Plant Transport Tissues, Transpiration and Translocation

Two transport tissues, two different jobs

A plant needs to move substances between organs. Its roots absorb water and mineral ions, while its leaves make sugars by photosynthesis. Growing tissues and storage organs may be far from the leaves, so transport cannot rely only on substances moving between neighbouring cells.

Plants have two specialised transport tissues: xylem carries water and dissolved mineral ions from the roots towards the stems and leaves; phloem carries dissolved sugars, mainly sucrose, and amino acids to places where they are needed. These tissues occur in groups called vascular bundles in roots, stems and leaves.

How xylem forms a strong, open pathway

Xylem vessels are formed from cells joined end to end. At maturity, these cells are dead and their contents have been lost. Their end walls break down, leaving a continuous hollow tube. The space inside the tube is called its lumen. With no cell contents or end walls blocking this pathway, water can flow through it.

The vessel walls are strengthened with lignin, a strong, woody material. This makes the walls stiff and helps prevent the vessels collapsing as water is drawn up the plant. It also helps support the plant.

Together, the hollow interior and strong walls make xylem suited to transporting water and dissolved mineral ions in the transpiration stream, from roots towards leaves.

How living phloem transports dissolved food

Phloem tubes consist of living cells called sieve-tube elements, joined end to end. Unlike xylem vessels, they retain end walls. These walls have holes and are called sieve plates. The holes allow dissolved sugars and amino acids to pass between successive cells.

Sieve-tube elements have no nucleus and are supported by adjacent companion cells, which carry out functions needed to keep the tissue working. Translocation requires energy supplied by living cells; the plant does not simply move sugars through dead, empty pipes.

Translocation is the movement of dissolved sugars and amino acids through phloem. Substances move from a source, where they are supplied, to a sink, where they are used or stored. A photosynthesising leaf can supply sucrose to a growing root or a storage organ. The sugar can be used in respiration and growth, or converted into starch for storage.

Transport is not restricted to one direction. During early spring, stored reserves in roots can supply developing leaves; during summer, leaves can supply roots. Phloem transport can therefore occur upwards or downwards, according to the plant's needs.

How water loss draws water through the plant

Transpiration is the loss of water vapour from a plant, mainly through its leaves. Inside a leaf, water evaporates from moist cell surfaces into the air spaces. Water vapour then diffuses through open stomata into the surrounding air.

This loss of water causes more water to be drawn into the leaf through the xylem. Water moves up from the roots to replace it, and the roots absorb further water from the soil. This connected movement forms the transpiration stream.

The important sequence is therefore evaporation inside the leaf → diffusion of water vapour out → replacement of water through xylem. Mineral ions dissolved in the water are carried along with it.

Stomata balance gas exchange and water conservation

A stoma is a small pore, usually in the surface of a leaf; the plural is stomata. Each pore is surrounded by two guard cells. Opening the pore allows carbon dioxide to diffuse into the leaf for photosynthesis and oxygen to diffuse out. It also allows water vapour to escape.

Guard cells control this trade-off. In many plants they are kidney-shaped, with thicker walls next to the pore and thinner outer walls. When they take in water by osmosis, they become turgid, meaning swollen and firm. Their uneven wall thickness makes them change shape so that the pore opens. When they lose water, they become flaccid, meaning less firm, and the pore closes.

Closing stomata reduces water loss when water is scarce, but also limits carbon dioxide entry. In many plants stomata normally open in daylight and close at night, conserving water when photosynthesis is not taking place.

Using a light microscope to examine transport tissues

A light microscope passes light through a specimen and uses lenses to produce a magnified image. The specimen must be thin enough for light to pass through; a thick piece of stem will hide rather than reveal its internal cells.

Use a prepared thin section of plant tissue, or a teacher-prepared section mounted in a drop of water on a glass slide. A suitable stain can improve contrast between tissues. Lower a coverslip gently from one edge to reduce trapped air bubbles. Handle slides and coverslips carefully, take care with sharp cutting tools, and avoid skin or eye contact with stains, following the teacher's precautions.

Place the slide on the stage and select the lowest-power objective. Use the coarse focus to locate the tissue, then the fine focus to sharpen the image. Centre the area of interest before changing to a higher-power objective. At higher power, use the fine focus and keep the objective clear of the slide.

Look for vascular bundles, then examine the cells within them. Xylem vessels have conspicuous thick walls around hollow spaces. Phloem contains living sieve-tube elements with associated companion cells. The section's direction matters: a cross-section shows the spaces inside vessels, while a suitable lengthways section can reveal their tube-like arrangement and end-wall features. Not every feature will be visible in every slide.

Record a clear labelled drawing of the structures you can distinguish. The observation is ready to record when the relevant tissue is sharply focused and its cell boundaries can be seen, rather than when the image is merely large.

Using a light microscope to observe stomata

To examine stomata, use a prepared slide of leaf epidermis, or use forceps to remove a thin piece of epidermis and lay it flat in a drop of water on a slide. Add a coverslip gently, avoiding folds and air bubbles. Keep the tissue moist so it does not dry out during observation.

Find the epidermis at low power, focus and centre a suitable area, then increase magnification using fine focus. Look for pairs of guard cells surrounding pores among the other epidermal cells. Identify and draw the guard cells and stomatal pore where visible.

A surface view is useful because it shows the two guard cells around the opening. A still slide shows their structure and the pore's appearance at that moment; observing one pore alone does not demonstrate its whole opening-and-closing process.

Transport tissues

Processes and stomata

• Transpiration: water evaporates from moist leaf cells, then water vapour diffuses out through stomata. Replacement water is drawn up .