The stalk of a leaf, supporting the leaf so that it rests in a particular position (see leaf angle index), connecting the leaf’s vascular tissue to that of the parent stem and, in deciduous species, participating in leaf fall. The petiole of aspen, both the European and the North American species (Populus tremula and P. tremuloides respectively), is flattened crosswise, allowing exceptionally free movement of the leaf so that it quivers even in a light breeze and may reduce wind resistance and wind damage in high winds

This definition is abridged from A – Z of tree terms: A companion to British arboriculture.
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