Abstract: iffraction provides a sensitive non-contacting method for measuring the diameter of a thin plant axis, and for monitoring changes related to growth or altered water status. A 2 mwatt HeNe laser beam diffracted by the plant axis produces a series of diffraction fringes which are related to the diameter (d) of the axis by the equation d $EQ (nR$lambda@/y), where n is the order of the diffraction fringe measured, R is the distance between the plant and the projection screen, a ground glass plate, $lambda is the wavelength of the beam (0.6328 $mu@m), and y is the distance between the center of the diffraction pattern and the fringe of order n. The advantages of this optical technique for measuring the diameters of small cylindrical plant axes are: (1) it is non-contacting, (2) it does not require calibration, (3) no lenses are required, (4) rigid body movement of the plant axis has no effect on measurement, provided that the object remains inside the coherent and collimated light field, and (5) measurement accuracy of $POM 5% is relatively easy to achieve. !9
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