Write a descriptive note on phloem and its elements in angiosperms.

Phloem is a complex permanent tissue responsible for translocation of organic nutrients (primarily sucrose) from sources (leaves) to sinks (roots, fruits, storage organs).

Components of Angiosperm Phloem:

Phloem comprises four types of elements:

1. Sieve Tube Elements
2. Companion Cells
3. Phloem Parenchyma
4. Phloem Fibres
5. Sieve Tube Elements:

Structure:

• Long, tube-like cells arranged end-to-end longitudinally
• Form continuous sieve tubes
• End walls modified into sieve plates (perforated areas with pores)
• Mature sieve element:
  • Living but lacks nucleus (enucleate)
  • Peripheral cytoplasm (thin layer)
  • Large central vacuole
  • Specialized ER (sieve element reticulum)
  • P-protein bodies (block sieve pores upon injury)

Function:

• Primary conduit for photoassimilate transport
• Maintains high-pressure flow (pressure-flow hypothesis)

1. Companion Cells:

Structure:

• Specialized parenchymatous cells
• Closely associated with sieve tube elements (arise from same mother cell)
• Connected via plasmodesmata (pit fields in common walls)
• Living cells with:
  • Prominent nucleus
  • Dense cytoplasm
  • Numerous mitochondria (high metabolic activity)

Function:

• Maintain sieve tube metabolism (nucleus absent in sieve tubes)
• Regulate loading/unloading of sugars
• Pressure gradient maintenance

Evolutionary Note: Companion cells are unique to angiosperms; gymnosperms have albuminous cells instead.

1. Phloem Parenchyma:

Structure:

• Living parenchymatous cells
• Elongated, tapering, cylindrical shape
• Dense cytoplasm and nucleus

Function:

• Storage of:
  • Food materials (starch, proteins, fats)
  • Secondary metabolites (resins, latex, mucilage, tannins)
• Lateral conduction of nutrients

Distribution:

• Present in most dicots
• Absent in most monocots (diagnostic feature)

1. Phloem Fibres:

Structure:

• Sclerenchymatous cells
• Much elongated, unbranched
• Pointed, needle-like apices
• Thick lignified walls
• Dead at maturity (protoplasm lost)

Function:

• Mechanical support to phloem tissue

Commercial Importance:

• Jute (Corchorus capsularis) - Bags, sacking
• Flax (Linum usitatissimum) - Linen fabric
• Hemp (Cannabis sativa) - Ropes, textiles

Phloem Transport Mechanism:

Pressure-Flow (Mass Flow) Hypothesis:

1. Source (Leaf):
   • Photosynthesis produces sucrose
   • Active loading into sieve tubes (via companion cells)
   • High solute concentration → water influx (osmosis)
   • High turgor pressure created
2. Sink (Root/Fruit):
   • Sucrose actively unloaded
   • Low solute concentration → water efflux
   • Low turgor pressure
3. Flow:
   • Pressure gradient drives mass flow from source to sink

NEET Focus: Phloem composition differs between angiosperms and gymnosperms; companion cells vs. albuminous cells is a key distinction.