Supporting observations
The mass flow hypothesis predicts pressure within sieve tubes, differences between source and sink regions, and ATP-dependent processes that establish translocation.
| Observation | How it supports mass flow |
|---|---|
| Phloem sap escapes when a sieve tube is punctured. | The release of sap indicates that the contents of the sieve tube are under pressure. |
| Phloem sap collected near a source has a greater sucrose concentration than sap collected near a sink. | This is consistent with sucrose loading near sources and unloading near sinks, producing the water movements needed to establish a pressure gradient. |
| Metabolic inhibitors prevent translocation. | This supports the involvement of ATP-dependent processes such as active loading in establishing mass flow. |
| Removing a ring of bark causes sugar-rich material to accumulate above the removed region. | Because phloem lies beneath the bark, breaking this transport pathway prevents sugars moving through the ringed region. |
Together, these observations support the presence of pressure in sieve tubes, differences between source and sink regions, and an important role for active loading.
loading at source + water entry → high pressure → mass flow → unloading at sink
Exam Tip: In an evidence question, follow every observation with its significance. For example: sap escapes from a punctured sieve tube → the sap was under pressure → supports pressure-driven mass flow.