Eckenwalder, J.E. (1980), “Foliar Heteromorphism in Populus (Salicaceae), a Source of Confusion in the Taxonomy of Tertiary Leaf Remains”, 《Systematic Botany》 5 (4): 366–383, Bibcode:1980SysBo...5..366E, doi:10.2307/2418518, JSTOR2418518
Eckenwalder, J.E. (1980), “Foliar Heteromorphism in Populus (Salicaceae), a Source of Confusion in the Taxonomy of Tertiary Leaf Remains”, 《Systematic Botany》 5 (4): 366–383, Bibcode:1980SysBo...5..366E, doi:10.2307/2418518, JSTOR2418518
Eckenwalder, J.E. (1980), “Foliar Heteromorphism in Populus (Salicaceae), a Source of Confusion in the Taxonomy of Tertiary Leaf Remains”, 《Systematic Botany》 5 (4): 366–383, Bibcode:1980SysBo...5..366E, doi:10.2307/2418518, JSTOR2418518