pinnation

Pinnation is a botanical term describing a feather-like arrangement of leaflets along a central axis, or rachis, creating a pinnate or compound leaf. The term comes from pinna, meaning feather.

In unipinnate leaves, leaflets are arranged along both sides of a single rachis, typically with a terminal leaflet at the end. When the leaf is even-pinnate (paripinnate), there is no terminal leaflet; when it is odd-pinnate (imparipinnate), a terminal leaflet is present. Pinnation can also be more elaborate: bipinnate leaves have secondary pinnae along a main rachis, and tripinnate leaves can show additional levels of subdivision.

Pinnate leaf forms occur in many plant groups. Examples include the compound leaves of ash trees (Fraxinus), walnuts (Juglans), and many legumes, as well as the imparipinnate leaves commonly seen on roses (Rosa spp.). Fern fronds can also exhibit pinnate patterns, though the term is most often used for flowering plants.

Pinnation is a descriptive feature used in plant identification and classification. It contrasts with other leaf arrangements such as palmate (where leaflets radiate from a single point at the leaf’s base) and simple leaves (where there are no leaflets). Some leaves may be described as pinnatisect if the leaf is deeply lobed to a point where divisions resemble separate leaflets but remain a single leaf.

In summary, pinnation denotes a feather-like, multi-leaflet arrangement along a central axis, with variations ranging from simple pinnate to highly subdivided bipinnate forms.