Mention two advantages of transpiration.

1. It creates the TRANSPIRATION PULL — the suction that draws water and dissolved minerals up from the roots to the leaves against gravity (the ascent of sap).
2. It COOLS THE PLANT. The evaporation of water from the leaf surface absorbs latent heat and prevents the leaf from overheating in strong sunlight.

Two advantages, one mark each. These two are the strongest and should be given first.

THE TRANSPIRATION PULL is the more important: without it, water could not be lifted to the top of a tall tree. The evaporation at the leaves creates a tension transmitted down the unbroken water column in the xylem.

A third advantage if three are asked for: it maintains the flow of mineral salts from soil to leaves, since minerals travel dissolved in that water.

THE WIDER POINT, worth a closing line: transpiration is sometimes called a necessary evil, because a plant loses enormous quantities of water by it. But the loss is unavoidable — the stomata must open to admit carbon dioxide for photosynthesis, and water inevitably escapes through the same pores.