Maintenance of tension in the transpiration stream requires coherence of the water molecules, but also their adherence to vessel walls (i.e. the lumen-facing surface must be wettable). As lignin is generally considered hydrophobic this presents a conundrum. McCully et al. observe wall contact with oil perfused into emptied vessels of maize (Zea mays) roots, contact angles of refilling sap, and wall substantivity to histochemical probes. Their observations reveal fine-scale heterogeneity of surface wettability, with lumen-facing bordered pits having hydrophilic aperture rims that repel oil, while oil adheres strongly to pit borders in empty vessels. As sap refills vessels through pits their borders become increasingly wettable by sap, which then spreads along the walls with low contact angles.
A study of wall contact with oil perfused into emptied vessels of maize roots, contact angles of refilling sap & wall substantivity to histochemical probes.