Evans, L. S. (Laboratory of Plant Morphogenesis, Biological Sciences Research Laboratories, Manhattan College, the Bronx, NY 10471). Stem surface injuries of Neobuxbaumia tetetzo and Neobuxbaumia mezcalaensis of the Tehuacan Valley of central Mexico. J. Torrey Bot. Soc. 132: 33–37. 2005.—Identical scale and bark stem surface injuries are present on surfaces of fourteen species of long-lived columnar cacti in both North and South America. The purpose of this research was to determine the amounts of injuries on south-, east-, north-, and west-facing stem trough surfaces for two species of columnar cacti, Neobuxbaumia tetetzo and N. mezcalaensis, from the Tehuacan Valley (18° N latitude) of central Mexico. Stem surface injury levels on south (equatorial)-facing surfaces were twice that of north (polar)-facing stem surfaces for both cactus species. Calculations indicate that twice as much total annual direct sunlight occurs to south-facing cactus surfaces than on north-facing surfaces at 18° N latitude. In general, surface injury levels for east-facing and west-facing plant surfaces were usually intermediate between the high levels of injury for south-facing surfaces and the low levels of injury for north-facing surfaces. It should be noted that published results from columnar cacti growing at 32 °N latitude have about four times more stem surface injuries and four times more direct sunlight exposure on equatorial-facing surfaces than on polar surfaces. So, for columnar cacti at 18° and 32° latitude, the ratios of injuires were coincident with ratios of sunlight on equatorial and polar surfaces. Overall, the results of these studies are consistent with the hypothesis that sunlight, possibly UV-B, cause injuries to stems of long-lived, columnar cacti.