phloem

http://en.wikipedia.org/wiki/Phloem#Companion_cells
Companion cellsThe survival of sieve-tube members depends on a close association with the companion cells. All of the cellular functions of a sieve-tube element are carried out by the (much smaller) companion cell, a typical plant cell, except the companion cell usually has a larger number of ribosomes and mitochondria. This is because the companion cell is more metabolically active than a 'typical' plant cell. The cytoplasm of a companion cell is connected to the sieve-tube element by plasmodesmata.
http://en.wikipedia.org/wiki/Plasmodesma

圖片參考：http://upload.wikimedia.org/wikipedia/commons/thumb/2/29/Apoplast_and_symplast_pathways.svg/300px-Apoplast_and_symplast_pathways.svg.png



即是, 有cell wall 阻隔所以要當成兩個cell: sieve tube element & companion cell. 不過, 其實d cytoplasm根本係連住既, 所有物質: atp, enzyme, 甚至sugars, 咩到過到去返到來, 唔使anaerobic respiration.