COMPARATIVE STRUCTURE OF PHLOEM

Abstract

During maturation, the protoplasts of sieve elements in all groups of vascular plants undergo a selective autophagy and the plasmodesmata in their common walls are enlarged to sieve-area pores, resulting in a greater degree of continuity between contiguous protoplasts. At maturity, the sieve elements lack nuclei and vacuoles, and the surviving protoplasmic components are entirely parietal in distribution. The sieve elements in all taxa are associated with parenchymatous elements, and together these two cell types constitute interdependent physiological units. The phloem of angiosperms, with its sieve-tube members and their ontogenetically related companion cells, is the most highly specialized among vascular plants. The open sieve-plate pores in their end walls provide a high degree of protoplast continuity between contiguous sieve-tube members. The sieve-tube members of dicotyledons and some monocotyledons contain a proteinaceous substance called P-protein, the function of which remains to be determined. Moreover, use of this protein category is in need of further scrutiny. The sieve cells in gymnosperm phloem typically are associated with albuminous cells, the counterparts of the angiospermous companion cell. The mature gymnospermous sieve cell contains a vast and complex network of tubular endoplasmic reticulum (ER), which forms massive aggregates opposite the sieve areas and is continuous from sieve cell to sieve cell via the sieve-area pores. With the exception of the sieve elements of lycopods, the sieve-element protoplasts of vascular cryptogams are characterized by the presence of refractive spherules, the function and exact chemical composition of which remain to be determined. The lycopod sieve elements are characterized by unoccluded sieve-area pores. By contrast, the sieve-area pores in Psilotum and eu- and protoleptosporangiate ferns are occluded with membranes, which are structurally distinct from the parietal ER of the cell lumen. The sieve-area pores in root and mid-internodal regions of aerial stems of Equisetum are traversed by large numbers of membranes, which are continuous with the parietal ER of the cell lumen. In leptosporangiate ferns, the sieve-area pores contain variable numbers of ER membranes, which also are continuous with those of the parietal network. Although specialized parenchyma cells analogous to companion cells and albuminous cells are lacking in vascular cryptogams, the sieve elements in this diverse group of vascular plants have numerous cytoplasmic connections with their neighboring parenchyma cells. Information on translocation velocities in selected vascular cryptogams would contribute greatly to an understanding of sieve-element structure in relation to function.