Taxonomy and Floral Biology of Scleropogon (Eragrostideae: Gramineae)

Scleropogon brevifolius is a monotypic, disjunct, New World, colony‐forming, perennial grass. Its nomenclature is relatively uncluttered. Its floral biology has always been confusing in its detail. Dioecism in S. brevifolius is expressed in sexually dimorphic inflorescences, and in adjusted floral sex ratios between male and female plants. At Uspallata in Precordilleran Argentina the primary sex ratio is met for female plants in the population (F/n=0.5), but male plants at 0.149 depart from expectation. Analysis of the remainder shows it to consist of monoecious variants among which andromonoecism (0.186) is the most significant. The contribution of ovules and pollen to the next generation by females, andromonoecids, and males is ca 80%—the balance is from very low frequency trimonoecious, monoecious, and hermaphrodite elements. The projected pathway to dioecism is via monoecism, and the numerous intermediate sex forms in the population allow the identification of many positions on that route. In the Uspallata population strict monoecism itself is <1% but premonoecism is at high frequency—ca 32%. Dioecism is still actively evolving on the male side, the early and characteristic fixation of male sterility genes having established the female element. Reasons for genetic tardiness in establishing complete control of female sterility are unknown. But the form of sexual expression displayed suggests a temporal aspect of evolution. The closest approximation in the grasses to the sex form diversity of S. brevifolius lies in New World Buchloe dactyloides which it matches in dimorphism, several sexual morphs, chamaechory, and in its colony form habit.