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ABSTRACTIntermixed populations of Arabis holboellii Hornem. var. pendulocarpa (A. Nels.) Roll., A. williamsii Roll., and their putative hybrids; and of A. holboellii var. retrofracta (Grah.) Rydb., A. drummondii Gray and A. divaricarpa A. Nels., the latter also considered to be a probable hybrid taxon, were studied in the field in Montana and Wyoming. In spite of the "intermediacy" of the putative hybrids in both cases, hybrid swarms were not found. In each instance, the "intermediate" as well as the taxa regarded as parental were remarkably uniform. The existence of three relatively uniform morphotypes in each of the several populations is contrary to the usual situation involving hybridizing taxa. A possible explanation is that seed production, at least in the "intermediate" morphotype individuals, involves apomixis, thus circumventing the usual sexual process that would result in a hybrid swarm. Amphiploidy as a possible mechanism for producing A. divaricarpa as a stable hybrid was ruled out in one population where the chromosome number 2n = 14 was found in all three taxa present in the population.
Arabis divaricarpa, widely distributed in northern North America, often occurs as a uniform morphotype where no other species of Arabis grows. If it is of hybrid origin, as the evidence suggests, a mechanism other than sexual reproduction must be present to keep up uniformity and at the same time perpetuate the taxon. It is suggested that facultative apomixis is involved in the reproductive process and that it controls the genetic variability that would otherwise be present by insuring maternal inheritance in each successive seed crop.
NATURAL INTERSPECIFIC HYBRIDIZATIONis not a rare phenomenon in the higher plants (Stebbins, 1950(Stebbins, , 1969. If hybrids are local, if they are the current product of hybridizing taxa and reproduction is uncomplicated, the parental species and the hybrids can easily be dealt with taxonomically. Lesquerella provides an example of this type (Rollins, 1957; Rollins and Solbrig, 1973). Introgression (Anderson, 1949; Heiser, 1949) complicates the taxonomic picture, particularly for the parent taxa. But when polyploidy and apomixis accompany interspecific hybridization and introgression, a most complicated taxonomic pattern results (Babcock and Stebbins, 1938; Rollins, 1949 Rollins, , 1950.The probability that interspecific hybridization is an important factor contributing to the complexity of certain taxa of Arabis was early recognized (Rollins, 1941: pp. 378-379). Since that time, field observations on many occasions (Rollins, 1946: pp. 367-369) have en-1
