"growth-promoting Substance" and Elongation of Roots

The vertical elongation of normal roots of Lupinus seedlings proceeds at constant rate over periods of 4 to 5 hours. The decapitation of a root stops its elongation for a variable length of time, followed by a period of renewed elongation at a rate lower than that of the normal root. The tipping of the decapitated root with a tip of a coleoptile of Zea induces a decrease in the rate of elongation of the root. The same effect can be obtained with the diffusate from tips of coleoptile of Avena and to a lesser extent with diffusate of root tips. The reduction in the rate of elongation of the root determined by diffusate from the lower half of the tip of a coleoptile placed horizontally is more pronounced than the inhibition elicited by the diffusate of the upper half of the same tip. Various experiments with the diffusate of tips support the idea that under the conditions used the growth-promoting substance of the coleoptile tip or root tip inhibits the elongation of the decapitated root.