Flowering plants with simple vessel perforations have evolved from species bearing scalariform perforations but there is a scarcity of integrative studies explaining why these transitions in wood evolution have happened.

Illustrations of light microscope wood sections (A, B, E, F) and scanning electron microscope surfaces (C, D) showing the marked wood anatomical difference between Viburnum (A, C, E) and Sambucus (B, D, F).
Illustrations of light microscope wood sections (A, B, E, F) and scanning electron microscope surfaces (C, D) showing the marked wood anatomical difference between Viburnum (A, C, E) and Sambucus (B, D, F). (A) V. prunifolium: transverse section, vessels mainly solitary, apotracheal axial parenchyma diffuse in aggregates (arrows). (B) S. glauca: transverse section, vessels in distinct clusters, axial parenchyma scanty paratracheal (arrows). (C) V. furcatum: radial section, scalariform vessel perforations with many bars (arrows). (D) S. javanica: radial section, simple vessel perforations (horizontal arrow). (E) V. opulus: tangential section, long multiseriate rays due to long uniseriate ends (arrows). (F) S. glauca: tangential section, multiseriate rays shorter and wider (arrows).

Lens et al. find that Viburnum, characterised by scalariform vessel perforations (ancestral), diversified earlier than the closely related Sambucus, characterised by simple perforations (derived). The lack of selective pressure for high conductive efficiency during early diversification resulted in Viburnum retaining the ancestral perforation plate type, while higher temperatures during early diversification of Sambucus triggered the evolution towards simple vessel perforations, resulting in more efficient long distance water transport mechanisms.

You can follow @Federic_Lens on Twitter.