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
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- 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
Tissue plan diagrams of a dicotyledonous leaf, stem and root
Worked example
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
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
Examiner 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)
