The Bases of Angiosperm Phylogeny: Vegetative Morphology

Abstract

Coherent patterns of morphology of apparent value in determining taxonomic and phylogenetic relationships are present in dicotyledonous leaves.Features of greatest value in assessing these affinities include leaf organization; marginal features, including morphology of the tooth; major vein configuration; characters of the intercostal venation; and gland placement.Of these, recognition of tooth morphology appears to be an overlooked tool of major systematic importance.Variation in these features is most coherent when analyzed in terms of the Takhtajan and Cronquist systems of dicot classification.Essential to our procedure was a recognition of the "basic" leaf features of each taxon.These were regarded as the most generalized type from which all of the more specialized types in a taxon could have been derived and they were derived from an analysis of the comparative morphology of modern leaves with limited input from the fossil record.The resulting scheme indicates strong correlation of leaf features with six of the seven Takhtajan subclasses, in addition to paralleling and clarifying both systems at the ordinal and familial levels.Conspicuous exceptions are the breakdown of the Asteridae into a possible rosid and a possible dilleniid group, reassignment of the Celastrales and Myrtales to the Dilleniidae, and of the Juglandales to the Rosidae.Affinities of numerous problem taxa, such as the Didymelaceae and Medusagynaceae, are resolved, as are some of the points of disagreement between the Takhtajan and Cronquist arrangements.This analysis also provides the first systematic summary of dicot leaf architectural features and the outlines of a regular systematic method for leaf determination.Inclusion of a paper dealing with vegetative morphology in a symposium on the Bases of Angiosperm Phylogeny may seem anomalous to many.Vegetative aspects such as branching patterns, phyllotaxy, growth form, leaf outline, and stem, bud, and root features have been extensively described and interpreted functionally and ontogenetically by workers such as Kerner (Kerner & Oliver, 1895), Goebel (1905), Troll (1967), and Radford et al. (1974).A limited systematic value has been recognized for vegetative features, especially within families and genera (see especially Hallés work on the architecture of trees, Hallé & Oldeman, 1970; Hallé, 1971), and they have been used, usually as adjunct features, in the construction of taxonomic keys.However, no meaningful application has ever been made of vegetative morphology to the systematic consideration of angiosperms at the higher taxonomic levels.Now our studies of modern and fossil angiosperm leaves indicate that coherent patterns of morphology of apparent value in determining taxonomic and phylogenetic relationships do exist among the leaves of the dicotyledons, and it is in order to elucidate these that we are making the following report.