AbstractA review is given of work on the movement of solutes into sieve elements, particularly with regard to investigations in which sieve tube exudate is obtained via the severed stylets of the aphidTuberolachnus salignus(Gmelin), sited on bark strips or stem segments of willow.Evidence is given showing that the uptake of naturally occurring solutes is an active and highly selective process, thus the rate of uptake is governed not by differences in concentration between the sieve tube and the source cells, but by “potential” gradients. It is pointed out that the phenomenon of selectivity by the sieve tube cannot yet be explained, though experiments are described which indicate the site of selection to be the tonoplast of the source cells, at least in the case of sugars.Calcium is generally considered to be a rather phloem‐immobile cation. Evidence is given which demonstrates that a factor determining the immobility of this substance may be its inability to enter the sieve elements.Some data are provided on the movement of certain synthetic growth regulators and herbicides into the sieve elements of willow. These were found to be loaded within a short time from their application to the cambial surface of bark strip. It is suggested that their ability to move into sieve elements is brought about by competition with naturally occurring solutes.The conclusion is reached that investigations on solute loading are probably of more practical importance than those on the mechanism of longitudinal transport, since the control of the overall rate of transport may be in the former, rather than the latter, pr
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