Transpiration is the process where plants release water vapor, primarily through small pores called stomata on their leaves. This vital process serves multiple functions.
It generates a “transpirational pull,” drawing water and essential minerals from the roots to the rest of the plant, facilitating nutrient distribution. Transpiration also acts as a natural cooling system, much like sweating, by releasing water vapor to regulate the plant’s temperature and prevent overheating. Furthermore, it helps maintain cell turgidity, crucial for the plant’s structure and metabolic activities like photosynthesis.
While stomatal transpiration is the most common, water can also be released through the waxy cuticle layer (cuticular transpiration) and lenticels on woody stems (lenticular transpiration). Specialized guard cells regulate stomatal opening and closing, influenced by environmental factors such as light, darkness, and water availability. Lower carbon dioxide levels generally encourage stomatal opening.
Several environmental factors impact the rate of transpiration: increased light intensity and higher temperatures accelerate it. Wind speed enhances transpiration by removing humid air around leaves, while lower atmospheric pressure can have a slight increasing effect. Conversely, limited water in the soil causes plants to reduce transpiration to conserve moisture.
In summary, transpiration is fundamental for plant survival, supporting nutrient transport, temperature control, and structural integrity. Understanding these mechanisms and influencing factors is key to appreciating how plants adapt and thrive.
EXERCISE
(A) MULTIPLE CHOICE TYPE:
1) Transpiration pull will be maximum under which set of the following conditions?
(a) Open stomach, dry atmosphere and moist soil.
(b) Open stomata, high humid atmosphere and well irrigated soil.
(c) Open stomata, high humid atmosphere and dry soil.
(d) closed stomata, dry atmosphere and dry soil.
Ans: (a) Open stomata, dry atmosphere and moist soil
2) With decrease in atmospheric pressure, the rate of transpiration will
(a) increase
(b) decrease rapidly
(c) decrease slowly
(d) remain the same
Ans:(a) increase
3) The rate of transpiration is more when
(a) atmosphere is dry
(b) temperature is high
(c) humidity is high
(d) atmosphere is dry and temperature is high
Ans: (b) temperature is high
4) One of the internal factors which affect the rate of transpiration is
(a) big size of the leaf
(b) colour of the leaf
(c) sunken stomata
(d) sunny day
Ans: (c) sunken stomata
5) Guttation takes place through
(a) stomata
(b) lenticels
(c) lower epidermis of leaves
(d) hydathodes
Ans: (d) hydathodes
6) The loss of water as water vapour from the actual parts of a plant is known as
(a) evaporation
(b) perspiration
(c) guttation
(d) transpiration
Ans: (d) transpiration
7) Transpiration will be fastest when the day is
(a) cool, humid and windy
(b) hot, humid and still
(c) hot, humid and windy
(d) hot, dry and windy
Ans: (d) hot, dry and windy
8) Most of the transpiration in tall trees occurs through
(a) stomata
(b) Lenticels
(c) cuticle
(d) Bark
Ans: (b) Lenticels
9) Transpiration is best defined as
(a) loss of water by the plant
(b) evaporation of water from the surfaces of a plant
(c) loss of water, as water vapour, by a plant
(d) release of water by a plant into the atmosphere
Ans: (b) evaporation of water from the surfaces of a plant
B. VERY SHORT ANSWER TYPE:
1) Name the following:
(a) openings on the stem through which transpiration occurs
(b) The process by which the intact plant loses water in the form of droplets.
(c) An instrument used to find the rate of transpiration
(d) A plant in which the stomata are sunken
(e) The apparatus to record the rate of transpiration in a cut shoot.
(f) Any two parts of a leaf which allows transpiration.
(g) The structure in a leaf that allows guttation.
(h) Loss of water as droplets from the margins of certain leaves.
Ans: (a) Lenticles are pores on plant stems, bark, and some fruits, allowing for gas exchange and water vapor release.
(b) Guttation is the exudation of water droplets from intact plants, often occurring when root pressure is high due to abundant soil moisture and humid conditions.
(c) A potometer is an instrument used to measure the rate of water uptake by a plant shoot, providing an estimate of its transpiration rate.
(d) Plants like Nerium (Oleander) have sunken stomata, an adaptation commonly found in xerophytes to reduce water loss in dry climates
(e) The apparatus commonly used to record the rate of transpiration in a cut shoot is a potometer. There are various types, such as the Ganong’s potometer or the simple bubble potometer, all designed to measure the rate of water uptake by a plant, which is then assumed to be roughly equivalent to the rate of transpiration.
(f) The two main parts of a leaf that allow transpiration are the stomata (tiny pores, primarily on the lower epidermis, regulated by guard cells) and the cuticle (a waxy, protective layer on the leaf surface). While most transpiration occurs through stomata, a small amount of water can also be lost directly through the cuticle.
(g). These are specialized pores, often found at the margins or tips of leaves, that are connected to the plant’s vascular tissue. Unlike stomata, hydathodes are always open and are involved in the exudation of xylem sap as liquid droplets, rather than water vapor.
