Schematic Reconstruction of an Arabidopsis Sieve Tube Schematic Reconstruction of an Arabidopsis Sieve Tube.Reconstruction of the structure of a sieve element-companion cell complex as found in in vivo confocal studies and after freeze substitution of whole plants. Schematic Reconstruction of an Arabidopsis Sieve Tube.Reconstruction of the structure of a sieve element-companion cell complex as found in in vivo confocal studies and after freeze substitution of whole plants. Sieve elements contain ER, mitochondria covered with clamp proteins, and electron-dense vesicles. While those structures are usually embedded in an amorphous ground matrix, SEOR1 filaments and sieve element plastids are always in direct contact with the sieve tube sap. A SEOR1 agglomeration is shown in front of a plate that does not fill the entire lumen of the sieve element. Companion cells contain all organelles typical for a plant cell, but only nucleus, vacuoles, chloroplasts, and mitochondria are shown. Blue lines indicate the location of a cross section for (A) to (C). C, chloroplast; Cl, clamp proteins; EV, electron-dense vesicles; GM, ground matrix; M, mitochondria; N, nucleus; P, plastid; SR, SEOR1 filaments; V, vacuole. Daniel R. Froelich et al. Plant Cell 2011;23:4428-4445 ©2012 by American Society of Plant Biologists