JOURNAL OF THEARNOLD ARBORETUMVOL. XLV OCTOBER 1964 NUMBER 4 A RE-EVALUATION OF THE GENUS AMBROSIA (COMPOSITAE) 1 WILLARD W. PAYNE IN SPITE OF THE FOCUS in recent years on the ragweeds and their rela-tives as sources of allergenic pollen, they are poorly understood biologically and taxonomically. Systematic controversy has centered particularly about the question of the possible congeneric nature of the so-called "true rag-weeds," Ambrosia spp., and "false ragweeds," Franseria spp. Linnaeus (1753) established the genus Ambrosia for modern use with the description of four species, Ambrosia trifida, A. elatior, A. artemisiifolia and A. mari-tima, all of which were based upon earlier descriptions. Cavanilles (1793) established the closely allied genus, Franseria, with the description of the single species F. ambrosioides. The similarity of the two genera was recognized by Cavanilles, as is shown by the specific epithet (ambrosioides) he chose for his species and by a statement in the description that he con-sidered the new genus to be intermediate between the ragweeds (Ambrosia) 1Publication number 38 on Atmospheric Pollution by Aeroallergens, BotanicalPhase, under Research Grant Number AP-00008-02 from the Division of Air Pollution,Bureau of State Services, Public Health Service. This paper was presented, in part, before the joint meeting of the American Societyof Plant Taxonomists and the Systematic Section of the Botanical Society of Americaat the 14th annual meeting of the American Institute of Biological Sciences, held atAmherst, Massachusetts, August 26, 1963. I wish to express my appreciation to Dr.Warren H. Wagner, Jr., University of Michigan, Dr. Bernice G. Schubert, HarvardUniversity, and Dr. Otto T. Solbrig, Harvard University, for assistance in the prepara-tion of the manuscript of this paper; to Dr. A. G. Norman, Director of The Universityof Michigan Botanical Gardens, for providing facilities for culture work; and toMiss Annetta Carter, University of California (uc), Dr. Elizabeth McClintock,California Academy of Sciences (cAs), and Dr. A. E. Schuyler, the Academy ofNatural Sciences of Philadelphia (PH), for assistance in locating types. I am alsoindebted to the directors and curators of the herbaria of the following institutions formaking types and other specimens available for this study: the Arnold Arboretum (A)and the Gray Herbarium (oG) of Harvard University, the University of Arizona(ARIZ), the University of Cincinnati (ciNc), the University of Illinois (ILL), IowaState University (Isc), the University of Michigan (MICH), the University of NorthCarolina (NCU), the New York Botanical Garden (NY), the Philadelphia Academy ofNatural Sciences (PH), the University of Arkansas (UARK), and the United StatesNational Museum (us).