Phloem is specialized for Carbohydrate Transport and is typically found adjacent to Xylem. In order for plants to evolve in size and complexity it was essential to have a rapid system for the movement of carbohydrates from one part of a plant to another.

This is obvious if you consider a large tree like the Giant Sequioa. Photosynthesis occurs in the leaves of the crown. These are several hundred feet above the soil. In order for sucrose to reach roots which may be a hundred feet below the soil, a rapid physiological transport system must be present. Diffusion operates efficiently over very short distances. Carbohydrate diffusion could sustain microscopic plants or very small macroscopic plants. Calculations show that it would take 1 year for a molecule to diffuse 25 cm through cells (Niklas 1997).

The conducting cells of Phloem are called Sieve Elements. There are two kinds of these, namely: Sieve Cells and Sieve Tube Members (STM). This is analogous to the situation in Xylem. Both types of Sieve Elements have Sieve Pores lined with Callose and both are Enucleate at maturity. Callose is a polysaccharide that is involved in the regulation of the Sieve Pore diameter. It is a biochemical hallmark of Sieve Elements.

STM are rectangular and have many sieve pores on their end walls which are called Sieve Plates.

Sieve Cells are elongate with tapered end walls. They lack Sieve Plates as their Sieve Pores are more widely distributed along their length. Sieve Cells have a smaller diameter than STM, and are less efficient at translocation.

Sieve Cells are ancestral and STM are advanced. The Magnoliophyta typically have STM while most other plants have Sieve Cells in their Phloem. A stack of STM make a Sieve Tube!

Sieve Cells: Note their Shape & Diameter.

Sieve Tube Members stained for Callose with Aniline Blue. The Callose accumulates at the Sieve Plates when the STM is injured. However, there is a lot of diffuse Callose, as well.

Aerial view of a Sieve Plate. The large holes are sieve Pores.

Cross-section of Cucurbita Phloem: The cells with large diameters are STM. Their dark contents may be Callose.