In botany, the petiole (pron.: //) is the stalk attaching the leaf blade to the stem. The petiole is the transition between the stem and the leaf blade, and usually has the same internal structure as the stem. Outgrowths appearing on each side of the petiole are called stipules. Leaves lacking a petiole are called sessile, or clasping when they partly surround the stem. Clasping leaves of the Poaceae have an extra structure called the ligule. Petiolate leaves are ones where the petiole connects to the leaf before its apex.
Populus is a genus of 25–35 species of deciduous flowering plants in the family Salicaceae, native to most of the Northern Hemisphere. English names variously applied to different species include poplar (//), aspen, and cottonwood.
In the September 2006 issue of Science, it was announced that the Western Balsam Poplar (P. trichocarpa) was the first tree to have its full DNA code sequenced.