Growing lily (Lilium longiflorum Thunb.) pollen always drive a current into their tubes and out of their grains. The only external ions needed for growth (and the growth current) are K(+), H(+), and Ca(2+). Increases in K(+) immediately stimulate the current; while decreases in K(+) immediately inhibit it. Comparable changes in H(+) have the opposite effect; while those in Ca(2+) have very little effect. We infer that most of the steady growth current is carried in by a potassium leak and out by a proton pump; but other considerations indicate that a minor, but controlling, component of the inward current consists of calcium ions.