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 require a transport system to move water and dissolved minerals from the roots to all parts of the plant (xylem) and sugars produced during photosynthesis from the leaves to other parts of the plant (phloem). This topic focuses on identifying the vascular tissues, xylem and phloem, within plant tissues using a light microscope.

Xylem

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

Phloem

Phloem is responsible for the transport of sugars (produced during photosynthesis) from the leaves to other parts of the plant where they are used for energy or growth. Phloem consists of living cells, which require energy for transport.

Microscopic Identification

When examining plant sections under a light microscope, xylem and phloem can be distinguished by their cellular structure.

Xylem Identification

• Cell Walls: Xylem cells have thick, lignified cell walls, which appear dark and rigid under the microscope.
• Cell Shape: Xylem cells are typically long and hollow, forming continuous tubes.
• Cell Contents: Xylem cells are dead at maturity and contain no cytoplasm or nucleus. This results in a clear appearance within the cell walls.
• Arrangement: Xylem vessels are often arranged in bundles or tracheids.

Phloem Identification

• Cell Walls: Phloem cells have thinner cell walls compared to xylem.
• Cell Shape: Phloem cells are living and have more variable shapes, often with rounded or rectangular ends.
• Cell Contents: Phloem cells contain cytoplasm and a nucleus.
• Companion Cells: Phloem often has companion cells associated with it, which are also living and support the phloem cells.

Examining Plant Sections

Root Section

In a root section, xylem forms a central core, with phloem located in the vascular bundles towards the periphery.

Stem Section

In a stem section, xylem and phloem are arranged in vascular bundles, which vary in number depending on the plant species. The arrangement can be ring-like or scattered.

Leaf Section

In a leaf section, xylem and phloem are found within the veins. Xylem is typically located towards the center of the leaf, while phloem is located towards the outer edges.

Suggested Diagram:

Conclusion

By carefully observing plant sections under a light microscope, students can identify xylem and phloem based on their distinct cellular structures. This understanding is crucial for comprehending the plant's transport system and its role in overall plant function.