Wide-band tracheids in leaves of genera in Aizoaceae: the systematic occurrence of a novel cell type and its implications for the monophyly of the subfamily Ruschioideae

Abstract.

 Stem and leaf anatomical characters of succulent plants in families of the order Caryophyllales were examined to determine phylogenetic and evolutionary trends within these families. Wide-band tracheids, novel tracheids having wide secondary walls hypothesized to prevent cell collapse under water stress, were found in three families–Aizoaceae, Cactaceae, and Portulacaceae. Wide-band tracheids are hypothesized to be a unique adaptation to severe drought conditions prevalent in the areas of origin for these families. Species of 89 genera of Aizoaceae were examined for the occurrence of these tracheids. If present, wide-band tracheids occurred only in leaves. Of the five subfamilies examined, these tracheids were found only in the subfamily Ruschioideae, but not in the putative basal genera of the ruschioids. The presence of wide-band tracheids is an apomorphic character for genera of Ruschioideae with the exception of the basal genera, and also suggests that further characters are needed to understand the alliance of the Apatesia and Cleretum groups with other ruschioid groups.