Cytogeography of the Rhizomatous American Species of Muhlenbergia

Abstract

The grass genus Muhlenbergia is estimated to contain 80 (Bews 1929) to 120 (Hitchcock 1936) species, confined to the western hemisphere with the exception of several species of Japan and the Himalaya. The genus belongs to the subfamily Chloridoideae, as evidenced by its plump bicellular hairs, rhombic stomata, siliceous cells, embryo characteristics (Reeder 1957), and chromosome number and size. Two rather distinct groups of species comprise the genus. The group which we have studied consists of broad-leaved, mesic, rhizomatous plants, primarily distributed in the deciduous forests of eastern North America. A few species extend far beyond the limits of this vegetational formation into the plains and western mountain states, and several closely related species are found in Japan and the Himalaya. The distribution pattern of the group and the association of many of the species with the deciduous forests suggest that its ancestors were part of the Arcto-Tertiary Geoflora. The plants of this group are readily recognized, even in vegetative condition, by their numerous rather lax, short culm leaves and by their peculiar scaly rhizomes. These species are radically different in vegetative structure and ecological relationships from the xeromorphic cespitose species of our western plains, and probably have had a different origin. The apparent basic chromosome number of the genus Muhlenbergia is x = 10. Diploids, tetraploids, hexaploids, and one octoploid have been found. Among the broad-leaf species, the two diploids are rare though wide-ranging, occurring in scattered relict stands in highly specialized habitats. Muhlenbergia asperifolia, which is also diploid, is a xeromorphic species of alkaline or saline habitats in the West. It bears no obvious relationship in habit, leaf structure, rhizomes, or habitat preferences to the other diploids. The tetraploids are common, abundant, and ecologically diverse. The single octoploid, M. californica, is a rare endemic in several mountain ranges in the Los Angeles area. All cytological observations were made on acetocarmine or propriocarmine squashes of microsporocytes. The slides were made permanent by the rapid liquid carbon dioxide method of Bowen (1955). Inflorescences were obtained from greenhouse-grown plants produced from living rhizomes collected in the field. The rhizomes grow readily and the plants ordinarily will bloom several times a year if they are cut back and fertilized after flowering. Supplemental light was provided during the winter. Meiosis follows a strong diurnal pattern, with diakinesis occurring most frequently around 7:50 AM in mid-June and 10 AM in late December. Since diakinesis lasts less than ten minutes, fixation at the correct time is important. Voucher specimens for the chromosome counts are deposited in the Iowa State University Herbarium. Gametic chromosome numbers for the rhizomatous American species of Muhlenbergia discovered in the course of this study or reported in the literature are summarized as follows, and some gametic chromosome complements are illustrated in Fig. 1-13.