Petioles

A petiole is the stalk that attaches the blade of a leaf to the stem, serving to position the leaf for light capture and to house the vascular tissues that transport water, minerals, and sugars between the stem and the leaf.

Anatomically, a petiole consists of epidermal and ground tissues surrounding a vascular strand that connects the leaf’s veins to the stem’s vascular system. In many dicot species the petiole is a distinct, often cylindrical stalk, though some leaves have flattened or slightly broadened bases. In some monocots the leaf blade is largely or entirely sessile, or a very short petiole is present, and in compound leaves the main stalk is the rachis with leaflets attached by their own petiolules.

Variation is common. Leaves may be petiolate (with a petiole) or sessile (blade attached directly to the stem). Some leaves are peltate, with the petiole attached to the underside of the blade away from the edge. In compound leaves, individual leaflets have their own small petiolules, while the leaf’s main stalk is the petiole. In ferns, the analogous stalk is typically called a stipe rather than a petiole.

Function and adaptation include not only support and transport but also mobility in some species. A pulvinate base of the petiole can enable leaf movements in response to light or touch. Petioles may be elongated to raise the blade above neighboring vegetation, or shortened to reduce damage in windy environments. In climbing plants, the petiole or its derivatives can be modified into tendrils to aid attachment.

In botany, petioles are a key feature used in leaf morphology and plant identification, with length, texture, and attachment patterns often characteristic of taxonomic groups.