Robert W. Scotland & Alexandra H. Wortley (2003), "How many species of seed plants are there?", Taxon, 52 (1): 101–104, doi:10.2307/3647306, JSTOR3647306
Robert W. Scotland & Alexandra H. Wortley (2003), "How many species of seed plants are there?", Taxon, 52 (1): 101–104, doi:10.2307/3647306, JSTOR3647306