Inside Leaf, Stem & Root

Explain means cause and effect, and the question adds a demanding condition: every claim must rest on something observed, not recalled. The examiner is testing whether you can read a slide as evidence.

6 marks, two per function. For each of gas exchange, water transport and nutrient transport: name the structure, describe what it looks like, and say how that appearance accounts for the function.

A peel of the lower epidermis, or a leaf cross-section, shows numerous small pores, the stomata, each bounded by a pair of curved guard cells. A cross-section also shows the spongy mesophyll beneath them as a loose layer of rounded cells with large air spaces between them.

The pores are openings through an otherwise sealed waxy surface, so they are the route by which carbon dioxide enters and oxygen and water vapour leave. Because each is bounded by two guard cells that can change shape, the aperture is adjustable, which lets the leaf limit water loss when necessary. The air spaces of the spongy mesophyll are continuous with those pores, so gases can diffuse freely to and from every photosynthesising cell rather than having to cross a solid block of tissue.

A stem or root cross-section shows xylem within the vascular bundles as large, thick-walled, hollow tubes with no cell contents, and when the stem has been stood in a dye such as eosin those tubes are stained while the surrounding tissue is not. A root section shows epidermal cells drawn out into long, narrow extensions, the root hair cells. The hollow, unobstructed lumen explains rapid low-resistance flow; the thickened lignified wall explains how the vessel resists collapsing under the tension of the transpiration stream; and the dye seen only in those tubes is direct evidence that this is the tissue carrying water and dissolved substances upward. The long extensions of the root hair cells greatly increase the surface area available for absorbing water and minerals from the soil.

The same vascular bundle shows phloem lying alongside the xylem, made up of smaller cells that still contain living contents, joined end to end with perforated cross-walls, the sieve plates, and accompanied by smaller companion cells. Being living and retaining cytoplasm is what allows phloem to load sugar actively using energy from the companion cells, and the perforated sieve plates let sap pass from one cell to the next along the tube. That the phloem is unstained by the dye, while the xylem beside it is stained, is itself evidence that the two tissues carry different substances.

Taken together, these observations explain all three functions from visible structure: pores and air spaces for gas exchange, hollow lignified tubes and root hairs for water uptake and transport, and living sieve tubes with companion cells for nutrient transport. Sections are best examined on low power first to locate the tissue and then on high power for detail, with a stain such as methylene blue or iodine used to make cell walls and tissue boundaries stand out.

Describe the observation, then explain the inference. Keep the two apart: what you can see is not the same as what it means, and the marks are awarded separately.

4 marks. Describe the appearance of the cut surface, identify the stained structures, then explain what the dye's journey proves about the tissue's function.

The cut surface shows a series of discrete red dots or short strands distributed across it, rather than a uniformly red surface. In celery these appear near the outer curved edge of the stalk, following the strands that can be felt as the stringy fibres. Cutting the stalk lengthwise instead shows continuous red lines running the length of the stalk.

These red dots are the xylem vessels within the vascular bundles. Each bundle contains xylem and phloem together, but only the xylem has taken up the dye.

The dye was dissolved in the water at the base of the stalk and has travelled upward to appear at the cut surface, which means water carrying dissolved substances moved through those vessels from the bottom of the stalk toward the leaves. Because only the xylem is stained, the observation is direct evidence that xylem is the tissue transporting water and dissolved minerals upward through the plant, drawn up by the transpiration stream. That the phloem in the same bundles is unstained is further evidence that it carries something else.