Phloem

Phloem is one of the two tissues that make up the plant's long-distance transport system, the other being xylem. Phloem translocates water and products of photosynthesis from source tissues to the sink regions where they are utilized or stored. Primary phloem originates from the procambial regions in meristems, while secondary phloem originates from the vascular cambium. Phloem contains specialized cell types. Sieve elements are long cells with restricted protoplasm and end walls penetrated by large pores. They are arranged vertically in series to form sieve tubes that allow the movement of water and assimilates with low resistance. Each sieve element is associated with a companion cell, which is metabolically active. The cells communicate through specialized plasmodesmata to establish the sieve element-companion cell complex. Companion cells facilitate the loading of assimilates in source tissue and unloading in sink tissue. In addition to transporting assimilates and minerals around the plants, an increasing role for the phloem in the long distance transport of peptides, proteins, and RNAs that act as signaling molecules is emerging.