Introduction and Objectives

In IJSO biology practicals, you are frequently given a prepared or freshly cut microscope slide and asked to identify and label specific tissue types or cell structures within plants or animals. Unlike the phyllotaxy experiment (which uses the naked eye), this task requires you to operate a compound microscope with precision, interpret what you see at different magnifications, and map your observations to your knowledge of histology.

The key challenge is not just memorizing what structures look like in a textbook, but also being able to recognize them under actual microscopic conditions, where colours, brightness, and clarity differ from printed diagrams.

Your Objectives

• Operate a compound microscope correctly and efficiently switch magnifications.
• Prepare or interpret cross-sections of stems, roots, and leaves.
• Identify and distinguish key plant tissue types: epidermis, chlorenchyma (palisade & spongy mesophyll), collenchyma, sclerenchyma, parenchyma, endodermis, pericycle, vascular bundles (xylem & phloem).
• Identify key animal tissue types when relevant (e.g. stratified squamous epithelium, smooth muscle).
• Annotate diagrams accurately using correct terminology.

Experimental Procedure

The procedure below covers identifying structures in a dicot stem cross-section (the most common IJSO specimen). The same logic applies to monocot stems, roots, and leaves.

Phase 1 : Orientation at Low Power (4x objective)

1. Survey the whole section. At 4x, you should see a circular or oval outline. Identify the outermost and innermost regions immediately.
2. Locate the major zones. Moving from outside in: epidermis → cortex → vascular bundles (ring) → pith.
3. Look for vascular bundles. In dicot stems, these are arranged in a ring. In monocots, they are scattered. This distinction is critical.

Phase 2 : Cortex Tissue Identification at 10x

1. Switch to 10x. Move to the outer cortex region (between epidermis and vascular bundles).
2. Look for collenchyma. Found just beneath the epidermis, especially at corners (ribs) in stems. Cells have thickened corners (uneven, non-lignified wall thickening). No staining = pale; safranin barely touches them.
3. Look for chlorenchyma (chloroplast-containing parenchyma). In young stems and the mesophyll of leaves. Large thin-walled cells containing green chloroplasts (or appearing more densely stained if chloroplasts are lost in fixation).
4. Look for sclerenchyma fibres. Often found as bundles or caps on vascular bundles. Cells have very thick, evenly lignified walls. With safranin, they stain intensely red/pink. The lumen (central cavity) is tiny or absent.

Phase 3 : Vascular Bundle at 40x

This is where the most detail is examined. Focus on one clean vascular bundle.

1. Identify xylem. Large, angular, thick-walled cells with heavily lignified walls (stain bright red with safranin). Xylem vessels may appear empty (dead at maturity) or show a distinctive spiral/annular wall pattern.
2. Identify phloem. Located on the outer side of the vascular bundle (opposite to xylem). Smaller cells with thin cellulose walls. Look for sieve tube members (large cells with porous end walls) and companion cells (small, densely cytoplasmic cells alongside sieve tubes).
3. Identify the endodermis (in roots). A single layer of cells with a distinctive Casparian strip, a band of suberin/lignin in the radial and transverse walls. With safranin staining, the Casparian strip appears as a bright red dot or bar when viewed in cross-section.
4. Identify the pericycle (in roots). The pericycle is a single layer just inside the endodermis.

The general anatomy of Monocot vs Dicot’s stem and root are as following:

Dicot vs monocot’s stem cross-sectioned anatomy

Dicot vs monocot’s root cross-sectioned anatomy

Leaf Cross-Section

Leaf sections reveal the most tissue diversity in a small area.

Layer	Key Features	Identification Tips
Upper epidermis	Single layer, no chloroplasts, covered with cuticle (appears as thin bright line)	Outermost, flattest cells; cuticle stains poorly (may appear as a clear rim)
Palisade mesophyll (= palisade chlorenchyma)	Elongated columnar cells, densely packed with chloroplasts; 1–3 layers	Tall narrow cells in neat columns just below upper epidermis
Spongy mesophyll (= spongy chlorenchyma)	Irregularly-shaped cells, large air spaces (intercellular spaces)	Looser, more rounded cells. Large gaps between them. Also contains chloroplasts.
Vascular bundle (midrib/vein)	Xylem (adaxial/upper) + phloem (abaxial/lower), often with bundle sheath	Xylem is closer to upper surface; look for thick red-stained walls vs. smaller pale phloem cells
Lower epidermis	Single layer; stomata + guard cells present	Guard cells are kidney-shaped and contain chloroplasts; look for the pore (stoma) between them

Leaf cross-section at 40x : upper epidermis, palisade mesophyll, spongy mesophyll, vascular bundle, lower epidermis with stomata

Structure Quick-Reference Guide

Use this as a rapid mental checklist during the practical.

