Anatomy of Dicotyledonous Plants
Structure🌿 The Plant's Internal Architecture
A dicotyledonous plant has a highly organised internal structure. Different tissues are arranged in predictable patterns in the root, stem and leaf — each perfectly positioned for its role in support, transport, and photosynthesis.
📌 What is a Dicot?
A dicotyledonous plant (dicot) produces seeds with two seed leaves (cotyledons). Examples include beans, roses, oak trees, sunflowers, and most flowering shrubs. Dicots have a distinct ring arrangement of vascular bundles in their stems — unlike monocots (e.g. grasses, maize) which have scattered bundles.
Dicot Stem — Cross Section (simplified)
| Tissue | Location | Cells alive? | Main Function |
|---|---|---|---|
| Epidermis | Outermost layer of stem, root, leaf | Yes | Protection; reduces water loss (waxy cuticle on leaves) |
| Xylem | Inner part of vascular bundle | No (dead) | Transport water and mineral salts upward from roots |
| Phloem | Outer part of vascular bundle | Yes (alive) | Transport manufactured food (sucrose) up and down |
| Parenchyma | Cortex and pith; leaf mesophyll | Yes | Storage; photosynthesis in leaves; packing tissue |
| Collenchyma | Beneath epidermis in stems | Yes | Flexible support in young, actively growing regions |
| Sclerenchyma | Stems and leaf veins | No (dead) | Rigid mechanical support; strengthening |
🌳 Secondary Growth & Annual Rings
Secondary growth produces annual rings in woody stems — one ring per year of growth. Count the rings to determine the age of the tree.
- Wide ring = favourable growing season (good rain, warmth)
- Narrow ring = unfavourable season (drought, cold)
- Used to study past climate patterns
💡 Economic Uses of Plant Anatomy
- Xylem fibres → wood for furniture, building, paper
- Sclerenchyma fibres → linen, rope, baskets, fabric weaving
- Secondary growth → timber industry
- Phloem fibres → some natural textiles
⚠️ Common Exam Mistake
In a dicot stem, xylem is on the INSIDE (facing the pith/centre) and phloem is on the OUTSIDE (facing the epidermis) within each vascular bundle. Students frequently swap these. Memory tip: X is at the centre — Xylem is inward.
Water & Mineral Salt Transport — Xylem
Xylem💧 Nature's Pipeline System
Water absorbed by root hairs travels all the way up to leaf cells through xylem vessels — a continuous hollow pipeline. Minerals hitch a ride dissolved in the water. The journey is driven by transpiration pull from above and root pressure from below.
Water Uptake — Root to Leaf Pathway
🔬 Xylem Vessel Structure — Why it Works
- Dead cells — no cytoplasm to block water flow
- No end walls — cells join end-to-end forming a continuous hollow tube
- Lignified walls — waterproofed with lignin; prevents collapse under tension
- Pits in walls — allow water to move sideways between adjacent vessels
| Substance | How it enters root cells | Why this process? |
|---|---|---|
| Water | Osmosis | Moves from high water potential (dilute soil solution) to low water potential (more concentrated cell contents) — passive, no energy needed |
| Mineral salts | Active transport | Mineral ions are in LOW concentration in soil and HIGH concentration inside root cells — must move AGAINST gradient, requires ATP energy |
The TACT Mechanism
Why water moves upT
Transpiration Pull
Water lost at leaves creates tension that pulls the water column upward through xylem — the main driving force
A
Adhesion
Water molecules stick to the inner walls of xylem vessels — helps maintain the water column along the tube
C
Cohesion
Water molecules hydrogen-bond to each other, forming an unbroken chain that can be pulled upward without breaking
T
Tension
Negative pressure (tension) develops in xylem as transpiration removes water from the top — this tension pulls water up
⚠️ Common Mistake — Mineral Salts
Mineral salts are NOT absorbed by osmosis. Osmosis only applies to water. Minerals enter root cells by active transport — against their concentration gradient — which requires energy (ATP). Students who write "osmosis" for mineral absorption will lose marks.
Transpiration
Water Loss🌬️ The Unavoidable Trade-off
Stomata must open to let in CO₂ for photosynthesis — but open stomata also let water vapour escape. This water loss is transpiration. Plants have clever structural adaptations to reduce excessive water loss while still allowing gas exchange.
📌 Definition — Must Know
Transpiration: the loss of water vapour from a plant, mainly through the stomata on the surfaces of leaves. (Note: water VAPOUR — not liquid water. This is a common exam error.)
