A new sensitive root auxanometer is described. The auxanometer represents an adaption of the position-sensor transducer method to measurement of intact root elongation and has the advantages of simplicity and high sensitivity. Experiments with the auxanometer show that auxin begins to inhibit intact pea root elongation within 10 minutes and continues to inhibit elongation for at least 1 hour following a 1-hour treatment with the hormone. Exposure of pea roots to pH 4 results in a 2-to 3-fold increase in elongation rate beginning about 1 minute after acid treatment. Acid-induced elongation continues at a steady rate for at least 160 minutes and can be reinitiated repeatedly by shifting between pH 4 and 6.5. Auxin inhibits acid-induced elongation whether given before or after acidification, and a transient exposure to auxin renders intact roots relatively insensitive to acid for at least 1 hour after withdrawal of the hormone.There have been numerous reports dealing with the interaction of acid and auxin in the regulation of stem tissue elongation (4,7,12 slightly smaller in diameter than the swollen seed and had a notch to one side through which the emerging radicle could grow. The trays were filled with Meyer's solution (11) to the bottom surface of the Plexiglas platform and the solution was aerated during the subsequent growth of the roots. Roots were allowed to grow at room temperature in the laboratory under normal room lighting for 3 to 6 days at which time straight roots from 3 to 4 cm long were selected for experimentation. Although light is reported to inhibit root elongation (16, 17), there was no obvious effect of darkening the room on root elongation in this system during these short term experiments so these initial experiments were carried out in the laboratory under room lighting.Measurement of Elongation. Root elongation was measured by adapting the position-sensor transducer method (5) of recording stem elongation so that it could be used with intact roots. The pea seedling was mounted (Fig. 1) in a Plexiglas platform (D) fixed to the side of a specially constructed growth chamber 450 ml in volume so that the root protruded through a 3-mm hole in the bottom of the holder. The tip of the root was positioned in a glass capillary (E) 1.5 mm high and 2 mm in diameter fixed to the end of a copper wire (F) which passed through the rotating arm of a Metripak model 33-04 angular position-sensor transducer (G) and extended beyond to a counterweight (H) of 102 mg. The glass capillary was fixed to the end of the copper wire with fingernail polish which was also used to coat the submerged portion of the copper wire. The position-sensor transducer was provided with a constant 25 v power input from a HewlettPackard model 6215A DC power supply while output from the transducer was recorded using a Sargent-Welch model SRLG recorder. The chamber was filled with half strength Meyer's solution to the bottom of the seedling platform (D). The solution was continuously oxygenated through a 21 gauge stainless st...