Electron Microscopic Observations of the Structure of Sieve-connexions in the Phloem of Angiosperms and Gymnosperms

Abstract

The sieve elements of Pinus silvestris L., Sorbus aucuparia L., Vitis vinifera L., and Cucurbita pepo L. have been examined electron microscopically in ultra-thin section, and the structures of the corresponding sieve areas or sieve plates have been described and compared. In Pinus the sieve areas contain groups of connecting strands which enter the wall from the lumen side as individuals but coalesce within it in the median cavity. This cavity has developed by wall breakdown in the middle lamella and primary wall region and corresponds to the median nodule visible under a light microscope. Neither in this nor in the other species observed is there any visible closing membrane. Structural differences between the four species are shown to suggest that the major evolutionary trend in the evolution of specialized conducting strands has been the enlargement of the connecting strands from groups of small separate strands to a smaller number of larger strands as the median cavity becomes enlarged to form a canal through the wall. The connecting strands appear invariably to be dense, highly osmiophilic and continuous with the cytoplasmic surface of the cell. No signs of micropores or of other tubular structure in the strands have been found. The structures thus revealed are more nearly compatible with active transport of materials across the sieve plate than they are with any purely physical mechanism. It is suggested that they are incompatible with any mass flow hypothesis.