Identify xylem and phloem in sections of roots, stems and leaves, using a light microscope.
8.1 Transport in Plants
Xylem – The Water Highway
Think of xylem as a network of pipes that carry water and dissolved minerals from the roots all the way up to the leaves. The main components are:
- Tracheids – long, narrow cells that act like tiny water‑tubes.
- Vessel elements – short, wide cells that connect end‑to‑end to form continuous channels.
- Fibers – provide support and help keep the plant upright.
Transport is driven by transpiration pull and root pressure. The equation for transpiration rate is often written as:
$E = \frac{D}{R}$
where $E$ is the transpiration rate, $D$ is the difference in water potential between leaf and atmosphere, and $R$ is the resistance to water flow.
Phloem – The Sugar Delivery Service
Phloem transports sugars (mainly sucrose) and other organic nutrients from the photosynthetic leaves to non‑photosynthetic parts like roots and fruits. It consists of:
- Sieve‑tube elements – cells with perforated end walls that allow flow.
- Companion cells – work with sieve‑tube elements to load and unload sugars.
- Phloem fibers – provide structural support.
The movement follows the pressure‑flow hypothesis: sugars are actively loaded into the sieve tubes at the source, creating a high osmotic pressure that draws water in. This generates a pressure gradient that pushes the sap toward the sink.
$P_{source} > P_{sink}$
Microscope Identification
When you look at thin sections of roots, stems, or leaves under a light microscope, you can spot the distinctive patterns of xylem and phloem:
| Feature | Xylem | Phloem |
|---|---|---|
| Cell type | Tracheids / Vessel elements (elongated, thick walls) | Sieve‑tube elements (short, perforated) + Companion cells |
| Arrangement | Usually in a radial pattern, often with a central pith in stems | Often adjacent to xylem, forming a ring or a strand |
| Colour (after staining) | Stains dark (e.g., with safranin) | Stains lighter or remains unstained |
Remember: “Xylem = water pipes, Phloem = sugar delivery trucks” – this analogy helps you recall the main functions and structures.
Exam Tips
- Use the “Xylem vs Phloem” table as a quick reference when answering identification questions.
- When asked to explain transport mechanisms, mention the pressure‑flow hypothesis for phloem and the transpiration pull for xylem.
- Draw clear, labelled diagrams. Use arrows to show direction of flow and label “source” and “sink” for phloem.
- For microscope questions, describe the key features you would look for (cell shape, wall thickness, staining).
- Practice writing concise, bullet‑point answers – examiners appreciate clarity.
Good luck, and remember: the plant’s transport system is like a well‑coordinated city – every part has a role!
Revision
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