Rachis

This article is about the biological term. For the genus of land snails, see Rhachis. For the king of the Lombards, see Ratchis. For the Romanian village of Rachiş, see Mirăslău.

The dark rachis of a turkey feather

Rachis /ˈreɪkᵻs/ is a biological term for a main axis or "shaft".

In zoology/microbiology

In vertebrates a rachis can refer to the series of articulated vertebrae, which encase the spinal cord. In this case the rachis usually forms 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.

In the gonad of the invertebrate nematode C. elegans, a rachis is the central cell-free core or axis of the gonadal arm of both adult males and hermaphrodites where the germ cells have achieved pachytene and are attached to the walls of the gonadal tube. The rachis is filled with cytoplasm.

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, flower-bearing portion of an inflorescence above a supporting peduncle.

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.

See also

Stipe (botany)it:rachide