🚪 How Stomata Control Water Loss
- Guard cells surround each stoma
- When turgid (full of water) → stomata open
- When flaccid (water lost) → stomata close
- Most stomata on the lower leaf surface (shaded from direct sun)
- Stomata open in light to allow CO₂ in for photosynthesis
- Close in drought to prevent excessive water loss
🌿 Wilting Explained
- Occurs when water loss > water uptake from soil
- Cells lose turgor pressure → become flaccid
- Leaves and stems droop
- Temporary wilting: water available in soil but plant temporarily can't absorb fast enough (hot midday)
- Permanent wilting: soil water depleted — plant may die
- Wilting causes stomata to close → reduces further water loss
Factors Affecting Transpiration Rate
EnvironmentalTemperature
Higher temperature gives water molecules more kinetic energy — evaporation from mesophyll cells increases, speeding up transpiration.
↑ Rate increases
Light Intensity
Light causes guard cells to become turgid, opening stomata wider. More open stomata = faster diffusion of water vapour out.
↑ Rate increases
Wind Speed
Wind removes water vapour from around the leaf surface, maintaining a steep concentration gradient — so water diffuses out faster.
↑ Rate increases
Humidity
High humidity means the air is already saturated with water vapour — the concentration gradient between leaf interior and air is reduced, slowing diffusion out.
↓ Rate decreases
| Factor | Change | Effect on rate | Reason |
|---|---|---|---|
| Temperature | ↑ Increases | Faster transpiration | More kinetic energy → faster evaporation from mesophyll |
| Light intensity | ↑ Increases | Faster transpiration | Stomata open wider → more water vapour escapes |
| Wind speed | ↑ Increases | Faster transpiration | Removes water vapour → steeper concentration gradient |
| Humidity | ↑ Increases | Slower transpiration | Reduces concentration gradient between leaf and air |
Adaptations to Reduce Transpiration
Xerophytes🕯️ Thick Waxy Cuticle
Waterproof wax layer on leaf surface prevents water evaporating directly through the epidermis. Most water loss occurs through stomata only.
🌵 Sunken Stomata
Stomata sit in pits below the leaf surface. Humid air trapped in the pit reduces the concentration gradient — less water vapour escapes.
🍃 Stomata on Lower Surface
Underside of leaf is shaded — cooler and less exposed to wind than upper surface, reducing evaporation from stomata.
🌾 Rolled Leaves
Some plants (e.g. marram grass) roll their leaves inward, trapping humid air around stomata to reduce the concentration gradient.
🌿 Hairy Leaf Surface
Tiny hairs (trichomes) trap a layer of moist air next to stomata, reducing the diffusion gradient and slowing water loss.
🔍 Fewer Stomata
Reducing the number of stomata reduces the total area through which water can escape. Fewer openings = less overall water loss.
Translocation — Phloem
Food Transport🍃 Moving the Factory's Products
Once leaves manufacture glucose during photosynthesis, that food needs to reach every other part of the plant — growing tips, roots, fruits, and storage organs. Phloem is the transport system that makes this possible, moving sucrose (and amino acids) in both directions.
📌 Definition — Translocation
Translocation: the transport of manufactured organic substances (mainly sucrose and amino acids) from areas of production or storage (sources) to areas of use or storage (sinks) through phloem tissue.
🔬 Phloem Cell Structure
- Sieve tube elements — living cells stacked end-to-end; perforated end walls called sieve plates
- No nucleus at maturity — controlled by companion cells
- Companion cells — have nucleus + mitochondria; supply energy (ATP) for active loading of sucrose
- Thin walls — no lignin
- Cytoplasm present (unlike xylem)
📍 Source vs. Sink
- Source = where sucrose is produced or released into phloem (e.g. leaves during photosynthesis, storage organs releasing starch)
- Sink = where sucrose is removed from phloem and used (e.g. roots, fruits, seeds, growing shoot tips)
- Sucrose is actively loaded at the source → high pressure
- Sucrose is unloaded at the sink → low pressure
- Pressure difference drives flow from source to sink
⚠️ Direction of Flow
A key difference: xylem transports water upward only. Phloem can transport sucrose up AND down the plant — from whatever is acting as source toward whatever is acting as sink. For example, sucrose moves downward from leaves to roots, but also upward from leaves to developing fruits above.
| Source Examples | Sink Examples |
|---|---|
| Mature leaves (photosynthesising) | Roots (growing and respiring) |
| Storage organs releasing starch | Developing fruits and seeds |
| Germinating seeds breaking down starch | Growing shoot tips and buds |
Xylem vs. Phloem
Comparison⚖️ Side by Side
The IEB frequently tests your ability to compare xylem and phloem. Knowing the key differences in structure, contents, direction, and energy requirements will earn you easy marks.
