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The student of phloem quickly learns that he is dealing with a tissue that functions normally only in the intact plant. Injury, such as an incision into the phloem, causes a disturbance that extends over great lengths up and down from the point of injury. The disturbance consists of several phases. First of all, there is an immediate and far-reaching turgor release which results in an accelerated shift of the sieve-tube content toward the point of injury (3). This shift may instantly seal the sieve plates with slime plugs (2). Even if no slime plugging occurs, rapid-though not instant-sealing of the sieve plates by callose formation may interrupt the conducting function of the sieve tubes. This can be seen if branches of certain trees, such as hickory, chestnut, etc., are detached and brought to the laboratory, for aphids may then not be able to feed on the phloem because of an interruption of sieve-tube continuity by heavy callose seals. Thus, sieve-tube transport has completely stopped in a branch that may still be alive and apparently healthy for weeks (11). In ash (Fraxinus americana L.) the sieve tubes are not immediately plugged when an incision is made o the conducting phloem. Exudation usually lasts oout an hour, sometimes longer. As far as concenation is concerned, only the very first samples can !e regarded as true samples of the assimilate stream Except for the possible presence of trace contaminations (9)]. Samples taken a few minutes after incision are osmotically diluted (7). It has also been shown that the concentration in the ash phloem increases above and decreases below an incision, but remains unchanged in tangentially adjacent tissue (10). In the present experiments two incisions were made, one exactly above the other; exudation occurred from both. Obviously, the upper cut was supplied from phloem tissue above, the lower one from tissue below. Transport toward the lower incision occurred therefore in a direction reverse to the direction of flow under natural conditions. This is a confirmation, on a larger scale, of the results of Weatherley et al. who found that phloem transport toward an aphid-stylet bundle can take place from either the apical or the basal end of a piece of stem (5).

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