Vascular Architecture: Structure of Xylem and Phloem

Vascular tissue consists of xylem, which transports water and minerals upward from the roots and provides support, and phloem, which transports sugars and other organic compounds from the places where they are made (sources) to the places where they are used or stored (sinks). In brief: xylem is made of dead, hollow cells with thick lignified walls (vessel elements and tracheids), and phloem is made of living sieve tube elements with companion cells. In a stem, xylem and phloem are grouped into vascular bundles, and in a root they form a central cylinder.

The vascular system allows land plants to grow tall and to transport materials over long distances. This guide describes the structure of xylem and phloem cells and how each structure suits its function, shows how the tissues are arranged in roots, stems and leaves, explains wood formation, compares xylem and phloem in a table, works through a flow-rate example and corrects common mistakes.

What is vascular tissue?

Plants with vascular tissue, the vascular plants (ferns, conifers and flowering plants), can reach heights that mosses cannot, because the tissue transports water over long distances and provides support. Vascular tissue runs through the whole plant, from root to leaf. Xylem and phloem are complex tissues made of several cell types.

What is the structure of xylem?

Xylem transports water and dissolved minerals in one direction, from the roots up to the leaves, and it also supports the plant. The main cells are:

• Vessel elements (in flowering plants): short, wide cells stacked end to end. When mature, they die, and the end walls break down or form large perforations, so that a continuous hollow tube, a vessel, is formed. Water can flow through with little resistance. Vessels can be up to several meters long in some trees, and 20 to 500 micrometers wide.
• Tracheids (in all vascular plants, and the only water-conducting cells in conifers and ferns): long, narrow cells with tapered ends, which are dead at maturity. Water passes from one to the next through pits, thin areas in the wall.
• Xylem fibers and parenchyma: fibers add strength, and living parenchyma cells store food and help to move materials sideways.

The walls of xylem cells are thickened with lignin, a hard, waterproof polymer, in patterns such as rings, spirals or a net. Lignin makes the walls strong enough to resist the negative pressure (tension) in the water column, which would otherwise collapse the tubes, and also provides support to the stem. Because the cells are dead and empty, there is no cytoplasm to slow the flow. The unlignified pits and the ring or spiral patterns allow the tubes to flex.

What is the structure of phloem?

Phloem transports sugars (mainly sucrose), amino acids and hormones, in both directions, depending on where they are needed. Its cells are alive. The main types are:

• Sieve tube elements: long cells joined end to end. The end walls have perforations and are called sieve plates. At maturity, they lose their nucleus, tonoplast and most organelles, which leaves a little cytoplasm with a few mitochondria along the walls, so that sap can flow through easily.
• Companion cells: each sieve tube element has one or more companion cells next to it, linked by many plasmodesmata. They have a nucleus, many mitochondria and dense cytoplasm, and they provide the metabolic support (including the ATP and proteins) for the sieve tube element, which cannot sustain itself. They also load sugar into the sieve tubes.
• Phloem fibers and parenchyma: support and storage.

Phloem sap contains about 10 to 25 percent sucrose, which is very concentrated compared with xylem sap, which contains very low concentrations of solutes. The mechanism of phloem transport is explained in a later article on translocation.

Xylem and phloem compared

How is vascular tissue arranged in organs?

• Dicot stem: vascular bundles form a ring, and each bundle has xylem on the inside, phloem on the outside and a layer of cambium between them.
• Monocot stem: bundles are scattered throughout the ground tissue, and there is usually no cambium.
• Root: xylem forms a central star-shaped or X-shaped core, with phloem between the arms. This arrangement gives the root strength to resist pulling. An outer layer called the pericycle can produce lateral roots.
• Leaf: vascular bundles form the veins. In the midrib, xylem is on the upper side and phloem on the lower side. Veins branch to supply every part of the leaf, and no cell is far from a vein.

How does wood form?

In trees, the vascular cambium divides to produce secondary xylem (wood) on the inside and secondary phloem on the outside. Each year, a new layer of xylem is added, which leaves growth rings. In the center of old trunks the xylem becomes heartwood: it no longer transports water, but is filled with resins and tannins that make it dark and resistant to decay. The outer, still active xylem is called sapwood. The phloem is a thin layer inside the bark, which is why removing a ring of bark from a tree (girdling) cuts the transport of sugar to the roots and eventually kills the tree, while the xylem can still bring water to the leaves for a time.

Worked example: how fast does sap move?

A tree has a trunk with a total cross-section of conducting xylem of 0.01 m² (100 cm²). The tree loses 400 liters of water through its leaves on a sunny day, over 10 hours. Estimate the speed of sap in the xylem.

1. Volume flow rate: 400 L = 0.4 m³. Over 10 hours = 36 000 s. Flow rate Q = 0.4 ÷ 36 000 ≈ 1.1 × 10⁻⁵ m³/s.
2. Speed: v = Q ÷ A = 1.1 × 10⁻⁵ ÷ 0.01 = 1.1 × 10⁻³ m/s, or about 1.1 mm/s.
3. Per hour: 1.1 × 10⁻³ × 3 600 ≈ 4 m per hour.

This is within the range reported for trees: in conifers with narrow tracheids, speeds are about 1 to 2 m/h, and in trees with wide vessels, such as oaks and vines, they can reach 40 m/h or more, because the flow rate is proportional to the fourth power of the radius of the tube (Poiseuille’s law), so wide vessels carry much more water. Doubling the radius gives 2⁴ = 16 times the flow, which explains the adaptive advantage of wide vessels, though they are more vulnerable to the formation of air bubbles (embolisms) in the water column during drought or freezing.

How was vascular tissue studied?

Marcello Malpighi showed in 1675 that removing a ring of bark from a tree stops the movement of sap, which indicated that nutrients move in the bark. Stephen Hales measured the flow of sap in the early 1700s. Ernst Münch proposed the pressure-flow hypothesis for phloem transport in 1930. Aphid stylets were used from the 1950s to collect phloem sap directly, and showed that it contains sucrose at high concentrations. Radioactive tracers have since confirmed the pathways.

Which mistakes are common?

Mistake 1: Saying that xylem transports sugars

Xylem transports water and minerals. Sugars are carried in the phloem.

Mistake 2: Saying that xylem cells are alive

Mature vessels and tracheids are dead, hollow tubes.

Mistake 3: Thinking that phloem is dead because sieve tubes lack a nucleus

Sieve tube elements are alive and depend on their companion cells.

Mistake 4: Saying that phloem transports only downward

It moves from sources to sinks, which can be up or down. Sugars move up to growing shoots and fruits, and down to roots.