Translocation of Organic Substances in Trees. II. On the Translocation Mechanism in the Phloem of White Ash (Fraxinus Americana L.).

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Abstract
Sieve tube exudate of Fraxinus americana was analyzed. The same substances are found in all parts of the tree. Their concentrations vary somewhat from tree to tree: sucrose 0.05 to 0.15[image]; raffinose 0.05 to 0.1[image]; stachyose 0.15 to 0.3[image]; a trace of verbascose; mannitol 0.05 to 0.2[image]; amino-acids and amides usually less than 0.001[image] mainly glutamine and glutamic acid; and traces of substances that are absorbing and fluorescent in the UV light. No reducing sugars and no sugar phosphates were found.[long dash]There are 3 basic requirements for Munch''s mass flow mechanism through the sieve tubes; thus the cytoplasm lining the longitudinal walls of the sieve tubes must be semi-permeable. The sieve plates must be permeable for the transported solution. The turgor gradient must be positive in the direction of flow. Experimental results with white ash are all in agreement with these requirements. The data presented suggest that there is a passive flow of solutes along a turgor gradient which is established and maintained by metabolic entry of assimilates into and removal from the sieve tubes.