Plants transport water through xylem and sugars through phloem. Xylem carries water and dissolved mineral ions mainly from roots to leaves. Phloem moves organic substances, especially sucrose, from a source where they are produced or released to a sink where they are used or stored.

Water in xylem

Water enters root hair cells by osmosis, moving from higher water potential in the soil to lower water potential inside the cells. In a leaf, water evaporates from the surfaces of mesophyll cells and diffuses out through the stomata. This is transpiration. As water leaves, it creates tension that pulls more water up through the xylem. Water molecules stick to one another by cohesion, so they form a continuous column. This explanation is called the cohesion-tension theory. Xylem vessels have thick, lignified walls that help prevent them from collapsing under tension.

Sugars in phloem

The movement of dissolved organic substances through phloem is called translocation. The mass flow hypothesis explains how it happens. At a source, such as a photosynthesising leaf, sucrose is loaded into phloem sieve tubes. This lowers the water potential in the phloem, so water enters from nearby xylem by osmosis. The resulting high pressure pushes phloem sap towards a sink, such as a growing root, where sucrose is unloaded. Water then moves out of the phloem, helping maintain the pressure difference.

For example, a storage organ may supply sucrose to growing shoots, making the storage organ the source and the shoots the sinks.

The key distinction is what drives each movement: transpiration creates a pull in xylem, while a pressure difference drives mass flow in phloem. Do not say that phloem always carries sugars downwards. A source can be above or below a sink, so translocation can occur in either direction in the plant.