Plant xylem is a unique evolutionary invention: It functions as a negative pressure hydraulic system that can move vast quantities of water and solutes over distances longer than 100 m. No other kind of organism transports liquids under negative pressure, and the most successful attempts to build an artificial negative pressure hydraulic system succeeded only to move a small amount of water over a few centimeters in a single microchannel (Wheeler and Stroock, 2008). That experiment proved that it is possible to move water under negative pressure, but it left the question unanswered how plants can do this so effectively and over such long distances. Somehow plants manage to move large volumes of water in heterogeneous channels that range in diameter from nanometers to a few hundred micrometers, that is, in nanofluidic and microfluidic systems where water moves very close to surfaces of other phases, both solid and gas (Eijkel and van den Berg, 2005;Squires and Quake, 2005). Plants move all this water efficiently along these phase surfaces without constantly creating gas bubbles in the system . What do we actually know about these surfaces in xylem conduits (i.e., vessels and tracheids)? INVITED SPECIAL ARTICLE PREMISE OF THE STUDY:Xylem sap in angiosperms moves under negative pressure in conduits and cell wall pores that are nanometers to micrometers in diameter, so sap is always very close to surfaces. Surfaces matter for water transport because hydrophobic ones favor nucleation of bubbles, and surface chemistry can have strong effects on flow. Vessel walls contain cellulose, hemicellulose, lignin, pectins, proteins, and possibly lipids, but what is the nature of the inner, lumen-facing surface that is in contact with sap?METHODS: Vessel lumen surfaces of five angiosperms from different lineages were examined via transmission electron microscopy and confocal and fluorescence microscopy, using fluorophores and autofluorescence to detect cell wall components. Elemental composition was studied by energy-dispersive X-ray spectroscopy, and treatments with phospholipase C (PLC) were used to test for phospholipids.KEY RESULTS: Vessel surfaces consisted mainly of lignin, with strong cellulose signals confined to pit membranes. Proteins were found mainly in inter-vessel pits and pectins only on outer rims of pit membranes and in vessel-parenchyma pits. Continuous layers of lipids were detected on most vessel surfaces and on most pit membranes and were shown by PLC treatment to consist at least partly of phospholipids. CONCLUSIONS:Vessel surfaces appear to be wettable because lignin is not strongly hydrophobic and a coating with amphiphilic lipids would render any surface hydrophilic. New questions arise about these lipids and their possible origins from living xylem cells, especially about their effects on surface tension, surface bubble nucleation, and pit membrane function.