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Hesperolinon (A.Gray) Small is a genus of herbaceous annuals in the flax family, Linaceae. Thirteen species have been recognized, occurring along the west coast of North America from southern Oregon to northern Baja California, Mexico. 12 species are confined entirely within California; only H. micranthum Small is known to occur outside the state. They are generally found in chaparral habitats of the coastal mountain ranges and western foothills of the northern Sierra Nevada. The name of the genus signifies its geographic distribution (Gr. hespero - “western” and linon - “flax”).

Most Hesperolinon were originally described as species of Linum ("flax") and placed in Linum section Hesperolinon by Asa Gray (1865). They were segregated as a separate genus by Small (1907), and that rank has been accepted in most subsequent taxonomic and floristic works. Morphologically, Hesperolinon are most obviously distinguished from Linum by their reduced number of carpels (two or three in Hesperolinon versus five in Linum). Molecular phylogenetic studies suggest that Hesperolinon is closely related to the monotypic genus Sclerolinon Rogers (which also bears bicarpellate flowers and occurs in western North America), and that both genera may be nested within Linum.

Hesperolinon is most famous for its high degree of edaphic endemism. All 13 species are ablserpentine soil outcrops, which are too inhospitable for most plants due to low nutrient concentrations, high levels of toxic elements such as cobalt and nickel, and seasonal extremes of heat, light intensity, and drought. Eight species of Hesperolinon occur only on serpentine soils, making the genus one of the most concentrated lineages of serpentine endemics to have evolved in the California flora. Hesperolinon has also been considered an example of speciation and adaptation in response to the development of the Mediterranean climate in the California Floristic Province over the last few million years. Hesperolinon has also been used as a model system in studies of plant-pathogen interactions.


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