The plant material including leaf samples, washed or unwashed, is dried and ground, then ashed, alkali fused, dissolved with perchloric acid and diluted with water to 50 mL. In the case of leaf samples, an appreciable amount of fluoride may be deposited on the external leaf surfaces. This fluoride behaves differently physiologically from fluoride absorbed into the leaf and it is often desirable to wash it from the surface as a preliminary step in the analysis. Details of a leaf-washing process are given in Section 7.1 . The dissolved digest and sulfuric acid are pumped into the Teflon coil of a microdistillation device maintained at 170°C. A stream of air carries the acidified sample through a coil of Teflon tubing to a fractionation column. The fluoride and water vapor distilled from the sample are swept up the fractionation column into a condenser, and the condensate passes into a small collector. Acid and solid materials pass through the bottom of the fractionation column and are collected for disposal. In the colorimetric method, the distillate is mixed continuously with alizarin fluorine blue-lanthanum reagent, the colored stream passes through a 15-mm tubular flow cell of a colorimeter, and the absorbance is measured at 624 nm. In the Potentiometric method, the distillate is mixed continuously with a buffer, the mixed stream passes through a flow-through fluoride ion electrode, and the differential millivoltage measured by an electrometer. The impulse is transmitted to a recorder. All major pieces of the apparatus are components of an automated analyzer such as the Technicon AutoAnalyzer (15) (Technicon Industrial Systems, Tarrytown, NY 10591) or the CFA-200 Analyzer (Orion Scientific Instruments Corporation, Pleasantville, NY 10570) or may be constructed from various laboratory equipment available from scientific supply companies. Details of construction of the microdistillation device are given in Section 5.9 . Earlier versions of this method have been published. (10,19,20).