Plan diagrams and tissue sections

The plan diagram convention

A low-power plan diagram records the positions, boundaries and relative proportions of tissues in a section. It does not show individual cells.

Draw tissue boundaries with single, clear lines. Do not shade or colour the diagram. Keep the regions in proportion, label them with straight ruled lines that do not cross, and include a magnification or scale bar. Use the eyepiece graticule where measurements are needed to preserve proportion.

A high-power biological drawing has a different purpose: it records the structure of a small number of representative cells. Do not combine high-power cellular detail with a low-power plan diagram.

Two further points of the convention: use a sharp pencil and unbroken lines rather than sketchy or feathered ones, and use no arrowheads on label lines.

Plan diagram

A low-power drawing of a section that shows the positions and boundaries of tissues, in correct proportion, without drawing any individual cells.

Transverse section

A section cut across an organ, at right angles to its long axis.

Longitudinal section

A section cut along an organ, parallel to its long axis.

The two section planes matter because a section is one slice through a solid object. Sectioning and artefacts are covered in Microscopy, magnification and resolution. A tube cut transversely appears as a ring; the same tube cut longitudinally appears as a pair of parallel lines; and neither view alone establishes that the structure is a tube. Interpret a section by relating the flat image to the three-dimensional organ it came from.

Xylem and phloem in root, stem and leaf

Plan diagrams are usually assessed on xylem and phloem, because a herbaceous dicot arranges the same two tissues in three different ways in its three organs, and each arrangement relates to the mechanical loading of that organ.

Describe the drawing

Three plan diagrams side by side, drawn as tissue boundaries with no individual cells. The root is a circle with a small central stele holding a star of xylem, coral, with four patches of phloem, amber, between its arms. The stem is a circle with eight vascular bundles arranged in a ring near the edge, each bundle keeping its xylem on the inner side and its phloem on the outer side. The leaf is a thin blade in section with one midrib bundle in which the xylem sits above the phloem. Text beneath notes that a transverse cut shows the stem bundles as a ring while a longitudinal cut shows them as parallel stripes.

In the root, the vascular tissue is gathered into a central column called the stele. The xylem forms a star, usually drawn with four arms in section, and the phloem sits in separate patches between the arms. A root is loaded in tension: the shoot above levers it and wind tries to pull it out of the ground, and a rope resists pulling best when its strength runs down the middle.

In the stem, the vascular tissue is split into discrete vascular bundles arranged in a ring near the outside. Each bundle has its xylem on the inner side, its phloem on the outer side, and a layer of dividing cambium between them. A stem is loaded in bending. The same amount of material resists bending far better spread into a hollow cylinder than gathered into a solid rod, so scaffolding poles are tubes, not bars. The ring of lignified bundles is the plant's version of the tube.

In the leaf, each vein is one bundle, seen most easily in the midrib. The xylem sits on the upper side of the bundle and the phloem on the lower side. The reason is continuity, not anything the leaf needs. The bundle runs out of the stem sideways into the leaf, and what was the inner face of the bundle in the stem becomes the upper face in the leaf. Use that continuity to determine the orientation of a leaf section.

The airway wall in plan

The same convention reads animal organs. The trachea and the bronchus are tubes whose walls are built from concentric layers of tissue, which makes them natural subjects for a plan diagram: each layer becomes one band, and the drawing is a set of rings.

Describe the drawing

Two airway walls drawn in plan as concentric layers around a lumen. The trachea has a ring of ciliated epithelium lining the lumen, mucous glands in the loose tissue outside it, and one C-shaped band of cartilage open at the back, where a strip of smooth muscle spans the gap. The bronchus has the same epithelium lining a narrower lumen, a complete ring of smooth muscle, and cartilage broken into five separate curved plates with gaps between them. In both organs the epithelium is the innermost layer and the cartilage never reaches the lumen.

Work from the lumen outwards and label the layers in that order. The ciliated epithelium lines the airway, with goblet cells scattered through it, secreting and moving the mucus that traps particles. Beneath it lies looser tissue holding mucous glands, which add to what the goblet cells secrete, and blood vessels. Elastic fibres run through this tissue. Then the cartilage, the layer that keeps the tube open against the pressure changes of breathing, and outside that the connective tissue that ties the organ to its neighbours.

The cartilage is also the identification feature. In the trachea it forms a single C-shaped ring in each segment, incomplete at the back, with smooth muscle spanning the gap; the opening faces the oesophagus, which needs room to bulge as swallowed food passes. In a bronchus the C is gone, replaced by separate curved plates of cartilage, with a complete ring of smooth muscle inside them, and the lumen is narrower. In a bronchiole there is no cartilage, only epithelium, smooth muscle and elastic fibres. Given an unlabelled airway section, look for the cartilage first: one C means trachea, plates mean bronchus, none means bronchiole.

Drawn as a plan diagram, the airway wall becomes five or six labelled concentric bands, which records the order of the layers without drawing any individual cell.

Four cells on a smear

A blood smear is made by putting one drop of blood at the end of a slide and drawing it out into a film one cell thick with the edge of a second slide, then staining it. The stains used for blood colour nuclei purple and cytoplasm pink or pale blue, so every identification below starts from the nucleus.

Describe the drawing

A field from a stained blood smear with every cell drawn to the same scale and a ten micrometre scale bar. Five erythrocytes about seven micrometres across appear as rings with pale centres and no nucleus. A neutrophil of about twelve micrometres has a nucleus of three lobes. A lymphocyte of about nine micrometres is almost filled by one round nucleus. A monocyte of about eighteen micrometres, the largest cell in the field, has a kidney-shaped nucleus pushed to one side.

Erythrocytes dominate every field, outnumbering white cells by roughly seven hundred to one. Each is about 7 µm across, stains pink, and has no nucleus: the biconcave shape makes the middle of the disc thinner, so it stains paler and the cell appears as a ring. Any nucleated cell on the smear is a white cell. Three kinds are described below; Edexcel B also names the eosinophil, about 12 to 17 µm across, with a nucleus of usually two lobes and large granules that stain red-orange.

A neutrophil is about 12 µm across with a nucleus in two to five connected lobes, and fine granules in its cytoplasm. It is the most abundant white cell and the first phagocyte to reach an infected tissue. A lymphocyte is smaller, about 9 µm, with a single round nucleus so large that only a thin rim of cytoplasm shows around it; these are the B and T cells of the specific immune response. A monocyte is the largest cell on the smear, up to about 18 µm, with a kidney-shaped or horseshoe nucleus pushed to one side; it leaves the blood to become a macrophage.

Blood smear

A single-cell-thick film of blood drawn across a slide and stained, used to examine and identify blood cells.

Neutrophil

A phagocytic white blood cell about 12 µm across with a lobed nucleus and granular cytoplasm.

Lymphocyte

A white blood cell about 9 µm across whose round nucleus almost fills the cell, responsible for the specific immune response.

Monocyte

The largest white blood cell of a smear, up to about 18 µm, with a kidney-shaped nucleus, which matures into a macrophage.