International A Level Biology / Plant structure and function

Core practical 7: plan diagrams of roots, stems and leaves revision notes

Edexcel International A Level Biology (YBI11), chapter 10: Plant structure and function, specification point 4.6. Free lesson.

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A plan diagram shows the layout of tissues in a whole plant section, without drawing individual cells. You need to prepare thin transverse sections and recognise the positions of tissues in roots, stems and leaves.

How to prepare a thin section

A transverse section is a slice cut straight across a plant part. It needs to be thin so light can pass through and cells do not overlap. You need a sharp blade, tile, dish of water, stain, slides, coverslips and a light microscope. Cut a short length of stem and stand it upright on the tile. Always cut away from your fingers. Put the sections in water straight away so they do not dry out, then choose the thinnest one.

What the stains show, and how to draw

Toluidine blue stains lignified cell walls blue-green. Other walls stain purple. Phloroglucinol with hydrochloric acid stains lignin red. Wear eye protection when using the acid. A plan diagram is made on low power. It shows the outlines and positions of tissues, but no cells. Labels name tissues such as xylem, phloem and cortex. A drawing of cells is made on high power. It shows a few cells and their walls, with labels for cell parts such as the wall, sieve plate or lumen. For both drawings, use a sharp pencil, smooth unbroken lines, no shading, ruled label lines and accurate…

How to recognise tissues in a root

In a root plan diagram, the epidermis is the one-cell-thick outer layer. A wide ring of cortex fills most of the section. The endodermis forms a ring around the vascular tissue. The xylem is in the centre, often shaped like a star or cross. The phloem lies in patches between the arms of the xylem. Vascular tissue in the middle helps the root resist being pulled.

How to recognise tissues in a stem

Stem vascular bundles form a ring inside the cortex. From the outside towards the centre, each bundle contains sclerenchyma, phloem and xylem. The pith is packing tissue in the middle. The epidermis is the outer layer, with a band of cortex inside it. Sclerenchyma forms a cap of fibres on the outside of each bundle. Phloem is just inside the cap, and xylem faces the centre. In a real section, sclerenchyma fibres have thick walls. Phloem cells are small and thin-walled. Xylem vessels are wide and empty, with lignified walls that stain red. The pith has large, thin-walled cells. In a…

How to recognise tissues in a leaf

A leaf section is cut through the midrib, the thick vein down the middle. A plan diagram shows tissue outlines rather than the individual cells. The upper epidermis runs along the top. Beneath it is the palisade mesophyll, made of tall cells packed with chloroplasts. It may be two rows deep. The spongy mesophyll lies below, with many air spaces. The lower epidermis runs along the bottom. In the midrib bundle, xylem is on the upper side and phloem is below it.

Avoid confusing this with a root-tip squash

A root-tip squash is a different practical. Squashing spreads cells into a layer and destroys the tissue layout, so it cannot be used to make a plan diagram. For a plan diagram, cut a thin transverse section with a sharp blade, stain it and view it on low power. If a stem tissue is blue-green after toluidine blue, it is lignified. If it also has wide, empty cells and faces the centre of a bundle, identify it as xylem. Sclerenchyma is lignified too, but lies on the outside of the bundle.