IN a recent letter to NATURE1 it was reported that strong cambial growth can be activated in young decapitated sun-flower seedlings by inserting their upper ends into tubes containing a weak solution in gelatine of the ether-soluble component of urine. With a similar method it has now been found that strong cambial growth can be activated by solutions of pure auxin- and of pure-indolyl-acetic acid (called by Kogl2 hetero-auxin) at concentrations of 1 or 2 in 106. The auxin- was kindly sent by Prof. Kogl from Utrecht, and the hetero-auxin was kindly synthesised by Dr. Weisberger at the Dyson Perrins Laboratory, Oxford. The cambium formed was in the normal position, and it extended for at least three centimetres below the part to which the hormone was applied. Controls had formed no cambium, or scarcely any.