Core practical 7: plant tissues under the light microscopeSpec 4.6
In short
A plan diagram shows the arrangement of tissues in a section using clear lines, without drawing individual cells. A high-power drawing shows the cells of one tissue. Thin sections are cut, stained, mounted and viewed with a light microscope, and the magnification of a drawing is calculated from image size divided by actual size.
Core practical 7: use a light microscope to make observations, draw and label plan diagrams of transverse sections of roots, stems and leaves; draw and label cells of plant tissues; and identify sclerenchyma fibres, phloem, sieve tubes and xylem vessels and their location.
Preparing a section
- Use a sharp razor blade or scalpel to cut a very thin transverse section of the stem, root or leaf. Cut away from your fingers, and place the blade in a sharps container or as directed after use.
- Float the sections in water in a dish. Choose the thinnest, most complete section with a fine paintbrush.
- Stain: toluidine blue stains lignified walls blue-green; phloroglucinol with hydrochloric acid stains lignin red. Treat hydrochloric acid as corrosive and wear eye protection.
- Mount on a slide in a drop of water or stain. Lower a coverslip at an angle, using a mounted needle, to avoid trapping air bubbles.
- Start with the lowest power objective, focus with the coarse focus and then the fine focus, then move to a higher power for detail.
Plan diagrams
- Draw with a sharp pencil, using clean single lines, with no shading and no colouring.
- Draw the outline of the whole section and the boundaries between tissues. Do not draw individual cells.
- Make the drawing large (using at least half the page) and in proportion.
- Label with straight lines ruled from the structure, not crossing each other, with a title, and record the magnification.
| Section | Tissues to outline and label |
|---|---|
| Root | Epidermis (with root hairs), cortex, endodermis, and a central vascular cylinder with xylem in a star or cross shape and phloem between its arms |
| Stem | Epidermis, cortex, a ring of vascular bundles (xylem on the inner side, phloem outside it, sclerenchyma cap outermost) and central pith |
| Leaf | Upper and lower epidermis, palisade mesophyll, spongy mesophyll, and the midrib vascular bundle with xylem on the upper side and phloem on the lower side |
Drawing cells of a tissue
Use a higher power objective and draw a small number of cells in one tissue with a continuous line, showing the cell walls, lumen and contents. Show thick walls as double lines. Identify the tissues from their appearance.
- Xylem vessels: large circular lumens, no contents, thick walls (stained blue-green or red by lignin stains).
- Sclerenchyma fibres: small lumens, very thick walls, in groups outside the phloem.
- Phloem: thin-walled; sieve tubes are larger and empty-looking, with companion cells smaller and with dense cytoplasm beside them. In a longitudinal section a sieve plate may be visible.
Calculating the magnification of a plan diagram
A student's plan diagram of a stem section is 96 mm wide. The real section, measured using an eyepiece graticule that was calibrated with a stage micrometer, is 3.2 mm wide. Calculate the magnification.
- Both lengths must be in the same units: 96 mm and 3.2 mm.
- magnification = 96 ÷ 3.2
Answer: ×30
To find the real size from a drawing, rearrange: real size = image size ÷ magnification. Convert units first: 1 mm = 1000 µm.
(opens full size in a new tab)A plan diagram must not show individual cells, and labels must not cross. Both are common ways to lose marks.
Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026