| Feature | Xylem | Phloem |
|---|---|---|
| Cells alive? | ❌ Dead at maturity | ✅ Alive (sieve tube elements) |
| What it transports | Water and mineral salts | Sucrose (sugar) and amino acids |
| Direction of flow | One way — upward only (roots → leaves) | Both directions — source to sink |
| Cell wall | Thick, lignified (waterproof) | Thin, no lignin |
| Energy required? | No — driven by transpiration pull (passive) | Yes — active loading requires ATP |
| End walls | Absent — forms hollow continuous tube | Sieve plates (perforated end walls) |
| Nucleus present? | No (cells are dead) | No in sieve tubes; yes in companion cells |
| Position in bundle | Inner — facing centre (pith) | Outer — facing epidermis |
| Companion cells? | No | Yes — provide energy and control |
📌 Memory Tricks
Xylem is at the centre — like an X marks the middleXylem = dead, one direction, water
Phloem = alive, both directions, food
X comes before P in alphabet — Xylem inside, Phloem outside
⚠️ Frequent Exam Errors
❌ "Phloem transports water" — No, that is xylem❌ "Xylem is alive" — No, xylem is dead at maturity
❌ "Phloem moves food upward only" — No, it moves in both directions
❌ "Phloem has lignin" — No, only xylem has lignified walls
Exam Tips & Memo Answers
IEB Style📝 How to Answer Like a Top Student
IEB questions on this topic tend to test your ability to explain processes, compare structures, and apply your knowledge to unfamiliar contexts (like graphs or data). Here are the most common question types with model memo answers.
❓ Explain why xylem vessels are well adapted for transporting water. (3 marks)
✅ Memo Answer
✓Xylem cells are dead / have no cytoplasm, allowing water to flow freely without obstruction.
✓There are no end walls between cells, so they form a continuous hollow tube for uninterrupted flow.
✓Cell walls are thickened with lignin, preventing the vessels from collapsing under the tension created by transpiration pull.
❓ Predict and explain the effect of increased wind speed on transpiration rate. (2 marks)
✅ Memo Answer
✓Transpiration rate will increase.
✓Wind removes water vapour from around the leaf surface, maintaining a steep concentration gradient between the moist air inside the leaf and the drier air outside — so water vapour diffuses out faster.
❓ Explain the difference between xylem and phloem transport in terms of direction and energy. (4 marks)
✅ Memo Answer
✓Xylem transports water and mineral salts in one direction only — upward from roots to leaves.
✓This is a passive process driven by transpiration pull — no energy (ATP) is required.
✓Phloem transports sucrose and amino acids in both directions — from source to sink.
✓Phloem transport requires energy (ATP) for the active loading and unloading of sucrose at source and sink.
⚠️ Language Precision
- Always say water VAPOUR leaves through stomata — not water
- Minerals enter by ACTIVE TRANSPORT — never osmosis
- Transpiration is water loss from the PLANT — not just from leaves
- Phloem transports SUCROSE — not just "sugar" or "glucose"
- Wilting is caused by loss of TURGOR PRESSURE — not dehydration alone
📌 Quick Recall Checklist
- ✅ Xylem = dead, water, upward, no energy
- ✅ Phloem = alive, sucrose, both ways, ATP needed
- ✅ Water enters root by osmosis
- ✅ Minerals enter by active transport
- ✅ Humidity DECREASES transpiration (reduce gradient)
- ✅ Wind INCREASES transpiration (steepen gradient)
- ✅ Annual rings = age of tree (1 ring = 1 year)
Test Yourself
Quiz🎯 Support & Transport Quiz
IEB and CAPS style questions. Select your answer — if you choose incorrectly, the correct answer will be highlighted so you can learn from it immediately.
Q1
Which process is responsible for the uptake of mineral salts into root hair cells?
Q2
A student examines a cross-section of a dicot stem under a microscope. Within a vascular bundle, where should she expect to find the xylem?
Q3
On a hot, dry, windy afternoon a plant shows signs of wilting. Which explanation BEST accounts for this?
Q4
Which statement correctly describes phloem transport?
Q5
A scientist places a potometer (a device that measures water uptake) on a plant and increases the humidity of the surrounding air. What result would she observe, and why?
Q6 — Application
A cross-section of a tree trunk shows 42 annual rings. The rings near the outside of the trunk are noticeably wider than the rings near the centre. What can be concluded from these observations?