THE MONOECIOUS AND DIOECIOUS CONDITIONS IN SAGITTARIA LATIFOLIA L. (ALISMATACEAE)

Field botanists recognize that many populations of plants, largely sexually monoclinic, may contain some individuals with diclinous flowers and that some generally monoecious species may include dioecious individuals in contradiction to descriptions in the taxonomic literature. These deviates, if they comprise a small proportion of the total population, are usually considered mere curiosities. Changes in sexual expression may, however, be a phenotypically plastic characteristic or may be the result of a genetic mutation. The latter may then, if selectively favored, result in a change from monoecious to dioecious structure. The positive results in attempts to control sexual expression in seed plants in general by either environmental modification or hormonal application seem to have little bearing on the problem of sex expression in Sagittaria latifolia. Experimental modifications serve to indicate environmental and genetic parameters which may affect the sex expression of a flower. They do not enable one to ascribe a causative factor or to predict changes in flower sex that may occur in field populations. The purposes of this research were first, to determine if sex expression is genetically or environmentally controlled in plants of S. latifolia L. Genecological experiments conducted on dioecious and monoecious populations of plants determine plasticity of sex expression. The second purpose was to ascertain if reproduction within populations is sexual or asexual. Dioecious spatially isolated plants without sex chromosomes, must maintain their population flower sex by asexual reproduction or natural selection. The third purpose was to plot distributions of plants to determine the frequency and geographical range of dioecious populations. Plant distribution can give an indication of the occurrence of differential selection and disruptive gene flow. The characterizations of Smith's (1894) five forms of S. latifolia include differences in sex distribution. S. latifolia proper is described as "monoecious or subdioecious. Form A is dioecious; New Brunswick to Minnesota, Louisiana and South Carolina; Form C is monoecious or dioecious; Prince Edward Island to British Columbia and southward to New York, Kentucky and Nebraska." All other forms are characterized as being monoecious. Recent treatments describe the flowers of species as "lower whorls pistillate or all unisexual" (Fernald, 1950); "sometimes the two kinds of flowers on different plants" (Fassett, 1940); "staminate above and the pistillate below on the same scape" (Bogin, 1955); "monoecious or polygamomonoecious" (Radford et al., 1968). Monoecious S. latifolia var. pubescens (Muhl.) J. G. Smith is a densely pubescent plant distributed primarily in the mountain and Piedmont areas of the southeastern United States (Radford et al., 1968).