Wide-band tracheids (WBTs) are novel variations of more common tracheids, and are characterized by a wide, intrusive secondary wall. Limited in systematic occurrence, WBTS are found only in three angiosperm families: Aizoaceae, Cactaceae, and Portulacaceae. These cells were hypothesized as evolutionary adaptations to water-stress events; during water-stress events, common tracheids would collapse whereas WBTs would retain functionality. Experiments using WBTs found in stems of species in the genus Anacampseros (Portulacaceae) show that light, not water-stress, is the primary controlling factor for WBT initiation and development. Plants exposed to low, moderate, and high light levels show correlative differences in WBT traits and numbers. Light intensity is thus proposed as a pre-adaptive factor that initiates WBT formation in preparation, not in response, to water-stress.

Key words: Anacampseros, light, Portulacaceae, tracheid, wide-band