Excised root tips of 2 inbred strains of Zea mays and their F1 hybrid were cultivated on a mineral salt-sucrose medium. When the roots reached a length of 2-3 cm, the apical portion was excised and transferred to a Petri dish containing the same media, and incubated. The root tips of one of the inbred strains, Oh 43, grew little, with growth ceasing in 1-2 days; the root tips of the other inbred, Hy 2, and the F1 hybrid increased in the course of 11-12 days about 8-12 times their original length. When a root tip of Hy 2 or of the F1 hybrid was cultivated alongside that of Oh 43, a substantial amount of growth was induced in Oh 43. The root tips of Hy 2 and of the F1 hybrid apparently excreted a potent growth substance required for the growth of the Oh 43 root tips which the Oh 43 strains was unable to synthesize itself. The growth substance is concerned with making available sucrose to the Oh 43 strain.