"Closed" minor vein (derived)

Plasmalemma surface in companion cells enlarged

Wall ingrowths present

Transmission electron micrograph of a derived (advanced) type of "closed" minor vein. Companion and phloem parenchyma cells have numerous wall ingrowths which greatly increase the absorbing surface of the plasmalemma. They may be designated as transfer cells. Four companion cells appear with very dark protoplasmic contents, and three phloem parenchyma cells (P) are with lighter contents.

Three tracheary elements with portions of helical secondary thickenings of xylem walls are represented by (X) and two sieve elements are marked by (S). The vein is surrounded by bundle sheath cells (B). Note the small size of sieve elements as compared to companion cells. This is a characteristic feature of minor veins.

Groundsel (Senecio vulgaris) is represented in this micrograph.