MEASURING THE DIAMETER OF XYLEM - A SHORT COMMUNICATION

The magnitude of error in estimating axial resistances was assessed using the diameters of xylem vessels of different shapes, measured using four different methods. Statistical analyses showed that the diameter measurements obtained using the different methods, were significantly different at the 0.001 level of significance. Using the Tukey test, the diameters obtained using the 'four axes' method were not significantly different from the actual diameters obtained using the Chromatic Color Image Analysis System. By using the 'Lewis' method for measuring the xylem diameter, axial resistance was overestimated by a factor of 1.6. Thus, the error caused by the method of measurement may partly explain why, in some studies, the estimated axial resistances have always been different from the measured resistances. The 'four axes' method is the simplest and most reliable and gives results very close to the actual diameters, compared with the other three methods.