Using the dual chambered bark strip system described by Peel et al. (1969), the relative longitudinal mobilities of MCPA, 2,4-D, 2·4·5-T, MH, paraquat and TCA in the sieve tubes of willow have been measured. It was possible to measure the quantity of a compound which entered one end of the sieve tube system and to compare this with the mass which moved to the other end of the same system. We have termed the ratio of these two masses the relative mobility.
Over a 24-hr application period, TCA and paraquat had the highest relative mobilities, 2·4-D and 2·4·5-T the least. However, the total mass of TCA and paraquat which was moved along the sieve tube system was less than certain of the other compounds, notably MH and MCPA. These differences in relative mobility are thought to be due to variations in the removal of the compounds from the sieve tubes as they are being transported. In experiments of 36-hour duration in which sieve tube exudate was sampled at 12-hour intervals, the relative mobility changed with time. After 36 hours the order of relative mobility was different from that after a 24-hour application time.
The relative mobilities of IAA and IAA asp were also studied, either by allowing the IAA asp to be produced by metabolism from applied IAA, or by simultaneous application of 3H-IAA and 14C-IAA asp. IAA asp had the greater relative mobility, though the mobility of IAA increased with time to approach that of the IAA asp after a 12-hour application period.