1. Changes in Tree Trunk Diameter

• During the day, transpiration pulls water up the xylem, creating tension in the xylem, making the tree trunk diameter smaller.

• At night, transpiration slows down, reducing tension, and the trunk diameter expands slightly.

• This supports the cohesion-tension theory, as water movement is linked to transpiration rates.

2. Broken Xylem and Air Intake

• If a xylem vessel is broken, air enters, and the plant can no longer draw up water.

• This shows that water transport relies on tension and cohesion, as an air break disrupts the continuous water column.

3. No Leakage When Xylem is Cut

• If water movement was due to high pressure, cutting the stem should cause water to leak out.

• Instead, air is drawn in, proving that water is being pulled up under negative pressure or tension.

1. Ringing Experiment

• A ring of bark (containing phloem vessels but not xylem) is removed from a tree stem.

• (Phloem vessels are found on the outer side of the stem, whereas xylem vessels are towards the middle).

• Sucrose accumulates above the ring, causing swelling, while the area below the ring dies due to lack of sugars.

2. Radioactive Tracer Experiment (Carbon-14 Labelling)

• A plant is exposed to radioactive carbon dioxide (¹⁴CO₂).

• The plant uses the carbon-14 in photosynthesis to produce radioactively labelled sucrose.

• Autoradiography (X-ray imaging) shows that radioactive sucrose moves through the phloem, confirming translocation occurs in the phloem and follows a source-to-sink pattern.

3. Aphid Stylet Experiment

• Aphids (sap-sucking insects) pierce the phloem with their mouthparts.

• The phloem sap continues to flow out, even when the aphid is removed, showing that translocation occurs under pressure.

• This supports the mass flow hypothesis, as movement is due to hydrostatic pressure gradients in the phloem.

While the Mass Flow Hypothesis is the most widely accepted explanation for phloem transport, some evidence suggests that translocation is more complex than simple pressure-driven mass flow.

1. Different Solutes Move at Different Rates

• According to mass flow, all solutes in the phloem should move at the same speed if transported by bulk flow.

• However, different organic molecules (e.g., sucrose, amino acids) move at different rates, suggesting that other processes might be involved in regulating transport.

2. Bidirectional Flow in the Same Phloem Vessel

• The mass flow hypothesis suggests all solutes move in one direction per sieve tube (from high to low pressure).

• However, some experiments show that solutes can move in opposite directions in the same sieve tube element, which mass flow cannot explain.

• This suggests that active processes or additional control mechanisms might influence translocation.

3. Phloem Transport is Faster Than Diffusion

• The speed of solute movement in the phloem is faster than can be explained by passive mass flow alone.

• This suggests that more active processes  may assist transport.

4. The Role of Sieve Plates

• The perforated sieve plates between sieve tube elements should slow down flow, acting as a resistance barrier.

• However, translocation still occurs efficiently, suggesting sieve plates may have a more active role in regulating flow, rather than just being passive channels.