IGCSE Biology 0610 - 8.1 Transport in Plants

Objective: Identify xylem and phloem in sections of roots, stems and leaves, using a light microscope.

Introduction

Plants have evolved specialized tissues for the transport of water and dissolved minerals (xylem) and sugars (phloem). These tissues are essential for plant growth, development, and survival. This section focuses on identifying xylem and phloem within plant tissues using a light microscope.

Xylem

Xylem is responsible for the upward transport of water and minerals from the roots to the rest of the plant. It consists of dead cells with thick, lignified walls, providing structural support.

• **Cell Type:** Primarily tracheids and vessel elements.
• **Cell Wall:** Thick and lignified (provides rigidity).
• **Living/Dead:** Dead at maturity.
• **Appearance under Microscope:** Appear as hollow, often wider cells with prominent, thick walls. The walls are usually darker in staining.
• **Location:** Found in all parts of the plant – roots, stems, leaves. In roots, it forms a central core. In stems and leaves, it forms vascular bundles.

Phloem

Phloem is responsible for the transport of sugars (produced during photosynthesis) from the leaves to other parts of the plant. It consists of living cells.

• **Cell Type:** Sieve tube elements and companion cells.
• **Cell Wall:** Thinner than xylem, cellulose-based.
• **Living/Dead:** Living at maturity (sieve tube elements). Companion cells are also living.
• **Appearance under Microscope:** Appear as living cells with less dense walls. Sieve tube elements have large sieve plates at their ends.
• **Location:** Found in vascular bundles within stems and leaves.

Microscopic Identification Guide

Feature	Xylem	Phloem
Cell Wall Thickness	Thick and lignified	Thin and cellulose-based
Cell Contents	Empty (dead cells)	Living cytoplasm
Cell Shape	Often wider and more irregular	More uniform and rounded
Presence of Sieve Plates	Absent	Present in sieve tube elements
Staining	Darker staining	Lighter staining

Practical Tips for Identification

1. Start with a low magnification to locate vascular bundles.
2. Increase magnification to examine individual cells within the bundles.
3. Look for the characteristic thick, lignified walls of xylem cells.
4. Look for the living cells with sieve plates in phloem.
5. Compare the features with known diagrams and images.

Examples of Plant Sections

The following are examples of plant sections where xylem and phloem can be observed:

• Root Section: Xylem forms a central core, surrounded by phloem in the vascular cylinder.
• Stem Section: Xylem and phloem are arranged in vascular bundles, often in a ring-like pattern.
• Leaf Section: Xylem and phloem are present within the veins of the leaf.