Case 1: The potometer readings the teacher would not accept

Weakness 1 - the shoot was cut in air. Air is drawn into the xylem and breaks the continuous water column, so water movement through the potometer is reduced by an unknown amount. Any reading may be too low, and by a different amount in each condition, so the ratio 31 : 12 becomes unreliable. The shoot should be re-cut obliquely under water and assembled while still submerged.

Weakness 2 - one reading per condition, taken in a fixed order, while the room warmed up. Temperature is a second variable that raises the rate of evaporation from the leaf. The fan reading was taken later, so part of the 31 mm may be a temperature effect, not a wind effect. A single reading also gives no idea of natural variation, so 31 versus 12 could exaggerate a real but smaller difference.

Other acceptable answers: leaves not dried between readings (surface water evaporates from the leaf surface, not through stomata, inflating the readings); the bag traps heat and condensation, so 4 mm does not show that transpiration has stopped, only that net uptake has almost stopped.

The arithmetic is correct (31 ÷ 12 = 2.6), so the statement is an accurate description of the two numbers obtained. As a conclusion about wind it is weak:

• Each condition was measured once, so there is no indication of the variability of the readings; a mean of at least five repeats per condition, with the spread shown, is needed.
• Temperature rose during the afternoon and the conditions were always run in the same order, so temperature and wind are confounded. The order should be randomised, and the room temperature and light intensity kept constant and recorded.
• If air entered the xylem when the shoot was cut in air, the water column may be partly broken, making every reading an underestimate.

The direction of the effect is nevertheless well supported by theory: moving air removes the layer of humid air next to the leaf, steepens the water potential gradient between the air spaces and the outside air, and so increases evaporation from the mesophyll and uptake through the xylem.

Redesign: same shoot for all conditions, cut under water, leaves blotted dry, 5-minute equilibration before each reading, randomised order, ≥5 repeats, temperature and light monitored, plus a control shoot with leaves removed or covered in petroleum jelly to show that the movement is due to transpiration through stomata.

A potometer measures uptake, not transpiration, and the two are linked but not equal:

• Some absorbed water is used in cells - for turgor, for growth, and as a reactant in photosynthesis in the mesophyll.
• If the shoot was slightly dehydrated when it was cut (likely, since it was cut in air and left on the bench), the first water drawn in simply rehydrates the tissue, so early readings overestimate transpiration. This is exactly why the first reading taken (still air, 12 mm) may be too high relative to the later ones.
• There is a short lag: water lost from the leaves is replaced from the xylem a little later, so uptake trails transpiration.

In the bag, the air quickly becomes saturated with water vapour, so the water potential gradient between the leaf air spaces and the outside air becomes very small and transpiration nearly stops; uptake then also falls, but not to zero instantly, because the shoot still uses water and the tissue can rehydrate. A 4 mm movement therefore shows that net uptake has fallen sharply - consistent with high humidity - but it cannot show that transpiration has stopped completely, and the trapped heat inside the bag complicates the comparison further.