CDS (II) 2024 General Knowledge · Question 5 of 120

Correct — B, Large number of stomata. A xerophyte is a plant built to survive where water is scarce, and every structural feature it carries is an answer to one problem — losing as little water as possible while still taking in carbon dioxide. Test each option against that single purpose. Smaller leaves shrink the surface from which water can evaporate. A thick waxy cuticle seals the leaf surface so that water cannot escape except through the stomata. Stomata sunk into pits sit in a pocket of still, humid air, sheltered from the wind that would otherwise carry the vapour away. Only the second option pulls the other way: stomata are the pores through which a plant loses water vapour, so having a large number of them would raise transpiration, which is exactly what a desert plant cannot afford. It is the one feature in the list that works against the xerophytic strategy rather than for it.

• (a)Smaller leaves — This is a genuine xerophytic feature. Reducing leaf area reduces the surface available for evaporation, and the reduction can go all the way to spines, as in a cactus, whose green stem then takes over the work of photosynthesis.
• (c)Waxy cuticle — Also genuine. A thick, often waxy cuticle waterproofs the leaf surface, and a pale waxy bloom additionally reflects sunlight so the leaf stays cooler and loses less water.
• (d)Stomata in pits — Also genuine. Sinking the stomata into pits or crypts, often lined with hairs, keeps them out of the moving air and traps humid air around the pore, so less vapour is swept away.

Plants are grouped by the water regime they are built for. Xerophytes live where water is scarce — deserts, sand dunes and dry rocky ground — and their whole design minimises transpiration. Hydrophytes live in or on water and have the opposite problem, so their stomata sit on the upper surface of floating leaves and their tissues are full of air spaces. Mesophytes, the ordinary plants of moderate conditions, sit between the two.

The item does not require any memorised list. Every option can be tested against one question — does this raise or lower water loss? Three lower it and one raises it, and that one is the answer. The care worth taking is that the strict physiology is more nuanced than the exam's phrasing: several desert plants carry a high density of very small stomata that stay shut through the heat of the day and open at night, so a raw count is not by itself the whole story. What is not in doubt, and what the item is really testing, is the direction of the adaptation — a xerophyte's structures are all arranged to restrict evaporation, and more open pores on an exposed surface would defeat that.

• Stomata are the pores through which a plant loses water vapour in transpiration, so their exposure governs water loss.
• Sinking stomata into pits, often among hairs, keeps them out of the wind and holds humid air around the pore.
• Many xerophytes reduce their leaves to spines, and the green stem then carries out photosynthesis, as in cactus.
• A thick cuticle, often with a white waxy coating that reflects sunlight, is a common xerophytic covering.
• Several desert plants keep their stomata shut through the day and open them at night, when the air is cooler and more humid.

• Assuming every desert plant simply has fewer stomata; the reliable point is that its stomata are protected and its evaporating surface reduced.
• Reading spines as a defence only; in a cactus they are reduced leaves and their first job is to cut water loss.
• Mixing up xerophyte and halophyte — one is built for dry ground, the other for salty ground, and a plant can face both.

Usually as an odd-one-out on desert adaptations, or as a cactus item on reduced leaves and photosynthetic stems; occasionally paired against hydrophyte features.

• practice — not a real PYQ

  In a floating aquatic plant such as the water lily, the stomata are found mainly on which surface of the leaf?

  • (a)The upper surface
  • (b)The lower surface
  • (c)Both surfaces equally
  • (d)Neither surface, as such leaves have no stomata

  Answer(a) The upper surface — the lower surface rests on water, so the pores must open on the upper side to exchange gases with the air.

• practice — not a real PYQ

  Sinking the stomata into pits on the leaf surface helps a desert plant chiefly because it

  • (a)increases the amount of light reaching the leaf
  • (b)traps humid air near the pore and shelters it from wind
  • (c)allows the leaf to absorb water directly from the air
  • (d)prevents insects from entering the leaf

  Answer(b) traps humid air near the pore and shelters it from wind — moving air would carry the water vapour away and speed up transpiration.