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It has been shown by various investigators that the substances in the sieve tubes are qualitatively the same throughout the plant. This, together with a number of physiological observations, seems to suggest that substances move through the sieve tubes without their being metabolized (2, 8, 23). There is good evidence, however, that the movement over the short distance from the leaf parenchyma to the sieve tubes is an active process. It has been shown by Phillis and Mason (13) that the sugar concentration of sap expressed from leaf tissue containing mainly mesophyll is much lower than that of sap expressed from the inner part of the bark or of leaf tissue containing mainly veins. This was later confirmed by a number of other workers using a wide variety of plants and different methods. Most important?since concentrations of expressed saps are not directly comparable?is the fact that leaf parenchyma cells have been plasmolyzed by the sieve tube ex?date of the same plant (7, 14). Transfer of substances into the sieve tubes must therefore occur metabolically. This process has been called "loading" by Barrier and Loomis (1). Huber and Bauer refer to it as "secretion" (9) because of the striking physiological similarity to the secretion of sugars in nectaries (10, 20, 21). There are a few reports on this movement dealing with both the chemistry of the transferred material and the physiology of the tissues involved (2, 18, 21), but the details are far from known. A parallel problem to that of the introduction of assimilates into the sieve tubes is that of how the assimilates are removed from these translocation channels. The mere fact that they do leave the sieve tubes has led some investigators to believe that the sieve tubes are "leaking." That this is not the case is indicated by the fact that they are, even at great distance from the leaves, under a high turgor pressure most of the time (see (5)). It will be shown in this paper that the turgor remains high for many days after the leaves have formed abscission layers in the fall, and also after artificial defoliation. The presence of large amounts of stachyose, raffi nose and sucrose make white ash a very interesting object for phloem translocation studies. It is significant that these sugars contain the sucrose unit within their molecules. In order to explain our previous observations it was proposed that there is an enzymatic removal of D-galactose units which results in a gradual break-down of the higher oligosaccharides, stachyose and raffinose, as they move down the tree (24). In the present investigation three ash trees have been defoliated, thus cutting off the supply of photosynthates to the phloem, and the changes that took place in the sieve tubes after this treatment have been followed. A similar study has been made of the changes in the phloem during the natural leaffall in autumn.

Translocation of Organic Substances in Trees. III. The Removal of Sugars from the Sieve Tubes in the White Ash (Fraxinus Americana L.).