Phloem – Plant Tissue

Made up from 4 different parts:

  • Phloem fibres
  • Phloem parenchyma
  • Sieve tubes
  • Companion cells


The sieve tubes and companion cells are both involved with the mass flow hypothesis.

Sieve Tubes

  • Cell membrane broken down
  • Fluid cytoplasm
  • No vacuole
  • No nucleus


With the end of the sieve wall broken down it allows inter cellular movement.

Translocation from Source (e.g. sugar in leaves) to Sink (roots)
  • Have plasmodesmata.
  • Very metabolically active.
  • High number of mitochondria.
  • May provide energy for translocation (mass movement).


Osmotic Pressure

The movement of solutes from a high hydrostatic pressure to a low hydrostatic pressure.

Simplified diagram of water and energy movement in a Plant

Evidence to support Mass Flow hypothesis

  • Solution under pressure.
  • Evidence for concentration gradient.
  • Observation of sieve tubes.
  • Movement of virus through plant.
  • No movement of virus when no photosynthesis.


Evidence against Mass Flow hypothesis

  • Get the impression of steady flow, this is wrong.
  • Certain things move different ways.


Phloem in a Root