In a dicot pot herb, vaseline/vegetable oil was applied on the upper surface of one leaf (Experimental leaf 1) and on the lower surface of another leaf (Experimental leaf 2). Vaseline/Vegetable oil was not applied on the control leaf. The plant was deliberately not watered for several days. Which leaf will dry up last?

The correct answer is B, Experimental leaf 2. In a dicot leaf most stomata are on the lower surface, so coating that surface blocks most of the water loss and the leaf stays fresh the longest.

Transpiration, the loss of water vapour from the plant, takes place mostly through stomata. In dicot leaves, which lie flat with one side facing the sun, the lower surface carries far more stomata than the upper, which keeps water loss down. Vaseline or oil seals the pores it covers. Leaf 2, sealed below, can lose water only through the few upper stomata and the cuticle, so it dries last. Leaf 1, sealed above, still loses water through its many lower stomata.

A is wrong because sealing the upper surface leaves most of the stomata open.

C is wrong because the control leaf loses water from both surfaces and dries first.

D is wrong because the leaves lose water at clearly different rates.

Exam tip: dicot leaves have more stomata on the lower surface; monocot leaves have them roughly equally on both.