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 vertical End Walls of the STM and have many large Sieve Pores which facilitate longitudinal translocation.

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

STMs and Sieve Cells have a greatly simplified cytoplasm with few organelles and they lack a vacuole membrane. Consequently, they are not able to regulate their own physiology. The latter is supplied by Companion Cells (STM) and Albuminous Cells (SC). These usually have a narrow diameter and resemble Parenchyma cells. They have many connections to the Sieve Elements and regulate their metabolism.

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.