Figure 1. Companion cells. Electron micrographs of leaf minor vein companion cells in different species. (a) Tomato (Lycopersicon esculentum Mill.). Minor veins from tomato are classified as type 2a with ordinary companion cells and few plasmodesmata at the interface between sieve element–companion cell complex and surrounding cells; × 6000 (b) Onion (Allium cepa L.). Type 2b species like onion have transfer cells with cell wall ingrowths (white arrowheads) and very few plasmodesmata; × 6000. (c) Squash (Cucurbita maxima). Squash is characterized by intermediary cells showing fields of plasmodesmata (black arrowheads). Note small vacuoles and dense cytoplasm; × 8000. BS, bundle-sheath cell; CC, companion cell; IC, intermediary cell; PP, phloem parenchyma; TC, transfer cell.

Figure 2. Schematic representation of different transport mechanisms in the phloem in relation to the classes of companion cells. Depending on the density of plasmodesmata and the type of companion cell, respectively, assimilates are loaded into the phloem via apoplasmic routes (upper and middle panels). In plants with intermediary cells, where plasmodesmata are frequent at the bundle-sheath cell–intermediary cell interface, assimilates are supposed to diffuse through plasmodesmata to the intermediary cell. Here, sucrose is transformed enzymatically to raffinose-type sugars, which are potentially too big to diffuse back via plasmodesmata. fru, fructose; glu, glucose; sta, stachyose; triose-P, triose phosphate; suc, sucrose.