Phloem
Made up from 4 different parts:
- Phloem fibres.
- Phloem parendyma.
- 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.
Note:
Translocation from Source (e.g. sugar in leaves) to Sink (roots)
Translocation from Source (e.g. sugar in leaves) to Sink (roots)
- Have plasmodesmata.
- Very metabolicly active.
- High number of mitochondria.
- May provide energy for traslocation (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. (click to enlarge)
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 to against Mass Flow hypothesis
- Get the impression of steady flow, this is wrong.
- Certain things move different ways.
Phloem in a Root. (click to enlarge)
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