rachis

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

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The dark rachis of a turkey feather

Rachis ( /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 lined with horizontal leaflets

In botany

In plants a rachis is the main axis of the inflorescence or spike. In ferns it is also the part of the axis to which the pinnae are attached. Rachis can also mean the main stem of a compound leaf, such as in Acacia.

Spikelets are small inflorescences bearing one or more florets, or small flowers, along with a set of miniature bractlike leaves. When the fruits of the wild-type wheat are fully ripened, a series of abscission layers forms that divides the rachis into dispersal units consisting of a single spikelet attached to a short segment of the rachis.

A ripe head of wheat is thus easily shattered into dispersal units when touched or blown by the wind. This is significant in the history of agriculture, and referred to by archaeologists as a "brittle rachis".