Structure	Location	Key visual features	Staining (safranin / TBO)
Epidermis	Outermost layer of all organs	Single layer, flattened cells, no chloroplasts	Pale / light green with TBO
Chlorenchyma (palisade)	Just below upper epidermis of leaf	Tall columnar cells, densely packed, many chloroplasts	Green / blue-green with TBO pale with safranin
Chlorenchyma (spongy)	Between palisade and lower epidermis	Rounded irregular cells, large air spaces	Green / blue-green with TBO
Collenchyma	Sub-epidermal in stems, petioles, ribs	Unevenly thickened corners (non-lignified); often elongated	Pink/red corners with TBO pale with safranin
Sclerenchyma (fibres)	Bundle caps, hypodermis of monocots	Very thick, evenly lignified walls; tiny or absent lumen; dead at maturity	Bright red with safranin blue-green with TBO
Parenchyma (cortex/pith)	Cortex and central pith of stems/roots	Large, thin-walled cells, large vacuole, round/polygonal	Light blue/green with TBO pale with safranin
Endodermis	Innermost cortex layer — roots primarily	Single layer; Casparian strip visible as bright dot in radial walls	Casparian strip stains bright red with safranin
Pericycle	Just inside endodermis (roots); outermost vascular tissue (stems)	One or more layers of parenchyma/sclerenchyma; origin of lateral roots	Variable — depends on cell type present
Xylem	Vascular bundle (inner/adaxial)	Large angular lumen, spiral/annular/pitted thick walls; dead cells	Bright red with safranin (lignin)
Phloem	Vascular bundle (outer/abaxial)	Small thin-walled sieve tubes + tiny companion cells	Pale / blue-green with TBO

Expected Results and Interpretation

What a Correct Identification Looks Like

At the end of the practical, you will typically be asked to:

• Draw and label a field of view at a specified magnification.
• Name a specific structure pointed to by an arrow on a printed micrograph.
• Match descriptions or features to structure names.

Characteristic patterns to learn

• Endodermis in roots: Look for the single ring of cells with what appears to be a bright red dot or bar at the junction of adjacent cells when stained with safranin. This is the Casparian strip, and no other layer produces this appearance.
• Pericycle: Immediately inside the endodermis. Difficult to distinguish unless a lateral root is initiating there (you will see a bulge of dividing cells).
• Chlorenchyma: The defining feature is the presence of chloroplasts. In a living or lightly fixed section, they appear as green organelles. If the section has been processed for permanent slides (fixing kills pigments), look for the distinctive tall-columnar (palisade) or loose rounded (spongy) arrangement instead.
• Collenchyma vs. sclerenchyma: Both provide mechanical support but differ critically. Collenchyma has uneven (corner) thickening and cellulose walls (no lignin). Sclerenchyma has even, full-wall thickening and is lignified. Collenchyma is alive at maturity; sclerenchyma is dead.

Dicot vs monocot’s root cross-sectioned anatomy

Dicot vs. Monocot Cross-Section – Key Differences

Feature	Dicot stem	Monocot stem
Vascular bundle arrangement	Ring (one ring near periphery)	Scattered throughout cross-section
Bundle sheath	Present (often sclerenchyma caps)	Always present (thick sclerenchyma sheath)
Pith	Distinct central pith	No distinct pith; parenchyma ground tissue
Epidermis	Single layer	Single layer; may have thick cuticle

Monocot’s stem cross-sectioned anatomy

Dicot’s stem cross-sectioned anatomy

Troubleshooting Table

Problem	Solution
Image blurry at 40×	Only use fine focus at 40×. Check the coverslip is properly seated. Ensure the section is fully flat under coverslip.
Cannot find cells / dark image	Increase light; open iris diaphragm. Check the objective is clicked into place (you will hear a click).
Everything looks the same colour	Your section may be too thick. Prepare a thinner section. Alternatively, re-stain or use TBO instead.
Air bubbles in mount	Lift coverslip, add more water to specimen, lower coverslip at 45° angle very slowly.
Cannot identify endodermis	Use safranin and look for the bright red Casparian strip. Reduce diaphragm aperture to increase contrast.