THE TAXONOMIC AND EVOLUTIONARY SIGNIFICANCE OF LEAF LONGEVITY

The half-lives of leaves of 202 taxa of vascular plants were estimated mostly from the literature and ranged from 1 to 350 wk. The longevity of leaves in the plant groups studied is in the order Pinophyta > Pte ridophyta > Liliatae > Magnoliatae. Substantial differences are eviden t between the superorders of flowering plants proposed by Dahlgren and the subclasses proposed by Cronquist. The mean leaf half-life for fam ilies of Magnoliatae was shown to be inversely related to their Advanc ement Index. Length of life of leaves, therefore, appears to have taxo nomic significance, with more primitive vascular plants tending to hav e longer-lived leaves than the more advanced. The ecological significa nce of leaf half-life is less clear, except in that annual leaf fall i s not necessarily associated with deciduousness. The capacity to shed individual leaves may give the angiosperms a flexibility in environmen tal response which allows them a competitive advantage over the gymnos perms except in the most stressful habitats.