Phloem is the tissue that transports carbohydrates throughout the plant body. The general term for the conducting cells is Sieve Element. There are two types of Sieve Elements. These are called Sieve Cells and Sieve Tube Members. This is reminiscent of the Xylem Tracheary Elements.

Sieve Tube Members (STM) have a Sieve Plate which is analogous to the Perforation Plate of Vessel Members. The Sieve Plates occur on the End Walls of the STM and have many large Sieve Pores which facilitate longitudinal translocation.

Sieve Cells (SC) do NOT have Sieve Plates. They only have Sieve Pores. Sieve Cells tend to have a narrow diameter compared to STM. The diameter of the cells has a profound effect on the rate at which fluids can move through them.

Sieve Cells also tend to be elongate and have tapered ends while STMs are shaped more like a wide tube with transverse end walls.

Consequently, STM are much more efficient at translocation than Sieve Cells.

This is analagous to the situation in Xylem where Vessel Vembers have traits that make them better for conduction than Tracheids.

STMs and Sieve Cells have a greatly simplified cytoplasm with few organelles and they lack a vacuole. Consequently, they are not able to regulate their own physiology.

The latter is supplied by Companion Cells (for STM) and Albuminous Cells (for SC). These usually have a narrow diameter and resemble Parenchyma cells. They have many plasmodesmatal connections to the Sieve Elements and regulate the metabolism of STM or SC.

Large Parenchyma cells can also be present.

Sieve Elements do not provide much structural support. However, Sclerenchyma Fibers can be found among the Sieve Elements of some species.

It is common to find Fiber Bundles just outside of the Phloem conducting cells. Thus, the Phloem is surrounded by these Fibers and by the Xylem. Both of the latter can protect the more delicate Phloem cells.