compoundleaf

A compound leaf is a type of leaf in which the blade is divided into multiple leaflets that resemble individual leaves but share a common stalk. The entire structure is considered one leaf and attaches to the stem by a petiole; at the leaf level, the elongated axis is called the rachis, to which leaflets are attached by small stalks called petiolules. In many species, the leaflets lack any further subdivision, while in others, some leaflets themselves are subdivided.

There are two common architectures: pinnate and palmate. In pinnate compound leaves, leaflets are arranged along one or more axes along the rachis on opposite sides, as in many ashes, walnuts, and roses. In palmate compound leaves, leaflets radiate from a single point at the end of the petiole, as in buckeyes, horse chestnuts, and Virginia creeper. Some species have tripinnate or bipinnate leaves, in which a primary axis bears secondary axes with leaflets still further subdivided, seen in ferns and some legumes.

Poison ivy forms a trifoliate, palmately arranged compound leaf; Virginia creeper forms a five-leaflet palmately compound leaf.

Functionally, the compound arrangement can increase total leaf area for photosynthesis while reducing mechanical stress from wind since leaflets are lighter and can flex independently. It also may influence leaf longevity and herbivore interactions.