Xylem and phloem Revision Notes

Xylem transports water and dissolved mineral ions absorbed from the soil by the roots upwards to the leaves and other parts of the plant. Besides transport, xylem vessels have thick, lignified walls that provide mechanical strength and structural support, helping the plant remain upright.

• Transports water and mineral ions from roots to leaves.
• Provides structural support through thick lignified walls.
• Supports plant rigidity and resistance to bending.

Phloem transports organic nutrients, mainly sucrose and amino acids, produced in the leaves during photosynthesis to other parts of the plant where they are used or stored. Unlike xylem, phloem can transport substances in multiple directions depending on the plant's needs.

• Transports sucrose and amino acids throughout the plant.
• Moves nutrients from leaves to growing and storage tissues.
• Can transport substances both upwards and downwards.

Xylem vessels are typically unidirectional, moving water and minerals upwards. Phloem tubes are bidirectional, distributing organic nutrients as required. The structural differences between xylem and phloem support their specific transport roles and additional functions like support in xylem.

• Xylem transport is mainly upwards.
• Phloem transport can be in multiple directions.
• Structural adaptations support their transport functions.

In roots of non-woody dicot plants, xylem forms a central star-shaped core. Phloem is located between the arms of this star. This arrangement supports efficient transport of water, minerals, and food substances within the root.

• Xylem forms a star shape in the center.
• Phloem lies between the arms of the xylem star.
• Arrangement visible in root cross-sections.

In stems, xylem and phloem are arranged in vascular bundles. Within each bundle, xylem is positioned towards the inside (center of stem), and phloem is towards the outside. These bundles are arranged in a ring around the stem's center.

• Vascular bundles contain both xylem and phloem.
• Xylem is on the inner side of each bundle.
• Phloem is on the outer side of each bundle.
• Bundles arranged in a ring in the stem.

In leaves, vascular bundles form the veins. Xylem is generally located on the upper side of the vein, while phloem is on the lower side. This arrangement supports transport of water to the leaf cells and distribution of sugars away from the leaf.

• Vascular bundles form leaf veins.
• Xylem is on the upper side of veins.
• Phloem is on the lower side of veins.
• Visible in leaf cross-sections.

Xylem vessels have thick walls strengthened by lignin, a substance that provides rigidity and waterproofing. These walls prevent the vessels from collapsing under the tension created during water transport, ensuring the tubes remain open for continuous flow.

• Provide mechanical strength to withstand pressure.
• Prevent vessel collapse during water transport.
• Make walls waterproof to contain water flow.

The cells forming xylem vessels lose their internal contents, becoming hollow tubes. This absence of cell contents removes any obstruction inside the vessels, allowing water to flow freely and efficiently from roots to leaves.

• Create hollow tubes for unobstructed water flow.
• Remove barriers inside vessels.
• Enhance efficiency of water transport.

Xylem vessel cells are joined end to end without cross walls (perforations), forming a continuous tube. This uninterrupted pathway allows water to move smoothly and rapidly through the plant.

• Form continuous tubes for water transport.
• Eliminate interruptions in water flow.
• Support efficient upward movement of water.