7.1 Structure of Transport Tissues

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Draw plan diagrams of transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants from microscope slides and photomicrographs
  2. Describe the distribution of xylem and phloem in transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants
  3. Draw and label xylem vessel elements, phloem sieve tube elements and companion cells from microscope slides, photomicrographs and electron micrographs
  4. Relate the structure of xylem vessel elements, phloem sieve tube elements and companion cells to their functions

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What Are Dicotyledonous Plants?

A dicotyledonous plant (or "dicot" for short) is a type of flowering plant whose seed contains two seed leaves (called cotyledons). Common examples include sunflowers, beans, and peas. In this topic, we study how water, minerals, and food are transported around these plants.

Plants need a transport system because:

  • They are made of many layers of cells, so diffusion alone is far too slow to move substances around
  • Different parts of the plant need different things — for example, roots cannot photosynthesise, so they need sugar delivered to them from the leaves
  • Water absorbed by roots must be moved all the way up to the leaves

This transport system consists of two types of vascular (transport) tissue: xylem and phloem. These tissues are grouped together into structures called vascular bundles.

Objective 1: Drawing Plan Diagrams of Transverse Sections

A transverse section (T.S.) is a cross-section — imagine slicing straight across a stem, root, or leaf like cutting a carrot into rounds. When you look at this cut surface under a microscope, you can see how the tissues are arranged inside.

A plan diagram is a simple drawing that shows the arrangement of tissue regions — not individual cells. Think of it as a map of the organ showing where each tissue is located.

Rules for Drawing Plan Diagrams

Follow these steps carefully:

  1. Use a sharp pencil — lines must be clean and neat, never sketchy or shaded
  2. Use continuous, smooth lines — no broken or wobbly lines
  3. Do not draw individual cells — only outline the regions of tissue (e.g. the cortex as a whole area, not each cell within it)
  4. Make your drawing large — this makes labelling easier and clearer
  5. Add labels using a ruler — draw straight horizontal lines from the structure to the label name; label lines must not cross each other and should stop exactly at the boundary of the tissue (no arrows)
  6. Label all tissues — include every region visible (e.g. epidermis, cortex, xylem, phloem)
  7. Keep proportions correct — make sure the relative sizes of regions match what you see in the image
  8. Add magnification if it is shown on the micrograph or asked for in the question

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