Part 2 — Long-Distance Transport of Water, Transpiration & Ascent of Sap (Q26–50)

Q26.

Which structure in the leaf is the main site of transpiration?
A. Epidermis
B. Cuticle
C. Stomata ✅
D. Mesophyll cell walls

Explanation:

A. Epidermis covers but is not primary site.
B. Cuticle reduces water loss.
C. (Correct) Most transpiration occurs via stomata (stomatal transpiration).
D. Mesophyll provides evaporative surface, but exit occurs through stomata.

Q27.

Which type of transpiration occurs through the cuticle?
A. Lenticular transpiration
B. Cuticular transpiration ✅
C. Stomatal transpiration
D. Guttation

Explanation:

A. Lenticular transpiration is through lenticels.
B. (Correct) Cuticular transpiration occurs through cuticle; minor in most plants.
C. Main transpiration pathway (stomata).
D. Guttation = root pressure-driven water droplets.

Q28.

What percentage of total transpiration in most plants is stomatal?
A. 5–10%
B. 20–30%
C. 80–90% ✅
D. 100%

Explanation:

A/B. Too low.
C. (Correct) ~80–90% of transpiration is stomatal.
D. Not 100%, as cuticular and lenticular transpiration occur too.

Q29.

Which statement is TRUE for guttation?
A. Occurs during high transpiration
B. Occurs through hydathodes at leaf margins ✅
C. Caused by cohesion-tension mechanism
D. Leads to vapor loss

Explanation:

A. Occurs when transpiration is low, not high.
B. (Correct) Guttation is exudation of liquid water through hydathodes due to root pressure.
C. Not cohesion-tension; it is root pressure.
D. Loss is liquid, not vapor.

Q30.

In cohesion-tension theory, which property of water ensures continuous column in xylem?
A. Compressibility
B. Cohesion ✅
C. Evaporation
D. Capillarity

Explanation:

A. Water is nearly incompressible, but that’s not the reason.
B. (Correct) Cohesion between water molecules maintains continuous column.
C. Evaporation creates tension but continuity requires cohesion.
D. Capillarity contributes little in tall trees.

Q31.

Which factor does NOT directly affect transpiration rate?
A. Light
B. Wind
C. Soil fertility ✅
D. Temperature

Explanation:

A. Light opens stomata → increases transpiration.
B. Wind removes humid air → increases transpiration.
C. (Correct) Soil fertility affects growth but not directly transpiration rate.
D. High temp increases transpiration.

Q32.

The ascent of sap is primarily explained by:
A. Capillary rise
B. Cohesion-tension theory ✅
C. Root pressure alone
D. Active pumping in xylem

Explanation:

A. Capillarity cannot explain tall trees.
B. (Correct) Cohesion-tension (Dixon-Joly theory) explains ascent in tall plants.
C. Root pressure insufficient for great heights.
D. Xylem is dead tissue, no pumping.

Q33.

Why is root pressure considered insignificant in tall trees?
A. Root pressure doesn’t exist in tall trees
B. Root pressure cannot generate enough pressure to push water up hundreds of meters ✅
C. Root pressure is absent during day
D. Root pressure only occurs in conifers

Explanation:

A. Root pressure exists but is small.
B. (Correct) Pressure from roots can move water only a few meters; transpiration pull is needed in tall trees.
C/D. Incorrect limitations.

Q34.

What is the role of adhesion in ascent of sap?
A. Prevents plasmolysis
B. Water molecules stick to xylem walls preventing column breakage ✅
C. Provides osmotic gradient
D. Opens stomata

Explanation:

A. Not related.
B. (Correct) Adhesion with xylem walls stabilizes water column under tension.
C. Osmotic gradient mainly root uptake.
D. Stomata regulation is separate.

Q35.

Lenticels are involved in:
A. Cuticular transpiration
B. Stomatal transpiration
C. Lenticular transpiration ✅
D. Guttation

Explanation:

A. Cuticle = cuticular.
B. Through stomata = stomatal.
C. (Correct) Lenticels are small openings in woody stems allowing gas exchange and water vapor loss.
D. Guttation occurs via hydathodes.

Q36.

Which condition favors guttation most?
A. Hot, dry afternoon
B. Cool, humid night ✅
C. Windy day
D. Cloudy noon

Explanation:

A. High transpiration, guttation suppressed.
B. (Correct) Low transpiration + high root pressure → guttation at night/early morning.
C/D. Increase or maintain transpiration.

Q37.

Capillarity in xylem contributes to ascent of sap because:
A. Vessel walls are hydrophobic
B. Narrow vessels enhance adhesion and cohesion ✅
C. Water is compressible
D. Root pressure is high

Explanation:

A. Walls are hydrophilic, not hydrophobic.
B. (Correct) Narrow vessels → stronger capillary rise.
C. Water incompressible.
D. Not related to capillarity.

Q38.

The highest negative pressure in the water transport pathway is found in:
A. Soil solution
B. Root xylem
C. Leaf mesophyll cells ✅
D. Stem cortex

Explanation:

A. Soil is relatively high water potential.
B. Root xylem under less tension.
C. (Correct) Evaporation in mesophyll cell walls creates most negative water potential.
D. Cortex stores, not highest tension.

