On the possible functions of helical thickenings in conductive cells in wood

The career of Sherwin J. Carlquist was marked by numerous pioneering contributions to botany and especially to ecological and evolutionary wood anatomy. He developed some of the most important modern functional hypotheses for wood, including postulating a biomechanical and fluid dynamic role for helical thickenings (HT) in seasonally dry environments. Here we endeavor to honor Carlquist's legacy by summarizing existing observations, explicitly acknowledging that HT represent a range of non-homologous and likely functionally disparate features, and exploring HT functional hypotheses in light of data from a pantropical genus, Croton, in which HT are associated with mesic rather than xeric conditions. This is noteworthy in part because HT are commonly associated with the flora of temperate mesic areas and seasonally dry areas, particularly in nontropical regions. Based on observations in Croton, the distribution of HT around the world, and interesting advances in fluid dynamics, we propose that diversity in this feature may serve two related functions in addition to the potential mechanical role previously articulated, namely, vessel refilling after cavitation and increased hydraulic efficiency.