# Transverse Sections: Stems, Roots & Leaves ## Dicotyledonous Plants - Dicotyledonous (dicot) plants possess specific structural features - Seeds contain **two cotyledons** (seed leaves) - Network of veins present throughout - Leaves typically have broad blades (leaf surface) and petioles (stalks) - Tap root with lateral branches - Herbaceous dicots have specific characteristics: - Relatively short life cycle (one growing season) - Non-woody tissue

Dicotyledonous plants are flowering plants with seeds containing two cotyledons, characterized by a network of veins and herbaceous or woody tissue.

## Transport Systems Overview - Plants require transport systems to meet their **metabolic demands** - Glucose, hormones, and mineral ions needed for various processes - Efficient movement of substances up and down the plant - Compensation for relatively small SA:V ratio - Plants generally cannot rely on diffusion alone for transport - A vascular system provides a network of vessels (vascular tissue) running through: - Leaves - Stem - Roots - These three parts are the main **organs involved in transport** ### Vascular System Components - The vascular system comprises two distinct types: - **Xylem** - transports water and mineral ions from roots to rest of plant - **Phloem** - transports substances from source (e.g. leaf) to sink (e.g. root) - Xylem and phloem are arranged together in **vascular bundles** - The bundles are laid out differently in leaves, stem, and roots ## Distribution in Plant Organs ### Vascular Bundle Arrangement in Roots - Vascular bundle found in the **centre** of the root - **Xylem tissue** forms the centre core - Helps roots withstand pulling strains - Supports the plant as it transports water upwards and grows - **Phloem tissue** located on the edges of the centre core ### Vascular Bundle Arrangement in Stems - Vascular bundles located **around the outside** of the stem - **Xylem tissue** found on the inside - Closest to the centre of the stem - Helps support the plant - **Phloem tissue** found on the outside - Closest to the epidermis ### Vascular Bundle Arrangement in Leaves - Vascular bundles form the **midrib and veins** - Spread from the centre of the leaf in a parallel line - **Xylem tissue** found on the upper side of the bundles - Closest to the upper epidermis - **Phloem tissue** found on the lower side of the bundles - Closest to the lower epidermis

In roots and stems, xylem is on the inside; in leaves, xylem is above phloem tissue.

## Tissue Plan Features | Organ | Vascular Bundle Position | Xylem Location | Phloem Location | |-------|-------------------------|----------------|-----------------| | Root | Central | Centre core | Edges of centre core | | Stem | Around the outside | Inside (towards centre) | Outside (towards epidermis) | | Leaf | Midrib and veins | Upper side (towards upper epidermis) | Lower side (towards lower epidermis) | ## Identifying Structures in Transverse Sections ### Root Cross-Section Features - Circular vascular bundle in centre - Xylem arranged in a distinctive cross or star shape in the middle - Phloem tissue between the xylem arms - Outer layer: epidermis - Cortex between epidermis and vascular bundle - Endodermis surrounds the vascular bundle ### Stem Cross-Section Features - Multiple vascular bundles arranged in a ring around the periphery - Each bundle contains xylem (inner) and phloem (outer) - Large central area: pith (parenchyma cells) - Cortex between epidermis and vascular bundles - Epidermis forms outer protective layer ### Leaf Cross-Section Features - Vascular bundles scattered through the mesophyll - Upper epidermis with palisade mesophyll beneath - Spongy mesophyll contains air spaces - Lower epidermis with stomata - Xylem positioned towards upper surface - Phloem positioned towards lower surface

Different plant organs show different arrangements of vascular tissue based on their functional requirements and the stresses they experience.

## Practical Skills: Using Microscopes for Measurement ### Eyepiece Graticule Calibration - An eyepiece graticule is a scale placed in the eyepiece of a microscope - It needs to be **calibrated** for each objective lens using a stage micrometer - Stage micrometer: a microscope slide with a scale of known measurement - Commonly has divisions of 0.1 mm or 0.01 mm #### Calibration Process 1. Place the stage micrometer on the microscope stage 2. Focus and align the eyepiece graticule scale with the stage micrometer scale 3. Count how many eyepiece graticule divisions fit into a known number of stage micrometer divisions 4. Calculate the actual size of one eyepiece graticule division **Example calculation:** - If 40 eyepiece graticule divisions = 4 stage micrometer divisions - And 1 stage micrometer division = 0.1 mm - Then 40 eyepiece divisions = 0.4 mm - So 1 eyepiece division = 0.4 ÷ 40 = 0.01 mm = 10 µm ### Measuring Structures - Once calibrated, use the eyepiece graticule to measure structures - Count the number of eyepiece divisions the structure spans - Multiply by the calibrated value of one division - Express measurements in appropriate units (typically µm for cells/tissues)

Always recalibrate the eyepiece graticule when changing objective lenses, as magnification affects the measurement scale.

