Xylem and Phloem

Water and the Environment

The xylem and phloem are the two kinds of tissues that transport water and other nutrients within plants. The xylem carries water up through the plant. The phloem transports nutrients, most notably glucose, down throughout the plant.

Xylem and Phloem: The Xylem

In Classical Greek, “xylem” translates to “wood.” This makes sense, as the most common xylem tissue is wood. The xylem supply all of the parts of a plant with water by transporting water up through the plant. Xylem are long tubes called vessels. They pump water from the roots up, replacing the water that plants lose to transpiration and photosynthesis.

Xylem and Phloem: How the Xylem Transport Water

Plants depend on xylem to replace the water that evaporates off of their leaves. The xylem can transport their sap through transpirational pull. In transpirational pull, water transpires, or evaporates, off of plant surfaces into the atmosphere. As transpiration pulls water out of the plant, the water tension within the plant pulls water from the plant’s roots and soil back into the leaves. This water tension is strong enough to lift water hundreds of meters above the ground into the highest branches of trees. However, for transpirational pull to work, the xylem vessels must be very compact in diameter, as this compactness maximizes pressure.

Xylem and Phloem: Other Ways That the Xylem Transport Water

The xylem can also pull water and nutrients up through the plant via root pressure. Through osmosis, plants absorb water into their roots. This osmosis then forces sap up the xylem and into the leaves. The xylem are also aided by capillary action, the force by which water adheres to the surface of xylem pipes. This capillary action balances gravity.

Xylem and Phloem: How the Phloem Work

Phloem is the second transporting tissue in vascular plants. The phloem carry nutrients, most notably glucose, down throughout the plant. Like “xylem,” “phloem” derives from Ancient Greek. “Phloem” translates to “bar,” which makes sense, as phloem is the innermost layer of bark in trees. Phloem transport the nutrients that plants produce in photosynthesis. The phloem’s transportation is called translocation. Translocation moves the phloem’s sugar-rich sap from sugar sources to sugar sinks. Plants generally store their sugars in their roots, and the phloem transports sugar from the roots to the growing areas in the plant, the sugar sink.

Differences Between the Xylem and Phloem

The xylem and phloem both transport vital commodities through plants. However, the xylem and phloem differ in several ways. While the xylem transport mostly water, the phloem transport nutrients, especially glucose. The xylem are made up of dead cells, while the phloem are made up of living cells. Xylem only transport sap upward, while the phloem are multidirectional—they move sugars wherever they’re needed. To work, the xylem rely on water tension, while the phloem rely on translocation.