OCR AS Biology

Revision Notes

3.3.5 Transverse Sections: Stems, Roots & Leaves

Transverse Sections: Stems, Roots & Leaves

Dicotyledonous (dicots) plants

  • Dicotyledonous (dicots) plants have:
    • Seeds that contain two cotyledons (seed leaves)
    • Network of veins
    • Leaves that typically have broad blades (leaf surface) and petioles (stalks)
    • Tap root with lateral branches
  • Herbaceous dicots have a relatively short life cycle (one growing season) and non-woody tissue

Transport systems

  • Plants need transport systems to meet their metabolic demands (glucose, hormones, mineral ions are required for various processes within plants), to efficiently move substances up and down and to compensate for their relatively small SA:V ratio (generally plants cannot rely on diffusion alone)
  • Plants have a vascular system which involves a network of vessels (vascular tissue) running through the leaves, stem and roots. These three parts are the main organs involved in transport
  • The vascular system is comprised of two distinct types:
    • Xylem (transports water and mineral ions from the roots to the rest of the plant)
    • Phloem (transports substances from the source (eg. leaf) to the sink (eg.root))
  • The xylem and phloem are arranged together in vascular bundles
  • The bundles are laid out differently in the leaves, stem and roots

Transverse sections_Stems, roots and leaves (leaf), downloadable AS & A Level Biology revision notes

Tissue plan diagrams of a dicotyledonous leaf, stem and root

Worked Example

Transverse Sections_ worked example 1, downloadable AS & A Level Biology revision notes

Step 1: Calculate the 1 eyepiece graticule unit

1 eyepiece graticule unit = 0.1 divided by 40 = 0.0025 mm

Step 2: Convert the answer to measurement with the unit most suitable for use in light microscopy

0.0025 multiplied by 1000 = 2.5 µm

Worked Example

Transverse Sections worked example 2, downloadable AS & A Level Biology revision notes

Step 1: Calculate the number of divisions between X and Y

There are 80 divisions between X and Y

Step 2: Calculate the value of each division

As each division is equal to 2.5 µm

Step 3: Calculate the actual width

80 x 2.5 = 200 µm

Exam Tip

When drawing tissue plan diagrams (which is common in the practical paper 3) 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
  • When using an eye-piece 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 line 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 (those requested)
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