Figure 1.

Schematic Representation of the Histology of the Sieve Element/Companion Cell-Complex in Vicia faba.

Four sieve elements (SE) connected by sieve plates (SP) are shown with their adjacent companion cells (CC). Only the companion cells contain nuclei (N), vacuoles (V), and chloroplasts (C); mitochondria (M) and endoplasmic reticulum (ER) are also indicated. Typical features of the sieve elements in this species are the sieve element plastids (Pl), the parietal P-proteins (PP), and the P-protein crystalloids (PC). One crystalloid is shown in the dense state, in which it permits the sieve element contents to flow past it. The other is depicted in the dispersed state (DPC), in which it blocks the tube. The direction of flow is downwards, as indicated.