Xylem and phloem

Plants need an internal transport system to move water, minerals, and food over long distances. This job is done by two specialized tissues: xylem and phloem. They are grouped together into structures called vascular bundles, which run through the roots, stem, and leaves like a plant's plumbing system. Xylem transports water and dissolved mineral ions from the roots up to the rest of the plant. Phloem transports sugars (like sucrose) and amino acids from where they are made (mainly the leaves) to where they are needed for growth or storage. The arrangement of these bundles also provides strength and support to the plant.

Xylem tissue is brilliantly adapted for its dual roles of water transport and support. It is composed of long tubes called xylem vessels. These vessels are formed from cells called vessel elements joined end to end, with their cross-walls completely broken down. This creates an uninterrupted, hollow tube for water to flow through easily. The cells are dead at maturity, meaning they have no cytoplasm, nucleus, or vacuole to obstruct the flow. The walls are thickened with a strong, waterproof substance called lignin. This lignin prevents the vessels from collapsing under the negative pressure of transpiration and provides mechanical support to the whole plant. The continuous column of water moving up from the roots through the xylem to the leaves is called the transpiration stream.

Phloem tissue is responsible for transporting the food made during photosynthesis (mainly in the form of sucrose) from the leaves to all other parts of the plant that need it. This process is called translocation. Unlike xylem, phloem is a living tissue. The main conducting tubes are called sieve tubes, formed from cells called sieve tube elements joined end to end. The end walls between these cells are not completely broken down; instead, they are perforated with pores, forming structures called sieve plates. These plates allow the sugary sap to pass from one cell to the next. To maximise space for transport, mature sieve tube elements have very little cytoplasm and no nucleus or large central vacuole. To keep them alive and manage their activities, each sieve tube element is closely associated with a companion cell. The companion cell has a dense cytoplasm, a nucleus, and many mitochondria to provide the metabolic energy needed for loading and unloading sugars.

Translocation is the directed movement of sugars 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 sugar is made by photosynthesis. A sink is a part of the plant that uses or stores sugar. Examples of sinks include growing roots, developing flowers, fruits, and seeds, as well as storage organs like potato tubers or the roots of a carrot. The key thing to remember is that the roles of source and sink can change depending on the season and the plant's needs. In summer, leaves are the source and roots are the sink. However, in early spring, the stored starch in the roots is converted back to sugar and the root becomes a source, transporting sugar up to the new developing leaves, which are sinks until they can photosynthesise for themselves.

Simple experiments can demonstrate the roles of xylem and phloem. To trace the path of water, you can place a plant stalk with white flowers, like a carnation or celery, into water containing a coloured dye. After a few hours, the dye will travel up the stem and into the petals or leaves, colouring them. When the stem is cut and examined, only the xylem vessels in the vascular bundles will be stained, proving they are the water-carrying tubes. To show phloem's role, a 'ringing experiment' is used. A ring of bark and phloem is carefully cut away from a woody stem, leaving the central xylem intact. Over time, the area above the ring swells up with a sugary fluid, while the growth of the roots below is stunted. This shows that sugars are transported downwards in the phloem and that removing it stops this flow.