8 - Diversity and Classification of Flowering Plants: Eudicots

Abstract

The eudicots are a large, monophyletic assemblage of angiosperms, comprising roughly 190,000 described species, or 75% of all angiosperms. The monophyly of eudicots is well supported from molecular data and delimited by at least one palynological apomorphy: a tricolpate or tricolpate-derived pollen grain. A tricolpate pollen grain is one that has three apertures, equally spaced and approximately parallel to the polar axis of the grain. Apertures are differentiated regions of the pollen grain wall that may function as the site of pollen tube exitus as well as to allow for expansion and contraction of the pollen grain with changes in humidity. Tricolpate pollen grains evolved from a monosulcate type (having a single distal aperture, which is considered to be ancestral in the angiosperms, as well as for many seed plant clades. Many eudicots have pollen grains with more than three apertures, of a great variety of numbers, shapes, and position (constituting important taxonomic characters). These are all thought to have been derived from a tricolpate type.