Classification

Substantial agreement between the classification systems of angiosperms (plants with true flowers and fruits) by Cronquist (1981) and Takhtajan (1980, 1997) led to their wide acceptance, but these schemes have been superseded by the accumulation of molecular evidence. These classification systems are still in frequent use, however; hence, I allude to them. The classification of flowering plants by the Angiosperm Phylogeny Group (APG; Figure 2-5) has been presented in the more modern form of clades, whose relationships in a cladogram serve the same purpose as a "tree." The APG classification recognizes a selected number of monophyletic suprafamilial groups, that is, clades supported by at least one and often several lines of evidence. Although APG employs the ranks of order and family, it is emphasized that categories of the same rank are not evolutionarily comparable units unless they are sister groups, that is, close relatives.

In general, APG has adopted a broad circumscription of orders compared to earlier classification systems. Their ordinal classification has been based on the principle of monophyly, with well-established and familiar taxonomic entities maintained to preserve some stability in nomenclature. In a phyloge-netic classification, names are given only to groups that are monophyletic. APG (1998) thus recognized 462 families in 40 orders of flowering plants whereas Cronquist (1981) had 321 families in 64 orders, and Takhtajan (1997), 589 families in 232 orders. These differences in numbers of orders and families clearly point out the somewhat arbitrary circumscription of families and orders. It may seem shocking to nonsystematists that there is no objective way to recognize such levels of taxa, that such decisions result partly from tradition, utility, and taste. The decision by APG, that classification is most useful as a reference tool with a limited number of orders, is accepted here.

For centuries, flowering plants have been divided into monocotyledons and dicotyledons (hereafter referred to as monocots and dicots) on the basis of several morphological characteristics, usually at the rank of class. The simplistic division of angiosperms into monocots and dicots, however, does not accurately reflect phylogenetic history. The monophyly of only two major angiosperm clades—monocots and eudicots—is supported. The remaining families form a largely unresolved complex. Nearly every one of these fami-

Was this article helpful?

0 0

Post a comment