In plants, what is a rachis?

In biology, a rachis (from the Ancient Greek: ῥάχις (rhákhis), "backbone, spine") is the main axis or "shaft".

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.