Xylem and phloem

Large, multicellular plants need specialised transport systems to move substances over long distances. Water and minerals are absorbed by the roots but are needed by all parts, especially the leaves for photosynthesis. Sugars are made in the leaves but are needed by all living cells for respiration, especially in the roots and growing points which cannot photosynthesise. These two transport jobs are carried out by two tissues: xylem and phloem. They are grouped together in structures called vascular bundles that run through the roots, stem, and leaves.

Xylem tissue is adapted for the transport of water and mineral ions, and for structural support. It is a one-way system, moving substances from the roots up to the leaves. Its structure is highly specialised for this role. Xylem vessels are formed from cells (called vessel elements) joined end to end. During development, the end walls between the cells break down completely, and the cell contents (cytoplasm, vacuole, nucleus) die and disappear. This creates a continuous, hollow tube called a xylem vessel, allowing water to flow with minimal obstruction. The walls of the xylem vessels are thickened with a strong, waterproof substance called lignin. This provides mechanical strength to support the plant and prevents the vessels from collapsing under pressure.

Phloem tissue is responsible for transporting the soluble organic products of photosynthesis, mainly sucrose and amino acids. This process is called translocation. Unlike xylem, phloem is a living tissue and can transport substances both up and down the plant. Phloem consists of two main cell types: sieve tube elements and companion cells. Sieve tube elements are long, thin cells joined end to end. Their end walls are perforated with pores, forming structures called sieve plates, which allow sap to flow between cells. To maximise flow, mature sieve tube elements have very little cytoplasm, no nucleus, and no large central vacuole. Because they lack these essential organelles, each sieve tube element is associated with a companion cell. The companion cell is connected to the sieve tube element by pores and contains a nucleus, mitochondria, and ribosomes. It actively manages the sieve tube element and provides the metabolic energy needed for translocation.

Translocation is the movement of sugars from where they are made to where they are needed. This is often described as movement from a 'source' to a 'sink'. A 'source' is a part of the plant that produces or releases sugar. The primary source is usually the leaves, where glucose is made by photosynthesis and converted to sucrose for transport. A 'sink' is a part of the plant that uses or stores sugar. Examples include growing roots, developing fruits, flowers, and storage organs like potato tubers. The key thing to remember is that the roles of source and sink can change depending on the season and the plant's needs. For example, in summer, the leaves are the source and the roots are the sink. But in spring, the stored starch in the roots is converted back to sucrose and the roots become the source, while the new growing leaves are the sink. Because translocation is an active process, it can move sap against gravity, from leaves down to roots, or from roots up to new buds.

Scientists use several techniques to investigate the function of xylem and phloem. These experiments provide direct evidence for what is transported and where.

1. Tracing Xylem: A simple and effective method is to place a plant shoot, like a stick of celery or a white carnation, into water containing a coloured dye (e.g., eosin or methylene blue). After a few hours, if you cut a cross-section of the stem, you will see small coloured dots corresponding to the xylem vessels. This shows that xylem is the tissue that transports water up the stem.
2. Ringing Experiment (for Phloem): A ring of bark, including the phloem, is carefully removed from a woody stem, leaving the central xylem intact. After a few weeks, the area above the ring swells up. Analysis of the liquid in this swollen area shows it is rich in sugars. This demonstrates that sugars are transported downwards in the phloem and cannot pass the gap.
3. Aphid Stylets (for Phloem): Aphids feed by inserting a sharp mouthpart, called a stylet, directly into a single phloem sieve tube. If the aphid is anaesthetised and its body removed, the pressurised sap continues to exude from the stylet. This pure phloem sap can be collected and analysed, proving it is rich in sucrose.

The position of xylem and phloem in vascular bundles differs between the root, stem, and leaf, and this arrangement is related to the function of each organ. In a Dicotyledonous Root: The vascular tissue forms a solid central core. The xylem is typically in a star or 'X' shape, with the phloem located in the gaps between the arms of the xylem. This arrangement provides a strong central column that helps the root to anchor the plant firmly in the soil and resist pulling forces. In a Dicotyledonous Stem: The vascular bundles are arranged in a distinct ring towards the outside of the stem. Within each bundle, the phloem is positioned towards the outside, and the xylem is towards the inside. This arrangement provides strength and support, a bit like the reinforcing bars in a concrete pillar, helping the stem to resist bending. In a Leaf: The vascular bundles form the veins. In a cross-section of a large vein, the xylem is found on the upper side (adaxial surface) and the phloem is on the lower side (abaxial surface). This allows the xylem to efficiently deliver water to the photosynthesising palisade cells and the phloem to easily collect the sugars produced.