Filaments but no Membranous Transcellular Strands in Sieve Pores in Freeze-etched, Translocating Phloem

Abstract

PLANT physiologists agree that organic substances are transported long distances in plants through sieve-tube elements and through pores in the sieve plates between them. But they disagree about how this transport is driven ; sieve tubes might be passive pipes between source and sink or might contain pumps all the way along. This controversy drags on because microscopists suspect that the contents of sieve elements may be altered or displaced as they are prepared for microscopy; thus their in vivo configuration remains uncertain. No electron micrograph has yet been published of a sieve element confirmed as translocating when it was fixed.