THE BASES OF ANGIOSPERM PHYLOGENY:CYTOLOGY1 PETER H. RAVEN2 ABSTRACT A broad review of chromosome numbers in the angiosperms is presented according to thephylogenetic system of Cronquist. Consideration of the results indicates that the original basicchromosome number for the class as a whole, and for ail but one of its subclasses, is or maywell be x -7. For Caryophyllidae, x = 9 is indicated. Families and taxa of higher rank canbe compared only if the original basic chromosome number for the group is known, and thereare many families where this is not the case. Evolutionary changes in chromosome numberand morphology, particularly in herbaceous plants, have tended to give the impression thatthese characteristics were of limited utility in classification, and have often led to numericalcoincidences between unrelated groups. In addition, many inaccurate counts have been reported,and vouchers, if present, are occasionally misidentified, giving rise to misleading conclusions.Electronic data processing should be applied to the field as soon as possible for efficientinformation retrieval, especially since the number of chromosome counts reported is growingannually. Well edited regional treatments, or those dealing with a particular taxonomic group,are encouraged. An initial burst of polyploidy is suggested for the angiosperms by the survivalof many polyploid lines, especially among Magnoliidae and Hamamelididae. Although manyfamilies and even orders are of polyploid origin, progressive evolution in the group seems tohave proceeded largely at the diploid level, and much of the major differentiation evidentlyoccurred even among plants with the original basic chromosome number, n = 7. For more than 50 years, chromosome cytology has been an important elementin evaluating relationships and deducing phylogenetic sequences in the angio-sperms. Data derived from this field are potentially useful, especially in woodyplant groups (Darlington & Mather, 1949; Darlington, 1956), but the use of suchdata is not simple, as will be illustrated in the following pages. Changes inchromosome number and morphology may be rapid even within a genus (Stebbins,1966), a tendency that had made many students of phylogeny mistrust or down-grade the importance of chromosomal information for broad considerations.Insufficient information, inaccurate information, and the necessity of under-standing the pattem in one taxon before it can be compared on this basis withanother taxon all contribute to the difficulty of using such information insystematic or evolutionary studies. MATERIALS AND METHODS The principal sources of information on the chromosome numbers of angio-sperms are the compendia of Darlington & Janaki Ammal (1945), Darlington & 1This work has been supported by grants from the U. S. National Science Foundation.P. Goldblatt has contributed a number of unpublished chromosome counts of great interest. Thefollowing individuals have generously contributed advice about particular groups: H. G. Baker,D. M. Bates, W. L. Bloom, G. B. Briggs, B. L. Burtt, W. G. D'Arcy, G. Davidse, R. Eyde,A. Gentry, S. A. Graham, P. S. Green, P. H. Khosla, R. A. H. Legro, R. M. Lloyd, D. M. Moore,H. E. Moore, Jr., G. A. Mulligan, R. Ornduff, D. M. Porter, J. A. Ratter, W. Rauh, R. W. Read,C. M. Rogers, R. C. Rollins, F. S. Santamour, Jr., L. B. Smith, B. T. Styles, H. J. Thompson,R. F. Thome, G. L. Webster, F. White. In addition, R. J. Moore very kindly made availablethe information assembled for the Index to Plant Chromosome Numbers for the years 1971,1972, and 1973 in advance of their publication. 2 Washington University and Missouri Botanical Garden, 2315 Tower Grove Avenue, St.Louis, Missouri 63110.ANN. MIssouRI BOT. GARD. 62: 724-764. 1975.