Plants possess two main types of vascular tissues that work together to transport materials throughout the plant body.

1.1 Xylem Tissue

Xylem conducts water and dissolved mineral salts from the roots to the leaves ( upward movement). It also provides mechanical support to the plant.

Structural components and their functions:

• Tracheids: Elongated dead cells with pointed ends and lignified walls containing pits. They allow vertical water movement through conduction.

• Vessel elements: Tubular structures joined end-to-end to form continuous vessels. They possess perforation plates that enable rapid water transport. Each vessel may be up to 10 cm long.

• Xylem parenchyma: Living cells that store food and conduct water sideways.

• Xylem fibers: Short, narrow cells with thick lignified walls that provide additional mechanical strength.

Adaptations for water transport:

• Narrow lumina allow capillary action
• Lateral pits permit lateral water movement
• Dead cells at maturity transport large water volumes
• Lignified walls prevent collapse and withstand pressure
• Perforation plates create continuous pathways

Evidence that xylem transports water:

• Dye experiments show only xylem tissue stains when cut stems are placed in colored solutions
• Removal of xylem causes wilting while phloem removal does not
• Metabolic poisons do not impede water flow, confirming passive transport

1.2 Phloem Tissue

Phloem translocates manufactured food (mainly sucrose) from source to sink areas in a bidirectional manner.

Structural components:

• Sieve tube members: Elongated cells joined end-to-end forming sieve tubes. They lack nuclei at maturity and possess sieve plates with large pores for rapid flow.

• Companion cells: Metabolically active cells with dense cytoplasm, mitochondria, and ribosomes. They regulate sieve tube function and provide ATP for active transport.

• Phloem parenchyma: Living cells that store carbohydrates, tannins, and resins.

• Phloem fibers: Elongated sclerenchyma cells providing mechanical support.

Evidence that phloem transports food:

• Cutting phloem causes exudation of sugar-containing solution
• Radioactive carbon tracing shows photosynthates move through phloem
• Aphid stylet experiments reveal sucrose solution in sieve tubes
• Ring barking experiments show food accumulation above the ring

2.1 Active Transport

This requires energy (ATP) to move materials against their concentration gradient. Mineral ions are often absorbed from low concentration in soil to higher concentration in root cells through this mechanism.

2.2 Passive Transport

Materials move along their concentration gradient without energy expenditure. This includes:

• Diffusion: Movement of molecules from high to low concentration
• Osmosis: Movement of water across a selectively permeable membrane from high to low water potential
• Facilitated diffusion: Passive transport aided by carrier proteins

Key differences:

Water moves from soil into the xylem through three pathways:

3.1 Apoplast Pathway

Water moves through cell walls and intercellular spaces without crossing cell membranes. This accounts for the highest percentage of water movement in plants. However, water cannot pass through the endodermis here due to Casparian strips (suberized bands in cell walls).

3.2 Symplast Pathway

Water moves through the cytoplasm of adjacent cells via plasmodesmata (cytoplasmic strands connecting cells). This pathway allows water and solutes to move without crossing membranes.

3.3 Vacuolar Pathway

Water passes from vacuole to vacuole across both pathways. This involves high resistance and is therefore less significant.

Role of Casparian Strips

These waxy bands in endodermal cell walls:

• Force water into the protoplast to reach xylem
• Allow active secretion of salts into xylem
• Create more negative water potential in xylem
• Protect against toxic substances

Water rises through xylem due to multiple forces:

4.1 Cohesion-Tension Theory

This explains how water reaches great heights in plants:

• Water molecules are cohesive (stick to each other through hydrogen bonds)
• Water molecules adhere to xylem vessel walls
• Transpiration creates tension (pull) at leaf surface
• This tension draws water upward through the continuous water column
• The unbroken water column transmits this pull from leaves to roots

4.2 Root Pressure

When transpiration is low, mineral ions accumulate in root xylem, lowering water potential. Water then moves into xylem by osmosis, creating positive pressure that pushes water upward. This is important in herbaceous plants and grasses but insufficient in tall trees.

4.3 Capillary Action

Water rises in narrow xylem vessels due to adhesion between water molecules and vessel walls. This contributes to upward movement but is limited to short distances.

4.4 Transpiration Pull

As water evaporates from stomata, it creates a water potential gradient that draws water from the xylem to replace lost water. This is the major force for water movement in tall plants.

5.1 Mass Flow (Pressure Flow) Hypothesis

This model, proposed by Münch in 1930, explains phloem translocation:

Process:

1. Photosynthesis produces glucose in mesophyll cells
2. Glucose is converted to sucrose
3. Sucrose is actively loaded into sieve tube elements at the source (leaves)
4. This creates hypertonic conditions, causing water to enter by osmosis
5. Hydrostatic pressure increases at the source
6. At the sink (roots, developing fruits), sucrose is actively unloaded
7. Water leaves the phloem, reducing pressure
8. Pressure gradient drives mass flow from source to sink

Worked Example:

Consider a maize plant in Morogoro during the rainy season:

• Source: Photosynthesizing leaves produce sucrose
• Loading: Sucrose enters sieve tubes in leaf veins
• Water entry: Water enters from adjacent xylem, creating pressure
• Flow: Sap moves through phloem to developing roots and young shoots (sinks)
• Unloading: Cells in roots actively take up sucrose for storage or respiration
• Result: Continuous pressure difference maintains steady translocation

5.2 Evidence for Mass Flow

• Phloem sap exudes from cut ends under pressure
• Rate of movement is faster than diffusion would allow
• Pressure differences between source and sink have been measured
• Aphid stylet experiments show continuous sap flow

5.3 Criticism of Mass Flow Hypothesis

• Does not fully explain why sieve tubes must be living
• Cannot account for bidirectional movement at the same time
• Does not explain different transport rates for different substances
• Does not address loading and unloading mechanisms adequately

6.1 Potassium Ions Hypothesis (Most Accepted)

This explains how stomata open and close:

Opening mechanism:

1. Light activates ATPase enzyme in guard cell membranes
2. ATPase pumps H⁺ ions out of guard cells
3. H⁺ gradient drives K⁺ and Cl⁻ ions into guard cells
4. Solute concentration increases, lowering water potential
5. Water enters by osmosis, making guard cells turgid
6. Differential wall thickening causes guard cells to curve, opening the pore

Closing mechanism:

1. Reverse process occurs in darkness
2. K⁺ and Cl⁻ leave guard cells
3. Water exits by osmosis
4. Guard cells become flaccid, closing the pore

6.2 Factors Affecting Transpiration

External factors:

• Light intensity: Stomata open in light, increasing transpiration
• Temperature: Higher temperature increases transpiration rate
• Humidity: Low humidity increases transpiration
• Wind: Removes water vapor, increasing transpiration
• Water availability: More soil water increases transpiration

Internal factors:

• Leaf surface area
• Cuticle thickness
• Stomatal number and distribution

In Tanzania, understanding plant transport systems directly benefits agricultural practices. For example, a coffee farmer in Mbeya applies this knowledge when deciding when to irrigate coffee trees during the dry season. By understanding transpiration and water movement, the farmer knows that watering in early morning minimizes water loss through stomata and maximizes water uptake through the roots. Additionally, understanding how minerals are transported helps the farmer apply fertilizers effectively—knowing that xylem transports minerals upward, fertilizer should be applied to the soil where roots can absorb them, rather than sprayed on leaves for quick absorption through phloem. This knowledge improves crop yields and conserves water resources, which is particularly important in regions facing drought conditions.