ARTIFICIAL AND NATURAL HYBRIDS IN THE GRAMINEAE, TRIBE HORDEAE I. ELYMUS, SITANION, AND AGROPYRON

THE OCCURRENCE of natural hybrids in the grass family has been noted by many authors, principally in Europe. Cugnac (1937) has pointed out that such hybrids are to be expected with greater frequency in the Gramineae than in other families because species of this family most often occur in large stands, and not infrequently two or more related species are closely intermingled. Furthermore, their habit of wind pollination increases the chances of cross pollination, both within and between species. These hybrids are important from both the purely scientific and the practical viewpoint. From the standpoint of the systematics of the Gramineae, it is important to note that a number of forms originally described as species, such as Festuca loliacea Curt., and Oryzopsis Bloomeri (Boland.) Ricker have later been demonstrated to represent collections of sterile F1 hybrids (Jenkin, 1933; Johnson, 1945; Johnson and Rogler, 1943). There is every reason to believe that other so-called "species" consist likewise of hybrids, and the evidence presented in this and suceeeding papers indicates that this is true of certain "species" described in the tribe Hordeae. It is also noteworthy that most of the hybrids to be described below, like the two just mentioned, are between species which according to the traditional system of classification belong in different genera. The frequent occurrence of such "intergeneric" hybrids and the ease with which nmany of them can be produced artificially suggest that a drastic revision of this and other tribes of the grass family may eventually be necessary. From the standpoint of the phylogeny and evolution of the Gramineae, these hybrids are important as the possible ancestors of new species and as indicators of the method by which many of the presentday species have originated. The origin of new plant species by means of hybridization and allopolyploidy is now widely recognized, and the actual parentage of many natural allopolyploids, including species of grasses, is now known (Clausen, Keck and Hiesey, 1945). Since polyploidy is particularly common in the Gramineae, reticulate evolution by this method has probably played a larger part in the development and differentiation into species within the grass family than in most other families of Angiosperms. The practical importance of these hybrids centers about the attempts of the senior author to obtain new allopolyploid species of grasses which will be of value in revegetating depleted stock ranges. The number of hybrid combinations possible even within a relatively small group of species of Gramineae is so large that some indication of the most vigorous and potentially successful combinations is desirable. This is amply provided by the 1 Received for publication January 8, 1946. existence of sterile hybrids growing beside their parents in natural habitats, often in considerable numbers, and forming more vigorous clumps than their fertile parents. It seems likely that such hybrids, if they can be rendered fertile by doubling their chromosome number artificially through the agency of colchicine, will give rise to particularly vigorous and well adapted new species. The identification of natural hybrids can be made with considerable certainty by means of careful field studies and even by examination of herbarium specimens, as has recently been brilliantly demonstrated by Johnson (Johnson and Rogler, 1943; Johnson, 1945). Nevertheless the final proof of the hybrid nature of a plant is its artificial reproduction through controlled emasculation and cross pollination. For this reason, the resemblance between the natural hybrids described in this paper and the corresponding artificial ones is of particular interest, and it seems justifiable to group them together in the discussion of their morphological and cytological characteristics. MIATERIAL AND METHODS.-For obtaining the artificial hybrids, the same technique was adopted as had previously been successful in Bromus (Stebbins and Tobgy, 1944), and the cytological methods, including fixation of material, making of smear preparations and observation and recording of results are as described in that paper. The chromosome drawings are reproduced at a magnification of 150OX. In those natural hybrids which were identified from herbarium specimens, pollen was examined by boiling florets in water, dissecting out the anthers, crushing them on a microscopic slide in a solution of cotton blue in lactic phenol and covering them with a cover glass. Under these conditions, the fertile pollen grains of the parent species become heavily stained in an hour or so, while those of the sterile hybrids remain clear and transparent. In addition, the pollen grains of the hybrids are smaller and often irregular in size, as has frequently been observed in hybrids. Both the parents of the artificial hybrids and the living material of the natural hybrids were collected by the senior author in the field, except for a few collections which were obtained from the U. S. Soil Conservation Service and from the Carnegie Institution of Washington laboratory at Stanford University. To the directors of this institution the authors give many thanks for this material. The exact source of each culture will be given below with the description of the hybrids. ORIGIN AND DESCRIPTION OF THE HYBRIDS.-1. Artificial hybrids. From a single pollination of 40 emasculated florets of Elymus glaucus with Sitanion jubatum, no hybrid seeds were obtained. This, however, is insufficient evidence on which to judge the