Transpiration Practice Questions
19 free Transpiration practice questions for the General Science. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
What is the primary definition of transpiration in plants?
- A The absorption of water from the soil by root hair cells
- B The loss of water as vapor from the aerial parts of the plant
- C The transport of prepared food materials through the phloem
- D The upward movement of sap through the xylem vessels of the stem
Correct answer: The loss of water as vapor from the aerial parts of the plant
Transpiration is the biological process where water is lost as water vapor through specialized structures like stomata. It is distinct from evaporation as it is regulated by the plant's physiological mechanisms.
Which of the following structures is responsible for the majority (about 90%) of transpiration in most terrestrial plants?
- A Lenticels
- B Cuticle
- C Stomata
- D Hydathodes
Correct answer: Stomata
Stomatal transpiration is the most significant form of water loss. Stomata are tiny pores found mostly on the underside of leaves that open and close to regulate gas exchange and water loss.
The cells that surround the stomatal pore and control its opening and closing are called:
- A Mesophyll cells
- B Cork cells
- C Guard cells
- D Subsidiary cells
Correct answer: Guard cells
Guard cells are specialized epidermal cells containing chloroplasts. Their turgidity, influenced by water and ion concentration, determines whether the stomatal pore is open or closed.
Which instrument is used to measure the rate of transpiration by measuring the amount of water absorbed?
- A Auxanometer
- B Potometer
- C Hydrometer
- D Barometer
Correct answer: Potometer
A potometer (such as Ganong's potometer) measures the rate of water uptake by a plant shoot. Since almost all water absorbed is transpired, it is a reliable tool for estimating transpiration rates.
How does an increase in external humidity affect the rate of transpiration?
- A It increases the rate.
- B It decreases the rate.
- C It has no effect on the rate.
- D It causes the stomata to open wider.
Correct answer: It decreases the rate.
Transpiration depends on the diffusion gradient between the leaf interior and the atmosphere. High humidity reduces this gradient, thereby slowing down the rate of water vapor loss.
The 'transpiration pull' or suction force that pulls water up from the roots is best explained by which theory?
- A Vital force theory
- B Godlewski's relay pump theory
- C Cohesion-tension theory
- D Root pressure theory
Correct answer: Cohesion-tension theory
Proposed by Dixon and Joly, the cohesion-tension theory explains that water molecules stick together (cohesion) and to xylem walls (adhesion), forming a continuous column pulled upward by transpiration.
During the day, stomata usually open due to the influx of which ions into the guard cells?
- A Sodium ions ($Na^+$)
- B Calcium ions ($Ca^{2+}$)
- C Potassium ions ($K^+$)
- D Magnesium ions ($Mg^{2+}$)
Correct answer: Potassium ions ($K^+$)
The active transport of potassium ions into guard cells lowers their water potential. This causes water to enter the cells via osmosis, making them turgid and forcing the stomatal pore to open.
What is the term for the loss of liquid water droplets from the margins of leaves, often mistaken for dew?
- A Transpiration
- B Guttation
- C Evaporation
- D Bleeding
Correct answer: Guttation
Guttation occurs through specialized structures called hydathodes when root pressure is high and transpiration is low, usually at night. Unlike transpiration, water is lost in liquid form.
Which of the following is an adaptation in xerophytes (desert plants) to reduce transpiration?
- A Thin cuticles
- B Large leaf surface area
- C Sunken stomata
- D Increased number of stomata on the upper surface
Correct answer: Sunken stomata
Sunken stomata are located in pits or depressions, which trap a layer of moist air near the pore. This reduces the diffusion gradient and effectively minimizes water loss in arid environments.
In dorsiventral leaves (dicots), stomata are typically more numerous on the:
- A Upper epidermis
- B Lower epidermis
- C Both surfaces equally
- D Leaf margins
Correct answer: Lower epidermis
The lower epidermis is usually shaded and cooler than the upper surface. Having more stomata on the bottom side helps the plant reduce excessive transpiration caused by direct sunlight.
Which plant hormone is known as the 'stress hormone' because it induces stomatal closure during water scarcity?
- A Auxin
- B Gibberellic acid
- C Abscisic acid
- D Cytokinin
Correct answer: Abscisic acid
Abscisic acid is produced under drought conditions. It signals the guard cells to release solutes, leading to a loss of turgidity and the rapid closure of stomata to conserve water.
Which of the following environmental factors will significantly increase the rate of transpiration?
- A Still air (no wind)
- B Darkness
- C Low temperature
- D High wind speed
Correct answer: High wind speed
Wind removes the saturated 'boundary layer' of air from the leaf surface. This maintains a steep concentration gradient between the inside and outside of the leaf, accelerating transpiration.
Stomata that remain open at night and closed during the day are characteristic of:
- A C3 plants
- B C4 plants
- C Hydrophytes
- D CAM plants
Correct answer: CAM plants
Crassulacean Acid Metabolism (CAM) plants, like cacti and pineapples, open stomata at night to fix $CO_2$ when it is cooler. This is an extreme water-conserving adaptation for desert survival.
The 'Wilting Point' of a plant refers to the stage where:
- A The plant begins to flower and set seed rapidly.
- B The soil holds more water than the roots can absorb.
- C The plant halts respiration but continues growing.
- D Transpiration exceeds the rate of water absorption.
Correct answer: Transpiration exceeds the rate of water absorption.
Wilting occurs when the plant cells lose turgor pressure. This happens when the water lost through transpiration cannot be replaced quickly enough by the roots from the soil.
What happens to the rate of transpiration when light intensity increases?
- A It decreases because the stomata close and the leaf cools.
- B It stays constant regardless of the light intensity.
- C It stops completely within minutes.
- D It increases because stomata open and the leaf warms up.
Correct answer: It increases because stomata open and the leaf warms up.
Light triggers the opening of stomata for photosynthesis and increases the temperature of the leaf. Both factors lead to a higher rate of water vapor diffusion out of the plant.
Which structure allows for 'Lenticular Transpiration'?
- A Openings in the bark of woody stems
- B Tiny pores in the epidermis of the leaf
- C Openings at the tips of veins
- D Waxy layers on the fruit surface
Correct answer: Openings in the bark of woody stems
Lenticels are small, permanent openings in the bark of woody stems and some fruits. Unlike stomata, they do not have a mechanism to open or close, allowing a small constant amount of water loss.
The movement of water through the cell walls and intercellular spaces (not crossing the plasma membrane) is called:
- A Apoplast pathway
- B Symplast pathway
- C Transmembrane pathway
- D Vacuolar pathway
Correct answer: Apoplast pathway
The apoplast pathway is the movement of water through the non-living parts of the plant. During transpiration, water moves via the apoplast until it reaches the endodermis of the root.
Which of the following chemical substances can act as an 'antitranspirant'?
- A Phenylmercuric acetate
- B Gibberellic acid
- C Magnesium sulphate solution
- D Sodium chloride
Correct answer: Phenylmercuric acetate
Antitranspirants such as phenylmercuric acetate are applied to leaves to reduce water loss. They work either by inducing partial stomatal closure or by forming a thin film that is impermeable to water vapor.
In a transpiring plant, the water potential is lowest in the:
- A Root hairs
- B Xylem of the stem
- C Leaf mesophyll cells
- D Atmosphere
Correct answer: Atmosphere
Water always moves from a region of higher water potential to lower water potential. The atmosphere usually has the lowest (most negative) water potential, which 'pulls' the water out of the leaf.