Origins and nature of vessels in monocotyledons. 10. Boryaceae: Xeromorphic xylem structure in a resurrection plant

Boryaceae are unusual in having xylem with an abundance of thick-walled tracheids, combined with relatively few vessel elements with scalariform perforation plates that have very slender bars. These features are relevant to the resurrection plant habit of Borya. The abundance of thick-walled tracheids provide sites for maintenance of water columns during hot dry months, whereas the perforation plates offer minimal impedance to flow during wet periods. This combination is unusual in monocotyledons. Root xylem is like stem xylem in the two species studied. Borya sphaerocephala occurs in the Mediterranean-type climate of south west Western Australia, whereas Borya subulata grows in a tropical summer-monsoon area, the Kimberley plateau of northern Western Australia. Despite differences in climatic regimes of the two species, survival of the dry season appears to be the primary factor in design of the xylem of both, and explains their similarity. Boryaceae are now placed in Asparagales, near Asteliaceae, Blandfordiaceae, Hypoxidaceae, and Orchidaceae. Xylem structure does not parallel the phylogenetic interconnections among these families, in which moist habitats, succulent habits, and other modifications seem to mitigate the effect of ecology in patterning xylem structure.