## Drawing Tissue Plan Diagrams ### Low-Power Plan Diagrams **Purpose:** - Show the distribution of tissues within an organ - Do not show individual cells **Key requirements:** - Read instructions carefully - Draw a large diagram using a sharp pencil - Use clear, continuous lines (no shading) - Do not draw individual cells - Keep proportions between tissues to scale - May only need to draw a portion of the image - Include magnification on the drawing **When using an eyepiece graticule:** - Use it to ensure correct proportions between different tissue areas - Maintain accurate relative sizes of different structures ### High-Power Detail Drawings **Purpose:** - Show cellular detail of specific tissues **Key requirements:** - Draw only a few of the required cells - Draw cell walls of plant cells - Use a sharp pencil with no shading - Use clear, continuous lines - Include magnification on the drawing ## Labelling Diagrams ### Labelling Rules - Use a ruler for label lines (and scale lines if appropriate) - Label lines should stop exactly at the structure - **Do not use arrows** on label lines - Don't cross label lines over each other - Label all tissues and relevant structures requested ### Tissues to Identify Common tissues in plant transverse sections: - Epidermis (upper and/or lower) - Palisade mesophyll (in leaves) - Spongy mesophyll (in leaves) - Vascular bundles - Xylem vessels - Phloem tissue - Cortex - Endodermis (in roots) - Pith (in stems) - Stomata and guard cells (in leaves)

When examining unfamiliar sections, look for characteristic features: lignified xylem vessels appear thick-walled with large lumens, while phloem contains smaller cells with companion cells.

## Worked Example: Measuring Plant Structures **Question:** A stage micrometer has divisions of 0.1 mm. When aligned with an eyepiece graticule, 40 eyepiece divisions equal 4 stage micrometer divisions. A plant stem measures 80 eyepiece divisions across. Calculate its actual width. **Step 1: Calculate one eyepiece graticule unit** - 4 stage micrometer divisions = 4 × 0.1 mm = 0.4 mm - 40 eyepiece divisions = 0.4 mm - 1 eyepiece division = 0.4 ÷ 40 = 0.01 mm **Step 2: Convert to suitable units** - 0.01 mm × 1000 = 10 µm per eyepiece division **Step 3: Calculate actual width** - 80 eyepiece divisions × 10 µm = 800 µm = 0.8 mm **Answer: 800 µm (or 0.8 mm)** ## Distinguishing Between Xylem and Phloem ### Xylem Identification Features - Larger diameter vessels - Thicker cell walls (lignified) - Appear darker/stained when lignin stains are used - No cell contents visible (dead cells) - Often arranged in groups - Distinctive patterns in cross-section ### Phloem Identification Features - Smaller diameter cells - Thinner cell walls (cellulose only) - Companion cells present with dense cytoplasm - Living cells with visible contents - Located adjacent to xylem tissue - Sieve plates visible in longitudinal sections | Feature | Xylem | Phloem | |---------|-------|--------| | Cell wall thickness | Thick (lignified) | Thin (cellulose) | | Cell diameter | Larger | Smaller | | Cell contents | None (dead cells) | Present (living cells) | | Appearance when stained | Often red/pink (lignin stain) | Lighter/unstained | | Associated cells | None | Companion cells nearby |

The position of xylem and phloem varies with organ type; always consider whether you're viewing a root, stem, or leaf section.

## Summary of Key Differences ### Positional Relationships - **In roots and stems:** Xylem is positioned towards the inside (centre) - **In leaves:** Xylem is positioned above phloem (towards upper surface) - **General rule:** Phloem is positioned towards the outer/lower regions in most organs ### Functional Considerations The arrangement of vascular tissues relates to function: - **Roots:** Central xylem provides structural support against pulling forces - **Stems:** Ring of vascular bundles around periphery provides flexible support - **Leaves:** Scattered vascular bundles supply all areas of the leaf blade efficiently