In vascular plants, phloem is the living tissue that carries organic nutrients (known as photosynthate), particularly sucrose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word φλόος (phloos) "bark". The phloem is concerned mainly with the transport of soluble organic material made during photosynthesis. This is called translocation.
Phloem tissue consists of less specialized and nucleate parenchyma cells, sieve-tube cells, and companion cells (in addition albuminous cells, fibres and sclereids).
The sieve-tube cells lack a nucleus, have very few vacuoles, but contain other organelles such as ribosomes. The sieve tube is an elongated rank of individual cells, called sieve-tube members, arranged end to end. The endoplasmic reticulum is concentrated at the lateral walls. Sieve-tube members are joined end to end to form a tube that conducts food materials throughout the plant. The end walls of these cells have many small pores and are called sieve plates and have enlarged plasmodesmata.
The 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.
There are three types of companion cell.
The first two types of cell collect solutes through apoplastic (cell wall) transfers, whilst the third type can collect solutes via the symplast through the plasmodesmata connections.
Unlike xylem (which is composed primarily of dead cells), the phloem is composed of still-living cells that transport sap. The sap is a water-based solution, but rich in sugars made by the photosynthetic areas. These sugars are transported to non-photosynthetic parts of the plant, such as the roots, or into storage structures, such as tubers or bulbs.
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