CIE A Level Biology

Revision Notes

Syllabus Edition

First teaching 2020

Last exams 2024

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7.1.1 Plant Transverse Sections

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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, e.g.
    • Glucose from photosynthesis needs to be transported to parts of the plant that cannot photosynthesise
    • Water from the roots needs to be transported to the upper parts of a plant
    • Plants may store glucose in storage organs, e.g. tubers, and need to release this glucose and transport it to metabolically active parts of the plant
  • Plants consist of many layers of cells and so cannot rely on diffusion from cell to cell to supply all their needs, as this would be too slow
  • 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
    • Xylem transports water and mineral ions from the roots to the rest of the plant
    • Phloem transports substances from the source (e.g. a leaf) to the sink (e.g. root)The vascular system is comprised of two distinct types:
  • 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

Transverse sections_Stems, roots and leaves (stem)

Transverse sections_Stems, roots and leaves (root)

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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Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.