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Summary: While total vessel length is widely recognized as being of fundamental functional significance, opinion is more divided regarding the potential functional importance of vessel element length, a variable that Sherwin Carlquist regarded as functionally significant. We show that vessel element length can, as Carlquist predicted, affect vessel resistance to deformation. Perforation plates are locally thickened annuli, so tubes with annuli resist deformation better than those without, and tubes with closely-spaced annuli resist deformation better than those with distantly spaced ones. However, there is a tradeoff between deformation resistance and conductance. With a comparative analysis across more than 1000 species of angiosperms, we show that both vessel element length and the areas of individual inter-conduit pits scale positively with vessel diameter. Such covariation is expected if plants are to maintain conductance as they grow taller. Congruent with Carlquist's thinking, we found that species with vessel elements that are exceptionally short tend to grow in drylands, whereas those with vessel elements that are exceptionally long tend to grow in moist climates. Finally, we show evidence suggesting that selection on vessel element length is an important determinant of imperforate tracheary element length. Conversely, the evidence for selection on imperforate tracheary element length affecting vessel element length appears to be weaker. These results seem sufficient to establish that, whatever the functional importance of total vessel length, vessel element length is a variable of functional significance in its own right, congruent with Sherwin Carlquist's long-held views. [ABSTRACT FROM AUTHOR]

Premise: Comparative anatomy is necessary to identify the extremes of combinations of functionally relevant structural traits, to ensure that physiological data cover xylem anatomical diversity adequately, and thus achieve a global understanding of xylem structure–function relations. A key trait relationship is that between xylem vessel diameter and wall thickness of both the single vessel and the double vessel+adjacent imperforate tracheary element (ITE). Methods: We compiled a comparative data set with 1093 samples, 858 species, 350 genera, 86 families, and 33 orders. We used broken linear regression and an algorithm to explore changes in parameter values from linear regressions using subsets of the data set to identify a threshold, at 90‐µm vessel diameter, in the wall thickness–diameter relationship. Results: Below 90 µm diameter for vessels, virtually any wall thickness could be associated with virtually any diameter. Below this threshold, selection is free to favor a very wide array of combinations, such as very thick walls and narrow vessels in ITE‐free herbs, or very thin‐walled, wide vessels in evergreen dryland pioneers. Above 90 µm, there was a moderate positive relationship. Conclusions: Our analysis shows that the space of vessel wall thickness–diameter combinations is very wide, with selection apparently eliminating individuals with vessel walls "too thin" for their diameter. Most importantly, our survey revealed poorly studied plant hydraulic syndromes (functionally significant trait combinations). These data suggest that the full span of trait combinations, and thus the minimal set of hydraulic syndromes requiring study to span woody plant functional diversity adequately, remains to be documented. [ABSTRACT FROM AUTHOR]