Bipinnation

Bipinnation is a leaf morphology in which the leaf blade is divided into secondary units called pinnae, each of which bears its own series of leaflets. In a bipinnate leaf, the main rachis forks into pinnae, and those pinnae themselves carry leaflets along their own shortened rachises. In other words, the leaf is twice pinnate or double pinnate.

Leaves that exhibit bipinnation are usually finely divided, giving a lacy or fern-like appearance. The primary rachis bears pairs of pinnae on either side; each pinna carries a row of small leaflets. The extent of subdivision varies among species, and some leaves may resemble tripinnate forms if further subdivision is noticeable.

Biologically, bipinnation increases leaf surface area relative to leaf mass, which can enhance photosynthetic capacity while keeping structural weight lower. It is common in several groups of flowering plants, especially in the legume family. Prominent examples include Mimosa pudica (sensitive plant) and Albizia julibrissin (silk tree).

Terminology and related concepts: unipinnate leaves are pinnate once; bipinnate leaves are pinnate twice; tripinnate leaves are thrice pinnate. Some authors use "double pinnate" or simply "bipinnate" to describe the same arrangement. Nyctinastic movements can occur in some bipinnate species, causing leaflets to fold at night in response to stimuli.