Describe the mechanism of transportation of materials in flowering plants (vascular system, absorption and movement of water and mineral salts, and transpiration)

The vascular system is a network of tissues that transports materials throughout the plant. It consists of two main types of conducting tissues: xylem and phloem. These tissues are arranged in bundles called vascular bundles.

Xylem Tissue

Xylem transports water and dissolved mineral salts from the roots to the stems and leaves. It also provides mechanical support to the plant.

Xylem is composed of four types of cells:

• Xylem vessel elements: Long, cylindrical, tube-like structures joined end-to-end to form hollow tubes. They are dead at maturity (no cytoplasm or nucleus), which allows them to transport large volumes of water. Their walls are strengthened by lignin.

• Tracheids: Elongated, dead, tube-like cells with tapering ends. They are less efficient at conducting water than vessel elements.

• Xylem fibres: Narrow, dead cells that provide mechanical support.

• Xylem parenchyma: The only living cells in xylem. They store water and food.

In the xylem of the main stem, materials move in one direction only — upward.

Phloem Tissue

Phloem transports manufactured food (such as sugars and amino acids) from the leaves (where photosynthesis occurs) to all parts of the plant. This process is called translocation.

Phloem consists of:

• Sieve tube elements: Tubular cells joined end-to-end with perforated end walls called sieve plates. They contain cytoplasm but no nucleus.

• Companion cells: Living cells associated with sieve tube elements. They provide energy (from mitochondria) for transporting food.

• Phloem parenchyma: Cells that transport sugars in dissolved form.

• Phloem fibres: Dead cells that provide mechanical support.

Unlike xylem, materials in phloem move in both upward and downward directions.

Comparison of Xylem and Phloem

Structure and Function of Root Hairs

Root hairs are extensions of epidermal cells in the root. They are long and slender, providing a large surface area for absorption.

Adaptations of root hairs:

• Large surface area for absorbing water and minerals
• Thin cell walls for short diffusion distance
• Permeable walls (hydrophilic) to allow water entry
• Cell sap is hypertonic (more concentrated than soil water), so water enters by osmosis
• If mineral salt concentration is higher inside, minerals are absorbed by active transport

Movement of Water from Root Hairs to Xylem

When root hairs absorb water, they become less concentrated than the adjacent cortex cells. Therefore, water moves from root hairs to cortex cells by osmosis. This process continues through the endodermis, pericycle, and finally into the xylem vessels.

How Water Moves Up the Xylem

Water and dissolved minerals move upward through xylem vessels by three forces:

1. Transpiration pull: As water evaporates from leaves through stomata (transpiration), it creates a tension that pulls water upward from the roots. This continuous column of water is called the transpiration stream.

2. Capillarity: Water rises through narrow tubes (like xylem vessels) due to capillary action. This is possible because of cohesion (water molecules stick together) and adhesion (water molecules stick to the xylem walls).

3. Root pressure: Mineral salts are actively pumped into the xylem by endodermis cells. This increases the osmotic pressure in the xylem, pushing water upward. When a plant is cut, sap oozing from the stump proves root pressure exists.

What is Transpiration?

Transpiration is the process by which plants lose water vapour through the stomata in the leaves. This water comes from the xylem in the leaves, which receives water from the roots.

Types of Transpiration

• Stomatal transpiration: Water loss through stomata (about 90% of total water loss)
• Cuticular transpiration: Water loss through the waxy cuticle on leaf surfaces
• Lenticular transpiration: Water loss through lenticels (pores on bark of woody plants)

Factors Affecting Transpiration Rate

Plant features:

• Leaf size (larger leaves = more stomata = higher transpiration)
• Number and position of stomata
• Thickness of leaf cuticle (thick cuticle reduces water loss)
• Root system size (extensive roots absorb more water, increasing transpiration)
• Epidermal hairs (trap water vapour, reducing transpiration)
• Sunken stomata (found in pits, reduce water loss)

Environmental factors:

• Temperature: Higher temperature increases transpiration
• Relative humidity: Higher humidity decreases transpiration
• Wind: Increased air movement increases transpiration
• Light intensity: Brighter light opens stomata, increasing transpiration
• Soil moisture: Low soil moisture reduces transpiration
• Atmospheric pressure: Low pressure at high altitudes increases transpiration

Importance of Transpiration

1. Maintains the transpiration stream for continuous water supply
2. Enables loss of excess water
3. Cools the plant
4. Helps absorb and distribute water and minerals
5. Maintains cell turgidity and osmosis

Guttation vs. Transpiration

In Tanzania, understanding plant transport systems helps farmers in Morogoro and other regions choose appropriate irrigation schedules. Knowing that transpiration rates increase on hot, windy days helps farmers water crops early in the morning or late in the evening to reduce water loss. For example, a tomato farmer in Arusha can use this knowledge to apply mulch (which reduces transpiration by cooling the soil and reducing wind effect on leaves) to conserve water during dry seasons, ultimately saving on irrigation costs and improving crop yield.