Abstract
The absence of epidermal trichomes has been used as a diagnostic character to distinguish Astrophytum myriostigma var. nudum from other infraspecific taxa. In the Huizache region of San Luis Potosi, Mexico, between the elevations of 1320 m and 1700 m, the local populations of A. myriostigma are polymorphic, comprised of nude, semi-nude, and flecked plants. However, this polymorphism is non-discrete as there is considerable variation and intermediacy among plants. Also, the proportion of nude plants increases gradually with elevation, which suggests clinal variation correlated with altitude. Exclusively nude populations are currently known only from near Santa Rita del Rucio (? 1710 m) and south of Entronque Huizache (? 1750 m). Farther south, flecked plants were recorded at 1726 m elevation at 6 km E of Guadalcazar, and to the north near El Realejo flecked plants occur as high as 1920 m. At localities southeast of the Sierra San Pedro (Sierra Guadalcazar), nude and semi-nude plants are absent or present at relatively low frequencies in local populations.
 Epidermal trichomes provide shading and reflectance that protects the cactus stem against excessive solar radiation and moderates stem temperature. The majority of nude A. myriostigma growing in exposed locations at the margins of Hechtia colonies were found to suffer from chlorophyll degeneration due to excessive exposure to solar radiation.
 A comparative morphological study did not find any characters which can be used in combination to diagnose the var. nudum. Given the lack of a suite of discrete, or nearly discrete diagnostic characters, and given that exclusively nude populations do not have a definable geographic distribution we suggest that the nude phenotype should be treated taxonomically as a “form” rather than as a geographical “variety,” or subspecies.
Published Version
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