From systematic to ecological wood anatomy and finally plant hydraulics: are we making progress in understanding xylem evolution?

摘要

Olson (2014; in this issue of New Phytologist, pp. 7–11) provides avalid criticism of our recent commentary (Nardini & Jansen,2013). The observation by Feild & Brodribb (2013) that highvenation density (> 12 mm mm2) in angiosperm leaves is associated with the evolution of simple perforation plates can indeedhelp to explain the Baileyan evolutionary trends, as pointed out byOlson, even though vessel diameter, a highly important hydraulictrait, was not measured by Feild & Brodribb (2013). We agree withOlson that neither Bailey nor other students from the Baileyanschool provided causal explanations for the transition fromscalariform perforation plates to simple ones (Bailey & Tupper,1918; Frost, 1930; Bailey, 1944), which readers could misinterpretfrom the title of our commentary. Nevertheless, the fact that Baileycorrectly described a trend even without providing a validmechanistic explanation reveals that not only evolution, butscience itself, is far from being a linear process. Clever and carefulobservations of patterns, even if not supported by rigoroushypotheses about the underlying functional processes, may turnout to be equally as important for the long-term progress of ourunderstanding of nature as ambitious process-based models devoidof observational and/or experimental support. Although we see nocontroversy surrounding Bailey’s functional and evolutionaryideas, we would like to pay attention in this Letter to two relatedconcerns: (1) the integration of experimental data from the field ofplant hydraulics into traditional, comparative wood anatomy; and(2) the reversibility of evolutionary transitions between perforationplate types that form a continuum.