 744-749

For the Romanian village of Rachiş, see Mirăslău.

The dark rachis of a turkey feather

Rachis (pron.: /ˈreɪkɨs/) is a biological term for a main axis or "shaft".

In zoology

In vertebrates a rachis can refer to the series of articulated vertebrae, which encase the spinal cord. In this case the rachis usually form the supporting axis of the body and is then called the spine or vertebral column. Rachis can also mean the central shaft of pennaceous feathers.

Vertical fern rachis with horizontal leaflets, "pinnae" attached, each with rachillae and pinnules

The inflorescence of Buddleja paniculata is arranged along a central rachis

In botany

In plants a rachis is the main axis of a compound structure. It can be the main stem of a compound leaf, such as in Acacia or ferns, or the main stem of the inflorescence.

A ripe head of wild-type wheat is easily shattered into dispersal units when touched or blown by the wind. A series of abscission layers forms that divides the rachis into dispersal units consisting of a small group of flowers (a single spikelet) attached to a short segment of the rachis. This is significant in the history of agriculture, and referred to by archaeologists as a "brittle rachis", one type of shattering in crop plants.