Stewart Herbarium, Gordon College, Rawalpindi.
Deciduous monoecious trees; bark greenish white, exfoliating in young trees; young shoots tomentose. Leaves simple, alternate, 3-9-lobed, palmatifid, palmatinerved, rarely elliptic-oblong and penninerved; petiole base calyptri¬form, enclosing an axillary bud; stipules sheathing, leafy or membranous, deciduous. Flowers in unisexual capitula. Perianth 3-6, scale-like. Male flower with 3-8 stamens; anthers sub-sessile, 2-locular. Female flower 3-9-carpellary; carpels free; ovary hairy at base, with 1-(2) pendulous ovules; style 1, hooked at the tip; stigma unilateral. Fruit a globose head of numerous achenes.
A unigeneric family of temperate and sub-tropical regions of N. America, S. West U.S.A., Mexico, S.E. Europe and Asia.
Acknowledgements: We are grateful to the United States Department of Agriculture for financing this research under P.L. 480 and to Mr. B.L. Burtt (Edinburgh) for his helpful suggestions.