Tissue found in vascular plants. Its main function is to transport sugars and other food materials such as amino acids (see protein) from the leaves, where they are produced, to all other parts of the plant. This could be from the leaves to the roots to provide the chemicals needed for growth. However, it could be from a leaf and up to a developing fruit that is rich in sugars. The sugars are made by photosynthesis, which occurs in green parts of plants, such as leaves (see leaf). The amino acids are made from sugars and minerals, such as nitrate absorbed from the soil. Phloem tissue is usually found close to the other transport tissue in plants, xylem, which transports water and minerals. In non-woody plants phloem and xylem are found in bundles, such as the veins of a leaf.
Phloem is composed of sieve elements and their associated companion cells, together with some sclerenchyma and parenchyma cell types. Sieve elements are long, thin-walled cells joined end to end, forming sieve tubes; large pores in the end walls allow the continuous passage of nutrients. Phloem is usually found in association with xylem, the water-conducting tissue, but unlike the latter it is a living tissue.
