"Closed" minor vein (ancestral)

Plasmalemma outline of companion cells is smooth

No plasmodesmatal fields or wall ingrowths

Transmission electron micrograph of a "closed" leaf minor vein from sugar beet (Beta vulgaris). In this type of vein, the companion cells--sites of sugar loading into the phloem--are symplastically isolated from bundle sheath cells and mesophyll. Consequently, the sugar produced in mesophyll cells during photosynthesis enter companion cells from the apoplast. Companion cells of the phloem have a smooth plasmalemma surface which is considered as a more ancestral type of closed minor veins.

1 = bundle sheath cell, 2 = fully mature (dead) tracheary element, 3 = differentiating tracheary element, 4 = vascular parenchyma cell, 5 = companion cell, 6 = sieve element

Micrograph courtesy of Sara Browning