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?
- (a)Experimental leaf 1
- (b)Experimental leaf 2
- (c)Control leaf
- (d)All the leaves will dry up simultaneously
Correct — B, Experimental leaf 2. In a typical dicot leaf, most stomata are on the LOWER surface. Vaseline/oil blocks stomata and stops water loss (transpiration) through them. Leaf 2 has its lower surface coated, so its majority of stomata are sealed and it loses water most slowly — meaning it stays hydrated longest and dries up last.
- (a)Experimental leaf 1 — Leaf 1 has only its upper surface coated. In a dicot the upper surface has few stomata, so most stomata (on the lower surface) are still open and it keeps losing water quickly — it dries faster than leaf 2.
- (c)Control leaf — The control leaf is uncoated, so stomata on both surfaces stay open and transpiration is fastest; it will dry up soonest, not last.
- (d)All the leaves will dry up simultaneously — They cannot dry at the same rate because the amount of open stomatal surface differs between the three leaves, so their rates of water loss differ.
Transpiration is the loss of water vapour from a plant, mainly through stomata on the leaf surface. In most dicot leaves the stomata are far more numerous on the lower (shaded) surface than the upper. Blocking a surface with oil/vaseline closes its stomata and reduces water loss from that side, so the leaf with the greater open-stomata area transpires — and dries — fastest.
The whole experiment turns on knowing where the stomata are. Because dicot leaves are stomata-rich on the underside, coating the lower surface (leaf 2) shuts off most transpiration and preserves that leaf longest. This 'controlled experiment' style — one variable changed, a control kept — is a favourite way to test the stomatal-distribution fact.
- Stomata are the main sites of transpiration in leaves.
- In most dicot leaves, stomata are far more numerous on the lower surface.
- Vaseline/oil blocks stomata and prevents water loss from the coated surface.
- Least open stomatal area = least transpiration = slowest to dry up (leaf 2).
Because dicot leaves have most stomata on the lower surface, coating the lower surface (leaf 2) preserves it longest.
- Assuming stomata are evenly spread on both surfaces — in dicots they cluster on the lower surface.
- Picking the control leaf because it 'has nothing done to it' — an uncoated leaf actually transpires fastest.
Asked through vaseline/petroleum-jelly experiments testing where stomata are, or by comparing rates of water loss from coated and uncoated leaves.
No directly related past PYQ was found.
- practice — not a real PYQ
In most dicot leaves, stomata are more numerous on the
- (a)upper surface
- (b)lower surface
- (c)leaf margin
- (d)midrib only
Answer(b) lower surface — which reduces water loss from the sun-facing upper side.
- practice — not a real PYQ
Applying petroleum jelly to a leaf surface reduces water loss because it
- (a)kills the leaf
- (b)blocks the stomata
- (c)adds water
- (d)increases photosynthesis
Answer(b) blocks the stomata — sealing the main routes of transpiration.