A phloem is a tube in a plant that functions to transport the outputs of photosynthesis. Together with xylem tubes, it is part of the vascular tissue system of nutrient transport in plants. Phloem tubes transport food (sugars) while xylem tubes transport water and minerals. Another difference between phloem and xylem tubes is that phloem tubes carry material in two directions while xylem tubes only carry materials up (away from the root). Phloem tubes always carry sugar from a sugar source (such as a leaf) to a sugar sink (such as a fruit, roots, buds, and stems). Many hypotheses for how the sugars move from sugar sources to sugar sinks have been tested. Currently, a widely accepted hypothesis is the pressure flow mechanism. This hypothesizes that the sugars move because of different areas of pressure.
Some of the types of cells that make up the phloem tube include parenchyma cells, fibers, companion cells, and sieve elements.