Stipules

Stipules are small leaf-like structures located at the base of the leaf stalk (petiole) in many dicotyledonous plants. They typically occur in pairs, one on each side of the petiole, but some species lack stipules altogether. Stipules may be leafy and resemble miniature leaves, or they can be scale-like, linear, or even spine-like. In some lineages the two stipules fuse to form a protective sheath around the stem or node; in others the stipules are short-lived and fall away early in leaf development.

Morphology and variation: Stipules come in a wide range of forms. Foliar stipules resemble small leaves, while other stipules are reduced to scales or bristle-like structures. In certain groups, stipules are modified into thorns or spines (for example, in many Acacia species, stipules become protective spines). Stipules can be persistent, remaining after the leaf has fully expanded, or deciduous, disappearing as the leaf develops.

Functions: The traditional view is that stipules protect developing leaf primordia and axillary buds. Some stipules bear glands and may contribute defensive chemicals or nectar-like secretions. In taxonomy, the presence, absence, size, shape, and arrangement of stipules are informative characters for identifying genera and species.

Development and evolution: Stipules originate from the leaf primordium at the base of the petiole and may be retained or shed during growth. Monocotyledons generally lack stipules, whereas many dicot lineages display a variety of stipular forms.

See also: leaf, petiole, bract, phyllotaxy, plant morphology.