A CYTOLOGICAL STUDY IN THE GRAMINEAE

NUMEROUS CYTOLOGICAL studies (Stiihlin, 1929; Avdulow, 1931; Armstrong, 1937; Flovik, 1938; Stebbins and Love, 1941) have been made of grasses native to regions of Europe, North America, and Asia, especially those of temperate zones. As a result the chromosome numbers of many hundreds of species in dozens of genera are known in such temperate zone tribes as Aveneae with sixteen genera studied cytologically, Hordeae, twelve genera, Agrostideae, twenty-seven genera, and Festuceae, thirty-two genera. In contrast to these temperate zone tribes is the tropical and warm temperate Paniceae, one of the largest tribes in the grass family. In this tribe of over 60 genera and 1400 species only 72 species in 11 genera have been studied cvtologically. The present paper is primarily concerned with a cytological study of the Paniceae of the southeastern United States. In addition, a number of species in certain genera of the tribes Oryzeae, Zizaneae and Hordeae were studied. A special effort was made to study as many species in as many taxonomic sections of the genus Panicum as possible, especially in the subgenus Dicanthelium. This subgenus is a group of some eighty closely related species forming a very homogeneous and taxonomically difficult group with the greatest concentration of species in the southeastern United States. The present study was made in an attempt to determine certain interrelationships of species, genera and tribes of the family Gramineae from evidence offered by cytology. Because of the large number of species and varieties studied, these totalling 115, the description of results and discussion of groups are combined. In the genera Elymus and Panicum many species pertinent to the discussion are not mentioned in the description of results but they are all listed in table 1. MATERIALS AND METHODS.-All plants studied in the Paniceae were collected in the southeastern United States except Panicum obtusum from Colorado. Root tips were collected in the field when possible or the plants were potted and tips collected after a period of growth in the greenhouse. The same collecting method applies to Elymus canadensis, E. Wieyandii, E. virginicus, Hystrix patula, Zizaniopsis miliacea and Hydrochloa carolinensis which were collected in the eastern United States from New Hampshire to North Carolina. All other species of Elymus as well as Sitanion hystrix were grown from seed furnished by the U. S. Dept. of Agriculture, Soil Conservation Service. All root tips were fixed in Navashin's 1 Received for publication February 2, 1948. The writer wishes to express his deep appreciation of the stimulating guidance received from Dr. Lewis E. Anderson throughout the course of this research. Sincere thanks for helpful criticism and advice are also extended to Dr. (eor(e L. Church and Dr. Karl Sax. fixative, embedded in paraffin, sectioned and stained with the iodine-crystal-violet method. Material for the meiotic studies of Zizania aquatica, Zizaniopsis milicea and the three species of Leersia was collected in the field, fixed in absolute-acetic (70 per cent absolute ethyl alcohol and 30 per cent glacial acetic acid), and smeared in aceto-carmine. When possible, the original collections were retained as herbarium specimens and these are deposited in the University of Texas herbarium. All Paniceae were verified or determined by Dr. H. L. Blomquist of Duke University. The nomenclature of Hitchcock (1935) is used throughout this work except for: Elymus Wiegandii Fernald (1933); Panicum mundum Fernald (1936); and Panicum bennettense Brown (1942). Drawings were made at various magnifications and reproduced at 1,850 X for Paniceae and 1,360 X for all others. DESCRIPTION OF RESULTS.-Table 1 lists the chromosome numbers obtained. The author's collection numbers of the collections examined cytologically are also listed except for plants obtained from seed furnished by the U. S. Dept. of Agriculture in which case their accession numbers are used. In many cases root tips were collected from more than the one plant listed under the collection number for the particular species. In a few cases root tips were obtained from several morphologicallv indistinguishable plants in a single colony, yet, unfortunately, only a single plant from the colony was retained for an herbarium specimen. This accounts for more than one chromosome number being reported for the same collection of root tips of Digitaria sanguinalis and of Panicum virgatum. The listing of 2n numbers indicates that root tips were examined; that of the n number that meiosis was studied. In citing references for previously recorded numbers, one reference only, usually the first report of the chromosome number for the species, is listed. In cases where more than one number has been reported for a species, two or more references may be listed. Chromosome numbers of species previously reported are not listed unless the same species was studied by the author. An exception is made in the case of Panicurn subgenus Dicanthelium in which all species so far reported are listed. Hordeae.-The genus Elymus is cytologically typical of the tribe Hordeae with its large chromosomes, with a wide range of polyploidy including numerous cytogenetic strains and, according to Stebbins et al. (1946), with considerable intergeneric hybridization. Of the fifteen species and varieties investigated fourteen are polyploid. E. junceus, 2n 14 chromosomes, a species native to the Kasakstan region of the U. S. S. R., is the only diploid species found. This is the second diploid species reported in this genus. According