Core Practical 7: Identifying Tissue Types Within Stems (Edexcel International A Level Biology) : Revision Note

Identifying Tissue Types Within Stems

• In order to identify tissue types within stems, a permanent pre-prepared slide could be used

• Alternatively, a section of a plant stem could be cut and stained before preparing a temporary slide

Apparatus

• Plant stem

• Scalpel

• Suitable stain

• Microscope slide

• Cover slip

• Forceps

• Dissecting needle

• Light microscope

• Gloves

Method

1. Cut a very thin cross-section of the stem using a scalpel

2. Carefully transfer each section into a dish containing a suitable stain and leave for one minute

   • E.g. toluidine blue O (TBO) stains xylem and sclerenchyma fibres blue-green, and phloem will appear pinkish purple

3. Rinse off each section in water and mount onto a microscope slide, before adding a cover slip

   • take care to lower the coverslip slowly over the sample from one side to the other to avoid trapping air bubbles, which could be mistaken for plant tissue structures

4. View under a microscope and adjust the focus to form a clear image

5. Make a labelled drawing of the positions of the xylem vessels, phloem sieve tubes and sclerenchyma fibres

Plan diagrams

• When drawing tissue plan diagrams you need to:

  • Read the instructions carefully

  • Draw a large diagram

  • Use a sharp pencil and do not shade (including the nucleus)

  • Use clear, continuous lines (no sketching)

  • When using an eyepiece graticule, use it to ensure you have correct proportions or if you are not using a microscope then endeavour to keep the proportions between tissues to scale

• If drawing from a low-power image:

  • Do not draw individual cells

  • Read the question carefully as you may only have to draw a portion of the image

  • Include the magnification on the drawing

• If drawing from a high-power image:

  • Draw only a few of the required cells

  • Draw the cell wall of the plant cells 

  • Include the magnification on the drawing

• When labelling, remember:

  • Use a ruler for label lines (and scale lines if appropriate)

  • Label lines should stop exactly at the structure (do not use arrows)

  • Don't cross label lines over each other

  • Label all tissues and relevant structures that are requested