Xylem and phloem Revision Notes

Xylem is a vascular tissue responsible for moving water and dissolved mineral ions absorbed by the roots upwards through the plant to the leaves and other parts. This movement occurs mainly by transpiration pull, cohesion, and adhesion of water molecules within the xylem vessels.

Water enters root hair cells by osmosis and passes through the root cortex to the xylem vessels. From there, it moves upward through hollow, dead xylem vessels that form continuous tubes. Mineral ions such as nitrate (NO3-) and potassium (K+) are actively absorbed by root cells and transported into the xylem along with water.

For example, after a rain, mineral ions dissolved in soil water enter the roots and are transported via xylem to leaves where they are used in processes like photosynthesis and protein synthesis.

• Xylem vessels are hollow and dead cells forming tubes for water transport.
• Water moves up by transpiration pull and cohesion between water molecules.
• Mineral ions are absorbed actively by roots and transported with water.

Besides transport, xylem provides mechanical strength to plants. The walls of xylem vessels are thickened with lignin, a complex polymer that waterproofs and strengthens the cell walls.

This lignification prevents the vessels from collapsing under the tension created by transpiration pull and supports the plant to stand upright against gravity and wind.

For example, in tall trees, the extensive xylem tissue forms wood, which supports the heavy branches and leaves.

• Lignin thickens xylem walls, providing rigidity and waterproofing.
• Xylem supports the plant structurally, preventing collapse under tension.
• Wood is mainly composed of xylem tissue in trees.

Phloem is another vascular tissue responsible for transporting organic nutrients, mainly sucrose and amino acids, from the leaves where they are produced by photosynthesis to other parts of the plant.

This process is called translocation and occurs in living sieve tube elements of the phloem, assisted by companion cells that load and unload sugars.

Sucrose is transported to growing regions such as roots, shoots, and fruits where it is used for energy or converted into starch for storage. Amino acids transported by phloem are used for protein synthesis in cells throughout the plant.

• Phloem transports sucrose and amino acids from leaves to other parts.
• Transport in phloem is called translocation and occurs in living cells.
• Companion cells help load sugars into sieve tubes for transport.

Xylem transport is unidirectional, moving water and minerals only upwards from roots to leaves. It relies on physical forces like transpiration pull and cohesion.

Phloem transport is bidirectional, moving sucrose and amino acids from sources (usually leaves) to sinks (growing or storage tissues) depending on the plant’s needs.

For example, during spring, sugars may move from storage organs like roots to growing shoots, reversing the usual direction.

• Xylem transport is unidirectional (roots to leaves).
• Phloem transport is bidirectional (sources to sinks).
• Xylem vessels are dead; phloem sieve tubes are living cells.

Xylem and phloem work together to maintain plant health and growth. Xylem supplies water and minerals essential for photosynthesis and cell function, while also providing mechanical support.

Phloem distributes the products of photosynthesis to all parts of the plant, supporting growth, storage, and metabolism.

Understanding these roles helps explain how plants survive, grow tall, and reproduce effectively.

• Xylem: water/mineral transport + structural support.
• Phloem: transport of sugars and amino acids for nourishment.
• Both tissues are vital for plant survival and development.

In non-woody dicotyledon plants, vascular tissues are specialised for transporting substances essential for plant survival. The two main vascular tissues are xylem and phloem. Xylem transports water and mineral ions from the roots upwards, while phloem distributes sugars and amino acids produced by photosynthesis to various parts of the plant. Recognising these tissues in cross-sectional images is fundamental for understanding plant structure and function.

• Xylem and phloem form vascular bundles in roots, stems, and leaves.
• They have distinct appearances and positions in cross-sections.
• Correct identification aids comprehension of plant transport systems.

Xylem vessels appear as relatively large, thick-walled, hollow tubes in cross-sections. Their walls are lignified, providing structural support. In roots, xylem often forms a star-shaped pattern at the centre. In stems, xylem is usually located towards the inside of the vascular bundle. In leaves, xylem is found on the upper side of the vascular bundle within the vein.

Because xylem vessels are hollow and large, they appear as clear or lightly stained circular or polygonal shapes in microscope images or diagrams.

• Xylem vessels are large and hollow with thick walls.
• Located centrally in roots, inner side in stems, upper side in leaf veins.
• Lignified walls provide mechanical support.

Phloem tissue consists of smaller cells compared to xylem and is located adjacent to the xylem but towards the outer side of the vascular bundle. Phloem cells have thinner walls and are more compact. In roots, phloem is found between the arms of the xylem star. In stems, phloem lies outside the xylem within the vascular bundle. In leaves, phloem is positioned on the lower side of the vascular bundle.

Phloem cells transport sugars and amino acids, but their smaller size and position help distinguish them from xylem in cross-sections.

• Phloem cells are smaller and more compact than xylem vessels.
• Located outside xylem in stems and roots, lower side in leaf veins.
• Thinner walls and less lignified than xylem.

The arrangement of xylem and phloem varies between roots, stems, and leaves but follows consistent patterns in dicotyledon plants. In roots, xylem forms a central star shape with phloem located between the arms. In stems, vascular bundles are arranged in a ring with xylem on the inner side and phloem on the outer side. In leaves, vascular bundles form veins with xylem on the upper side and phloem on the lower side, reflecting their roles in transport and support.

Recognising these patterns helps locate xylem and phloem accurately in different plant parts.

• Roots: xylem star-shaped centre; phloem between arms.
• Stems: vascular bundles in ring; xylem inside, phloem outside.
• Leaves: veins with xylem upper side, phloem lower side.

When examining cross-sectional diagrams or microscope images, look for the size, shape, and position of vascular tissues. Xylem vessels are larger, hollow, and often stained differently due to lignin. Phloem cells are smaller, more densely packed, and adjacent to xylem but towards the outside or lower side depending on the organ. Labelled diagrams can help confirm identification. Practising with various images of roots, stems, and leaves improves accuracy.

Using these visual clues ensures correct recognition and supports understanding of plant transport systems.

• Identify large, hollow xylem vessels first.
• Locate smaller phloem cells adjacent to xylem.
• Use position relative to organ (root, stem, leaf) to confirm.
• Practice with labelled diagrams for confidence.