Structure and function in the sieve tube - An electron microscopic study

Abstract

This thesis starts with a brief historical background to the modern study of translocation and discusses how different hypotheses of its mechanism have emerged and taken their present shape. This is followed by a review of the literature on the ultrastructure of the phloem tissue (especially the sieve tubes) down to the present year, with an emphasis on the state of the normal functioning sieve plate. At the moment a strong difference of opinion exists about this: The Californian School have argued the view that the sieve plate pores are normally 'open', while the contrary view has been maintained as energetically in London. The work reported consists of an investigation mainly of the sieve tube ultrastructure of Helianthus annuus hypocotyl and Saxifraga sarmentosa stolon. Material of these excised in 1 mm slices by twin cuts direct into fixative invariably showed the pores filled with slime fibrils. No open pores were ever encountered. The pores sometimes had callose cylinders and were sometimes without, but this seemed to make no difference to the degree of compaction. In a second approach the plant axes were plunged into boiling water. Subsequent excision, fixing and e.m. examination indicated that the pores were filled with coagulated slime. This indicates that the filling of the pores is not an enzymatic reaction artefact. It also discounts the view that it is due to turgor release. Finally material was wilted for several hours prior to surgery and fixation, contrary to the findings of the Californian school, the pores still appeared filled. The slime fibrils of both species have the typical banded structure and tend to 'crystallise' in the pores or when close together. Tubular material was rarely found, but membranous aggregates of several types were frequent. Low power surveys embracing several sieve tubes at once revealed a similar polarisation state in all; and photomontages of whole sieve tubes indicated the relationships of the centre and plate regions of functioning elements. On the whole, the work reported does not support the pressure-flow hypothesis, nor the transcellular streaming theory.