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2.7.7Additional lessons

Drawing Root and Stem Structure — IB Biology SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 40 of 142

📚 IB Biology SL Hub / Topical Revision
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Part 1 of 6Tissues in a Stem

2.7.7 Drawing Root and Stem Structure
Epidermis
  • The outer protective layer of the stem, often covered with a waxy cuticle that reduces water loss.
Cortex
  • A layer of packing tissue beneath the epidermis that supports the stem and may store food.
Vascular bundles
  • Bundles of xylem and phloem arranged in a ring near the outside of the stem.
Xylem and phloem
  • In each bundle the xylem lies towards the inside and the phloem towards the outside.
Pith
  • Parenchyma tissue in the centre of the stem that stores substances and gives support.

Part 2 of 6Tissues in a Root

Epidermis
  • The outer layer in the root carries root hairs that increase the surface area for absorbing water and ions.
Cortex
  • A wide layer of cells through which water crosses from the epidermis to the centre.
Central vascular tissue
  • The xylem and phloem are grouped together in the middle rather than in separate bundles.
Xylem
  • The xylem often forms an X or star shape in the centre, with phloem between its arms.
Strength
  • The central position of the xylem resists the pulling forces on a root.

Part 3 of 6Drawing a Plan Diagram

1
Use a sharp pencil, draw a large diagram and give it a title and the magnification.
2
Draw only the outlines of tissue regions; a plan diagram shows no individual cells.
3
Use single, continuous lines and do not shade or colour.
4
Draw tissues in proportion to their size and position on the section.
5
Add straight, ruled label lines that touch the structure and do not cross each other.

Part 4 of 6Using the Drawing

Using the Drawing
Comparison
  • Compare the stem and the rootseparate bundles in a ring in the stem and a central xylem in the root.
Function
  • The root cortex stores food and carries water inward, whereas the stem cortex gives support.
Scale
  • Add a scale bar or state the magnification so that the real size of each tissue can be found.

Part 5 of 6Worked Example

Question

  • A plan diagram of a stem is drawn 80 mm wide.
  • The real stem section is 4 mm wide.
  • Calculate the magnification of the drawing.

  1. Magnification = drawing size ÷ actual size.
  2. 80 mm ÷ 4 mm = 20.

Answer The drawing has a magnification of ×20.

Part 6 of 6Exam Checklist & Tips

2.7.7 – Key Points for the Exam

  • Plan diagram A low-power drawing showing the arrangement of tissues but not cells.
  • Vascular bundle A group of xylem and phloem running together in the stem.
  • Cortex Tissue between the epidermis and the vascular tissue.
  • Root hair An extension of an epidermal cell that increases absorption surface area.

Common Mistakes

  • Drawing individual cells in a plan diagram, when only tissue outlines are needed.
  • Placing the vascular bundles of a root in a ring like those of a stem.
  • Shading the drawing, which hides detail and is not allowed.
  • Leaving out the magnification or title of the drawing.

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