Editing Xylem (section)

=== Measurement of pressure ===
[[Image:Pressurebomb.svg|upright=1.3|thumb|A diagram showing the setup of a [[pressure bomb]]]]

Until recently, the differential pressure (suction) of transpirational pull could only be measured indirectly, by applying external pressure with a [[pressure bomb]] to counteract it.<ref>[https://web.archive.org/web/20090918110039/http://bugs.bio.usyd.edu.au/learning/resources/plant_form_function/external_sites/PAP/Lab08_WaterPot/08Lab_14.html The pressure of the water potential of the xylem in your plant's stem can be determined with the Scholander bomb.] bio.usyd.edu.au</ref> When the technology to perform direct measurements with a pressure probe was developed, there was initially some doubt about whether the classic theory was correct, because some workers were unable to demonstrate negative pressures. More recent measurements do tend to validate the classic theory, for the most part. Xylem transport is driven by a combination<ref>{{Cite web|url=https://www.nature.com/scitable/knowledge/library/water-uptake-and-transport-in-vascular-plants-103016037|title=Water Uptake and Transport in Vascular Plants|last=Andrew J. McElrone, Brendan Choat, Greg A. Gambetta, Craig R. Brodersen|date=2013|website=The Nature Education Knowledge Project}}</ref> of transpirational pull from above and [[root pressure]] from below, which makes the interpretation of measurements more complicated.