Q39.

Transpiration increases with:
A. High atmospheric humidity
B. High wind velocity ✅
C. Closing of stomata
D. Thick cuticle

Explanation:

A. High humidity lowers gradient.
B. (Correct) Wind removes boundary layer humidity → higher rate.
C. Closed stomata reduce transpiration.
D. Thick cuticle reduces cuticular transpiration.

Q40.

Which statement supports cohesion-tension theory?
A. Sap exudation by root pressure
B. Pressure probe shows xylem sap under negative pressure ✅
C. Capillarity rises water 120 m in tree
D. Vessel elements actively pump water upward

Explanation:

A. Root pressure is not the main mechanism.
B. (Correct) Direct measurements show negative pressures (tension) in xylem sap.
C. Capillarity cannot rise so high.
D. Xylem dead, cannot pump.

Q41.

Which instrument directly measures water potential?
A. Potometer
B. Pressure chamber (Scholander bomb) ✅
C. Psychrometer
D. Porometer

Explanation:

A. Potometer measures water uptake (transpiration rate), not water potential.
B. (Correct) Pressure chamber estimates balancing pressure = water potential of xylem sap.
C. Psychrometer measures relative humidity (indirect).
D. Porometer measures stomatal conductance.

Q42.

Which is the main driving force for long-distance water transport in xylem?
A. Root pressure
B. Transpiration pull ✅
C. Active pumping by living xylem cells
D. Capillary rise alone

Explanation:

A. Minor force.
B. (Correct) Transpiration-induced tension is the main driver.
C. Xylem cells are dead.
D. Insufficient for tall plants.

Q43.

Why does guttation not occur in large trees like banyan?
A. They lack hydathodes
B. Root pressure is too low to force water up such long distances ✅
C. Stomata are closed at night
D. Cohesion between water molecules is absent

Explanation:

A. Hydathodes exist but guttation limited.
B. (Correct) Root pressure is not enough to force guttation in tall trees.
C. Stomata closing is irrelevant to guttation.
D. Cohesion is always present.

Q44.

During daytime, water potential gradient across plant is:
A. Highest in atmosphere, lowest in soil
B. Highest in soil, lowest in atmosphere ✅
C. Equal in all tissues
D. Highest in stem, lowest in leaf

Explanation:

A. Reverse of reality.
B. (Correct) Soil ψ > root ψ > stem ψ > leaf ψ > air ψ (most negative).
C. Gradient exists.
D. Stem is not highest.

Q45.

The opening and closing of stomata depends mainly on:
A. Photosynthesis in guard cells
B. Turgor pressure changes in guard cells ✅
C. Root pressure changes
D. CO₂ concentration in mesophyll only

Explanation:

A. Photosynthesis indirectly affects ion status, but main factor = turgor.
B. (Correct) Guard cells open stomata by becoming turgid due to K⁺ and water influx; closing = loss of turgor.
C. Root pressure not direct cause.
D. CO₂ affects, but mechanism = turgor.

Q46.

Transpiration pull is strongest in:
A. Desert night
B. Humid rainforest
C. Sunny, dry windy day ✅
D. Underground stem

Explanation:

A. Low transpiration.
B. High humidity → low pull.
C. (Correct) Hot/dry/windy conditions maximize evaporation → strong pull.
D. Underground stems have little transpiration.

Q47.

Guttation differs from dew in that guttation:
A. Is condensation of atmospheric water vapor
B. Is exudation of liquid water from leaf hydathodes ✅
C. Forms only in cold weather
D. Is visible on stem only

Explanation:

A. Dew is condensation.
B. (Correct) Guttation is internal water exudation via hydathodes.
C. Dew forms in cold, guttation not limited to cold.
D. Guttation occurs on leaves.

Q48.

The transpiration ratio refers to:
A. Ratio of CO₂ gained to water lost
B. Ratio of water lost to CO₂ gained ✅
C. Ratio of root pressure to leaf pressure
D. Ratio of osmotic pressure to turgor pressure

Explanation:

A. Inverted.
B. (Correct) Transpiration ratio = water lost / CO₂ fixed (hundreds of water molecules lost per CO₂ gained).
C/D. Not standard terms.

Q49.

Which method would best reduce transpiration experimentally in a potted plant?
A. Increase light intensity
B. Coat leaves with vaseline ✅
C. Provide abundant fertilizer
D. Increase wind velocity

Explanation:

A. Increases transpiration.
B. (Correct) Vaseline blocks stomata → prevents transpiration.
C. Fertilizer doesn’t directly affect transpiration.
D. Wind increases transpiration.

Q50.

Which structure is the final barrier that forces water and solutes to cross a membrane before entering the stele?
A. Epidermis
B. Cortex
C. Endodermis with Casparian strip ✅
D. Pericycle

Explanation:

A. Epidermis is initial barrier, not selective final checkpoint.
B. Cortex is ground tissue.
C. (Correct) Casparian strip in endodermis blocks apoplastic path → ensures selective membrane transport.
D. Pericycle lies inside stele, not the selective barrier.

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Part 2 — Long-Distance Transport of Water, Transpiration & Ascent of Sap (Q26–50)