Plant Transverse Sections (Cambridge International AS Biology)

Transverse Sections: Stems, Roots & Leaves

Spec point — 7.1.1 · draw plan diagrams of transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants from microscope slides and photomicrographs

Transport systems

• Plants need transport systems to:

  • Meet their metabolic demands (glucose, hormones, mineral ions are required for various processes within plants)

  • Efficiently move substances up and down, e.g.

    • Sugars made in photosynthesis need to be transported, as sucrose, 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 starch in storage organs, e.g. tubers, and need to convert it to sucrose 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 therefore 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 (e.g. a leaf) to the sink (e.g. root)

• 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 The micrograph below shows a transverse section of root tissue.

Draw a plan diagram of this section of root tissue.

Light micrograph (photomicrograph) of transverse root tissue

Step 1 Draw the outlines of each section

Use a sharp pencil with no shading or sketching

Use a continuous line

Step 2 Add the outlines of the remaining tissues

A plan diagram shows tissues only, so do not draw any individual cells; for example, outline the xylem tissue at the centre of the root

Step 3 Add labels

Use a ruler to draw horizontal lines to each structure of interest

Label clearly with correct spelling

Step 4 Add magnification(if present on micrograph or question)

Plan diagram of transverse root tissue

The 'before-and-after' images look like this side-by-side:

Root transverse section 'before-and-after' images side-by-side

Exam Hint 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 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 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)