A new technique for studying extension growth in the root is described which is based on excising a zone which extends 1·5–3·0 mm. from the tip. Large numbers of these segments are cultured with different nutrient fluids in the dark at 25° C. with continual shaking.The effects of a large number of nutrients on the growth of the segments have been studied, but only two, sugar and potassium ions, have been found to have stimulating effects.The effects of water, three concentrations of sugar, and one of potassium in air, and with an atmosphere containing 5 per cent. oxygen have been studied in detail in connexion with length increase, sugar absorption, content of free sugar, cellulose content, dry weight, and respiration.It has been shown that with increasing concentration of sugar in the medium, the rate of growth, the time during which growth proceeds, the internal concentration, respiration, dry weight, and cellulose formation all increase. Also that potassium stimulates the rate of growth and respiration, and that with per cent, oxygen all the aspects studied are depressed.It is suggested that the stimulation due to sugar may be attributed to an acceleration of water absorption with a complementary increase in cellulose formation. It is further suggested that sugar accelerates water absorption by accumulating in the vacuole and thus sustaining the osmotic pressure of the vacuolar sap. It is further suggested that potassium stimulates growth by increasing water absorption through an effect on respiration. The effect of respiration in this connexion may be to promote the transport of water directly to enhance the osmotic pressure of the sap by inducing an accumulation of inorganic ions in the vacuole.