Elasticity of Phloem Tissues1

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Abstract
Linear displacement transducers and a pressure transducer were used to simultaneously monitor pressure reduction and shrinkage associated with phloem exudation following incision of the bark of Fraxinus americana L. The shrinkage associated with the pressure reduction indicated that the sieve tubes had elastic walls. Mathematical models of a mass flow system of phloem transport have assumed rigid walls for the sieve tubes, but results demonstrated that the dimensional changes associated with pressure potential changes would produce significant differences in volume flow rates if the pressure gradient were maintained at a constant value, and thus indicate a requirement for modification of the models to accommodate elastic cell walls.