Sieve Tube Strands in Tree Bark

Abstract

IF the observations of Thaine¹ are valid, they may be of great significance to an understanding of organic solute translocation in vascular plants. Not only were transcellular strands seen, but also particles and threads moving through the strands. But Esau² criticized this work on two main grounds: strands may be artefacts produced by diffraction of the light beam by adjacent cell walls, and the cells he examined may not be sieve tubes. Although diffraction is sometimes a serious problem, and although it is indeed easy to confuse parenchyma cells of the axial type with adjacent sieve tubes, strands in sieve tubes of leaf petioles of some common tree species appear to be the rule rather than the exception³. Most investigators have failed to see strands, probably because of too much dependence on elaborate fixation and embedding procedures. In using the electron microscope we have found that many cytoplasmic structures may be completely destroyed during embedding (not necessarily during fixation). In spite of this, tubules (Ger. Röhren) or strands (Ger. Stränge) have been visible in young coniferous sieve cells using electron microscopic techniques^4,5.