By sequential visual and photographic recording, the courses of the initiation of bending and the subsequent rates of curvature of the petiole and stem have been followed when a growth regulator is applied to (a) the leaf surface above the midrib of one primary leaf, (b) a partially excised short tongue of the midrib, and (c) the cotyledonary node at the junction of a cotyledon. In a comparison of 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) it was found that though the times taken to initiate bending are greatly dependent on the site of application, there are no significant differences between the two compounds. On the other hand, for indole-3-acetic acid (IAA), the initiation times are shorter and the equi-effective doses are much smaller. Once bending has been initiated, for 2,4-D and 2,4,5-T the degree of curvature increases progressively, whereas for IAA the initial positive rate eventually becomes negative.
Using 2,4-D and 2,4,5-T, labelled with carbon-14 in the carboxyl group, autoradiographs demonstrated that entry into the vascular strands via the tongue of midrib take place in about 5 minutes, while for applications to the cotyledonary node radioactivity is present in the tip of the primary leaf within the same time.
On the basis of these and other observations, it is concluded that under the controlled conditions selected, the initiation time for the bending of the stem following an application of either 2,4-D or 2,4,5-T to the leaf surface I cm from the tip can be broken down into (a) 79 minutes for enough of the auxin to pass from the leaf surface to the underlying vascular bundles, (b) 61 minutes to reach the first internode, and (c) 24 minutes to induce curvature. For IAA the corresponding figures for (b) and (c) are 31 and 17 minutes.
The velocity of downward transport in the phloem is estimated at 10–12 cm/hour for 2,4-D and 2,4,5-T and 20–24 cm/hour for IAA. From differences in the initiation times for applications of 2,4-D to the tip and base of the leaf the velocity was estimated to be 14–25 cm/hour.
Simultaneously with observations on the course of bending, the amounts of [14C]2,4-D and [14C]2,4,5-T. present in the tissues were followed. When bending is initiated the petiole and first internode contain 0.0016 and 0.008 μg respectively.
The distribution of 2,4-D applied to one of the primary leaves was examined for periods up to 27 hours. Penetration into the leaf and subsequent export from the leaf are greatest when the application is made to the tongue of midrib, less for an application to the lower leaf surface and least for the upper surface. The pattern of accumulation is dependent on the amount exported. In general, accumulation is highest in the first internode and upper hypocotyl. At the end of 27 hours no 2,4-D is yet present in the opposite leaf.