Structural features of sieve elements and companion cells

Sieve tube and companion cells in the phloem of squash (Cucurbita pepo) (diagram, left), and a transmission electron micrograph of adjoining ends of two sieve tube members with fragments of companion cells and phloem parenchyma cells in tobacco (Nicotiana tabacum) (right). Compare the two illustrations.

In the mature conducting state, sieve elements are devoid of a nucleus, Golgi apparatus, most cytosol, ribosomes, and vacuole, but retain a functional plasmalemma. The remaining highly modified plastids and endoplasmic reticulum, as well as mitochondria, occupy a parietal position which leaves maximum space in the center of the cell for moving sugar solutions. By contrast, companion cells are rich in cytoplasm, and contain a nucleus and other organelles. Both squash and tobacco have simple sieve plates.

The small transmission electron micrograph at the bottom right (red bordered inset) shows a sieve plate of Cucurbita with numerous sieve pores in surface view. Both the upper left and the lower right portions of the sieve plate appear missing due either to a tearing artifact or to the remaining portions being out of the plane of sectioning.

1 = companion cells; 2 = sieve tube content; 3 = parenchyma cell; 4 = sieve plate (oblique view); 5 = one sieve element; 6 = sieve plate (side view); 7 = plasmalemma of the sieve tube; 8 = original site of moving sieve tube sap, but possibly P-protein due to artifact of fixation; 9 = plastids; 10 = phloem parenchyma cell

Large right illustration from: R. Anderson & J. Cronshaw, 1970, Planta, 91: 173-180