Both cell types have a secondary cell wall and are dead at maturity. These are sclerenchyma cells which generally fall into two categories: fibres and sclereids. Supportive cells Īlthough its primary function is transport of sugars, phloem may also contain cells that have a mechanical support function. Īlbuminous cells have a similar role to companion cells, but are associated with sieve cells only and are hence found only in seedless vascular plants and gymnosperms. Intermediary cells, which possess many vacuoles and plasmodesmata and synthesize raffinose family oligosaccharides.They are specialized in scavenging solutes from those in the cell walls that are actively pumped requiring energy. Transfer cells, which have much-folded walls that are adjacent to non-sieve cells, allowing for larger areas of transfer.Ordinary companion cells, which have smooth walls and few or no plasmodesmatal connections to cells other than the sieve tube.There are three types of companion cells. The common sidewall shared by a sieve tube element and a companion cell has large numbers of plasmodesmata. The dense cytoplasm of a companion cell is connected to the sieve-tube element by plasmodesmata. All of the cellular functions of a sieve-tube element are carried out by the (much smaller) companion cell, a typical nucleate plant cell except the companion cell usually has a larger number of ribosomes and mitochondria. The metabolic functioning of sieve-tube members depends on a close association with the companion cells, a specialized form of parenchyma cell. Other parenchyma cells within the phloem are generally undifferentiated and used for food storage. The pores are reinforced by platelets of a polysaccharide called callose. All sieve cells have groups of pores at their ends that grow from modified and enlarged plasmodesmata, called sieve areas. One of the few organelles they do contain at maturity is the rough endoplasmic reticulum, which can be found at the plasma membrane, often nearby the plasmodesmata that connect them to their companion or albuminous cells. Sieve tube cells do contain vacuoles and other organelles, such as ribosomes, before they mature, but these generally migrate to the cell wall and dissolve at maturity this ensures there is little to impede the movement of fluids. At maturity they lack a nucleus and have very few organelles, so they rely on companion cells or albuminous cells for most of their metabolic needs.

Sieve elements are the type of cell that are responsible for transporting sugars throughout the plant.

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