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We tested the effect of high temperature treatment (103 degrees C for 17 h) on seed survival for 26, 6 and 5 species in the Aizoaceae, Crassulaceae and Cactaceae, respectively. For both the Aizoaceae and Cactaceae species there was a significant positive correlation between the proportion of seeds surviving and the absolute maximum annual air temperature at the collection site. These results suggest that tolerance of extreme temperatures may have evolved as a mechanism to enable persistence in the soil in these predominantly desert species. Furthermore, these species may provide a useful model system for investigating the mechanism(s) of both desiccation- and thermo-tolerance in seeds. (c) 2007 SAAB. Published by Elsevier B.V. All rights reserved.