Flow in xylem vessels and Poiseuille's law

We measured water flow in simple xylem vessels of Sechium edule at various pressures. We found a disagreement with Poiseuille's law, which changes from vessel to vessel and becomes abruptly pronounced in some cases. We discuss our data in terms of an 'impediment coefficient,' K. It can be shown that the total flux in a stem, calculated as the sum of the flux in single vessels, agrees quite well with the data reported in literature.