VARIATION IN THE DEGREE OF CLEISTOGAMY WITHIN AND AMONG SPECIES OF THE GRASS DANTHONIA

North American species of the grass Danthonia bear distinct chasmogamous and cleistogamous flowers. Four taxa occurring in North Carolina (D. compressa, D. epilis, D. sericea, and D. spicata) were examined for variation in the degree of cleistogamy. The flowering culms bear a single terminal panicle consisting of 4 to 12 spikelets of chasmogamous flowers. In each leaf axil, at the nodes of the flowering culm, is a single spikelet of cleistogamous flowers completely surrounded by the leaf sheath. The percentage of cleistogamous flowers produced on any culm depends on the number of chasmogamous and cleistogamous spikelets and the number of flowers in each type of spikelet. All four characters vary among the taxa examined. Danthonia compressa produces, on the average, 50% cleistogamous flowers; D. spicata, 25% cleistogamous flowers; D. sericea and D. epilis, 5% cleistogamous flowers. The degree of cleistogamy in D. spicata is associated with certain habitat features. Populations from mountain sites, disturbed sites, and non-woodland sites produced higher percentages of cleistogamous flowers than did populations from piedmont, undisturbed, and woodland sites, respectively. Grazing may favor increased cleistogamy because cleistogamous flowers are produced lower on the plant. In North Carolina, the most frequently grazed Danthonia taxa also produce the highest percentage of cleistogamous flowers. FLOWERING PLANTS provide ideal subjects for the study of the evolution of sexual systems because of their diverse reproductive mechanisms (Maynard Smith, 1978; Charlesworth and Charlesworth, 1979; Lloyd, 1979). Individuals of many species are capable of reproduction by two or more distinct mechanisms (Stebbins, 1950; Levin, 1972; Grant, 1975; Jain, 1976; Schemske, 1978; Lord, 1981). In such cases, it is possible to directly compare the costs and benefits of different modes of reproduction without the confounding factors inherent in comparisons across taxonomic categories. Dramatic examples of mixed reproductive systems are found in plant species where individuals bear chasmogamous and cleistogamous flowers. Chasmogamous flowers open and are therefore potentially cross-fertilized where' Received for publication 27 May 1982; revision accepted 9 November 1982. This paper is based, in part, on a dissertation submitted in partial fulfillment of the requirements for the Ph.D. degree, Duke University, Durham, N.C. I would like to thank Dr. Janis Antonovics, who directed the dissertation, for his advice and assistance throughout the study. Janis Antonovics, Christopher Campbell, Norman Christensen, Donald Stone, Bruce Weir, Henry Wilbur, and one unidentified reviewer made valuable comments on an earlier draft of this paper. During the course of this study I was supported by a NSF Graduate Fellowship and a grant from the Highlands Biological Station, Highlands, N.C. 2 Present address: Department of Botany, University of Texas, Austin, TX 78712; reprint requests should be sent to this address. as cleistogamous flowers never open and are obligately self-fertilized (Uphof, 1938; Lord, 1981). Individuals bear chasmogamous and cleistogamous flowers in all North American species of the grass Danthonia (Chase, 1918; Weatherwax, 1928). Spikelets of chasmogamous flowers are borne on the flowering culm in a terminal panicle. Single spikelets of cleistogamous flowers are produced in each leaf axil, at the nodes of the flowering culm. These flowers are enclosed by the leaf sheath preventing cross-fertilization and reducing potential seed dispersal distances. Illustrations of the chasmogamous and cleistogamous spikelets and their locations on the flowering culm can be found in Weatherwax (1928). The chasmogamous and cleistogamous flowers are completely distinct from the earliest point of development of the flowering culm. There is no intergradation of flower types. The resulting chasmogamous and cleistogamous caryopses (and seeds) are dimorphic. Dispersal of the chasmogamous caryopses occurs 1 to 2 wk prior to maturation and dispersal of the cleistogamous caryopses. At maturity, the flowering culm disarticulates (breaks apart) at the nodes and the cleistogamous caryopses are scattered around the base of the parent plant within the internodes. The lowermost nodes of the flowering culm are typically subterranean; cleistogamous caryopses at the lowest nodes therefore remain in place and are not dispersed.