The stalk that attaches the leaf lamina to the stem. The point of attachment is often strengthened by a widening of the base of the petiole. Some leaves (sessile leaves) lack a petiole and are joined to the stem at the base of the lamina. Sessile leaves are characteristic of most monocotyledons. The petiole is generally similar in structure to a stem except that the vascular and strengthening tissues are asymmetrically arranged so as to bear the weight of the lamina. The different patterns of veins evident in petioles are useful taxonomically. Various modifications of the petiole are seen. Some are flattened and bladelike (see phyllode
). Others are inflated, as in Eichhornia crassipes (water hyacinth), where they aid buoyancy. In many species the base of the petiole sheaths the stem, as in Heracleum mantegazzianum (giant hogweed). The base of the petiole may be modified as a *pulvinus
. The petioles of some climbing species, e.g. Clematis, are haptotropic. The base of a fern rachis is sometimes termed a petiole.