Translocation of Organic Substances in Trees. V. Experimental Double Interruption of Phloem in White Ash (Fraxinus americana L.)

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Abstract
If 2 incisions are made into the conduction phloem, one above the other, exudation takes place from both. This indicates upward transport toward the lower incision, in other words a reversal of the normal direction of translocation. A series of samples taken from each of these incisions shows a gradual concentration decrease which is ascribed to osmotic dilution of the sieve-tube content. The concentrations of the upper incision are slightly higher than those of the lower incision, because the upper incision is supplied from phloem tissue above, where concentrations are higher, while the lower incision is supplied from tissue below, where concentrations are lower. In an attempt to find the effect of gravity on phloem transport, exudation rates from 2 similar incisions (one above the other) were compared in the right-side-up and upside-down position of detached trees. However, the method was not sensitive enough to discern the gravitational portion of the exudation pressure.