Studies on the Movement of Solutes between the Sieve Tubes and Surrounding Tissues4

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Abstract
It has been shown that the time lapse of 2–3 hours found by Peel (1963), between the introduction of an inactive ion into the xylem of a stem segment and its appearance in honeydew collected from that segment, is not an artefact caused by detection methods. A delay of similar magnitude was found in the present experiments when radioactive rubidium, 32P-phosphate, or 35S-sulphate was used. Pretreatment of stem segments with the respective inactive ion caused a very considerable increase (20 hours) in the time lapse when the radioactive ions were subsequently supplied to the segment. Experiments with bark strips demonstrated that the time lapse was due to the presence of a barrier to the radial movement of ions from the xylem of segments, since using bark strips the time lapse was reduced to less than one hour. This barrier could either be the xylem itself, the cambium, or both tissues. Pretreatment of bark stnps with inactive ions increased the time lapse in the case of 32P-phosphate and 35S sulphate, but had no detectable effect with radioactive rubidium. It is considered that these results indicate that rubidium may be able to pass directly into the sieve tube from the bathing solution in bark strips. Phosphate and sulphate, however, may have to move into the sieve tube via the storage cells of the bark. It has been shown that phosphorus-32 activity in the sieve tube is not confined to inorganic phosphate, but is certainly present in other compounds of which three sugar phosphates have been identified.