Grass studies. III. Additional somatic chromosome complements

THE WORK reported in this paper is a continuation of the investigations begun by Nielsen and Humphrey (1937). The material used in the study, taken from plants grown in the greenhouse, was fixed in chrom-acetic-formalin, the embedded material sectioned at 12 IL and stained with iodine-crystal-violet. The plants later were transplanted into the field where they were grown to maturity. The identifications were made with the aid of Hitchcock's (1935) "Manual of Grasses of the United States." The camera lucida drawings were made at a projected magnification of 1500X, 2100X, or 2600 X. The micro-morphological measurements reported for the nuclear diameters were made upon nuclei in metabolic condition located in the meristematic region of the root tip. Fifty measurements were used to determine the average figure given for each case discussed. These and similar data for the chromosomes are summarized in table 1. Included in this table also are the Arkansas accession numbers, the seed or clonal source, and the original collector and year of collection if known. FESTUCEAE.-Stahlin (1929) and Avdulov (1928) have reported complements of 42 and 56 chromosomes, respectively, for races of Bromus inermis Leyss. The writer has confirmed Avdulov's observations of 56 chromosomes in somatic tissue and examined material of an additional race of this series with complements of 70 chromosomes. The 56-chromosome race has thicker and shorter chromosomes and smaller nuclei than the 70-chromosome race (fig. 1, 2). Herbarium specimens prepared from the plants from which the cytological material was taken show little morphological difference other than a slight increase in size of the latter race. This is also true of other plants of these same selections now being grown in the grass nursery at this station. Bromus marginatus Nees is composed of a series of races closely paralleling that of Bromus inermis. This species has previously been reported to have races with complements of 28 by Horton and Beck (1932), of 42 by Nielsen and Humphrey (1937), and of 56 chromosomes by Avdulov (1931). To this series of races has been added an additional one with a complement of 70 (fig. 3). A comparison of the members of the complements of the 42and 70chromosome races shows that the former race has thicker chromosomes and smaller nuclei. Unfortunately, no herbarium specimens were prepared of the 42-chromosome strain previously reported so that no gross morphological comparisons can be drawn. Root tip cells of Bromus commutatus Shrad. and B. secalinus L. were characterized, respectively, by 'Received for publication April 3, 1939. Research Paper No. 646, Journal Series, University of Arkansas. complements of 56 and 14 chromosomes (fig. 4, 5). Although the diameters of the chromosomes of both species were similar, the lengths of those of the former species varied from 2.5 IL to 7.5 IL as compared with 3.5 IL to 6.0 IL for the latter. No other significant morphological differences such as constrictions and satellites were detected. Avdulov (1928) had previously reported B. secalinus to have 28 chromosomes, a complement which was subsequently verified by Stahlin (1929) and Nakajima (1931). This new number for this species thus suggests another polyploid series in members of this genus. Glyceria grandis S. Wats. was found to have 28 chromosomes (fig. 6), mostly about 0.5 Pu in diameter and from 1.5 IL to 3.5 IL in length. The individual members of the complements are irregularly rodlike to U-shaped and show no distinguishing morphological characters. The nuclei averaged 8.1 IL in diameter. Chromosome complements of 40 were observed in Eragrostis curvula (Schrad.) Nees and E. sessilispica Buckl. (fig. 7, 8). Likewise, material from Eragrostis spectabilis (Pursh) Steud., which was previously reported by Humphrey (1937) and by Nielsen and Humphrey (1937) as having 42 chromosomes, has been reexamined by the writer and found to have a complement of 40 (fig. 9), which thus places the basic number of this species in accord with those of other species of Eragrostis herein reported and with those previously reported by Avdulov (1928, 1931) and Hagerup (1932). The complements studied were made up of members characteristically about 0.3 IL in diameter which ranged from 1.0 IL to 2.2 ju in length. Their shape is generally rod-like, and they show no signs of such morphological features as constrictions. The average nuclear diameters were 7.9 IL for E. curvula, 8.0 ju for E. sessilispica, and 12.0 ju for E. spectabilis. The Festuceae just reported fall into two groups as shown by these species of Bromus, Glyceria, and Eragrostis. Bromus and Glyceria have rather long, sometimes rod-like, but more often vermiform to U-shaped chromosomes characteristically in multiples of 7; Eragrostis has short, more regularly rod-shaped or slightly curved chromosomes occurring in multiples of 10. Although these observations, and a majority of those recorded for other species of this tribe, indicate these distinct groups, there are some records that multiples of other numbers also occur in the tribe. Avdulov (1928, 1931) reported diploid numbers of 18 in Melica altissima L., M. ciliata L., M. micrantha Boiss. and Hohen., and M. nutans L. Kattermann (1930) found these numbers correct for M. altissima and M. nutans, and Hunter (1934) verified the report of 18 chromosomes in M. altissima.