Evolution of xylem physiology

This chapter summarizes the evolution of wood anatomical traits in woody angiosperms and the recognition of ecological preferences from past geological records for a number of arbitrarily defined wood functional types. Xylem evolution can be viewed in the context of a "trade-off" triangle, with different adaptive solutions to the structure/function problems depending on environmental demands as well as phylogenetic constraints (taxonomic history). Evolution of xylem physiology is complicated by the fact that, ever since the early evolution of land plants, xylem has simultaneously performed multiple functions. Regression analysis of various physiological and anatomical characters is used in the chapter to look for trends and possible "trade-offs" between various extant species. The temporal and spatial distribution of the wood types is largely explained by the "tradeoffs" between xylem conductive efficiency and vulnerability to embolism. The two major causes of embolism in plants appear to be freezing and water stress, but the mechanism for embolism formation differs in these two cases. Regression analysis results provide a basis for understanding the variations in the incidences of vessel diameters as related to freezing and drought. However, additional studies are needed to elucidate the physiological significance of variations in vessel wall thickness, vessel perforation type, wood density and parenchyma distribution. The chapter concludes with a discussion on the ecological patterns in xylem anatomy in terms of their functional significance.