The Movement of Sugars into the Sieve Elements of Bark Strips of Willow: II. EVIDENCE FOR TWO PATHWAYS FROM THE BATHING SOLUTION

Abstract

It is suggested that when 14C-labelled sucrose, or its constituent hexoses, are applied to the cambial surface of a bark strip, the sugars can move into the sieve elements by two pathways. The first is a direct one which probably involves the companion cells ; in it the labelled sugars do not mix with pools of unlabelled sugars before entry into the sieve elements. Entry by this pathway leads to activity in a wide range of compounds in the sieve-tube exudate. The second pathway is an indirect one, involving the storage parenchyma of the phloem, where consider able metabolic changes take place involving the sugar applied to the strip, but the products of these changes, apart from sucrose, are unable to move into the sieve elements. These results are discussed in relation to published work on the cytology of angiosperm phloem, and the results of other investigators on the movement of sugars into plant cells.