(h) The loss of water as droplets from the margins of certain leaves is called guttation. This phenomenon typically occurs when transpiration rates are low (e.g., at night, when humidity is high) and root pressure is high, forcing water out through the hydathodes. The droplets are not pure water but contain various dissolved substances from the xylem sap
2) Fill in the blanks:
(a) Transpiration is the loss of water as water …………… from the ………… parts of the plant.
Ans:(a) vapour, aerial
(b) Closing of …………… and shedding of leaves reduces …………..
Ans:(b) stomata, transpiration
(c) Transpiration helps in creating ………………. Force and in eliminating excess………….
Ans: (c) suction, water (heat)
C. SHORT ANSWER TYPE:
1) Given below is an example of a certain structure and its special functional activity: Chloroplasts and Photosynthesis.
In a similar way, write the functional activity against each of the following:
(a) Hydathodes and ……………………….
(b) Leaf spines and …………………………
(c) Lenticels and ……………………………
(d) Thick cuticle and ………………………..
Ans: (a) guttation
(b) protection and reduced transpiration
(c) transpiration
(d) reduced transpiration
2) (a) State whether the following statements are True (T) Or False (F)?
(i) Most transpiration occurs at midnight.
(ii) Transpiration creates a pull for upward movement of the sap.
(iii) Wind velocity has an effect on transpiration.
(iv) Voltmeter is an instrument used for measuring the rate of transpiration in green plants.
(b) Rewrite the false statements, in (a) above, in the correct form by changing either the first or the last word only.
Ans: (a) (i) False (ii) True (iii) True (iv) False
(b) (i) Most transpiration occurs at mid-day.
(iv) Potometer is an instrument used for measuring the rate of transpiration in green plants.
3) Give suitable explanation for the following:
a) A higher rate of transpiration is recorded on a windy day rather than on a calm day.
b) Excessive transpiration results in the wilting of the leaves.
c) Water transpired is the water absorbed.
d) More transpiration occurs from the lower surface of a dorsiventral leaf.
e) Cork and bark of trees help in preventing loss of water.
f) Perspiration and transpiration help to cool the body temperature of the organism.
g) On a bright sunny day, the leaves of certain plants roll up.
Ans: a) Wind speeds up transpiration by removing humid air around leaves, keeping the water vapor gradient steep.
b) Too much transpiration causes wilting because water loss outpaces root absorption, reducing cell turgor.
c) Healthy plants maintain a crucial balance between water absorbed by roots and water released through transpiration from leaves.
- Nutrient Transport: It carries dissolved nutrients from the soil to all parts of the plant, essential for growth and metabolism.
- Cooling: Transpiration acts like sweating, cooling the plant during hot weather and preventing cellular damage.
- Structural Support: Water’s properties and the negative pressure from transpiration create turgor pressure, keeping plant cells firm and leaves upright, which is crucial for photosynthesis.
In essence, the regulated balance of water uptake and release is fundamental for a plant’s health, growth, and survival.
d) Dorsiventral leaves lose more water from their underside due to a higher concentration of stomata there.
e) Cork and bark on trees prevent water loss as their water-resistant cells form a protective barrier.
f) Both sweating (perspiration) and plant transpiration cool by using the heat-absorbing process of water evaporation.
g) Some plants curl their leaves on sunny days to conserve water by reducing exposed surface area and enclosing stomata.
4) Which of the following statements are true and which ones are false? Give reason in support of your answer.
(a) Potometer is an instrument used for Demonstration of transpiration occurring from the lower surface of a leaf.
(b) Forests contribute to bringing rains.
(c) Hydathodes are similar to stomata in plant physiology.
(d) Atmospheric humidity promotes transpiration from a green plant.
(e) Some desert plants have sunken stomata on their leaves.
(f) Most transpiration occurs during midday.
Ans:(a) False. A potometer measures the overall rate of water absorption, indicating total transpiration from all plant surfaces, not specifically the lower leaf surface.
(b) True. Forests release significant water vapor through transpiration, contributing to cloud formation and rainfall.
(c) False. Stomata regulate gas exchange and transpiration, while hydathodes are always open and involved in guttation (liquid water release).
(d) False. High atmospheric humidity reduces the water potential gradient, thereby decreasing transpiration.
(e) True. Sunken stomata in desert plants create humid microclimates, reducing water loss in arid environments.
(f) True. Transpiration peaks at midday due to higher temperatures and light intensity, which increase evaporation and promote stomatal opening.
5) Differentiate between guttation and bleeding in plants.
Ans:
| guttation | bleeding |
| The exudation of water droplets from the tips or margins of leaves. | The oozing or leakage of sap from a cut or wounded part of a plant. |
| Occurs through specialized pores called hydathodes | Occurs when the plant’s vascular system (xylem or phloem) is damaged. |
| Typically happens during the night or early morning when root pressure is high and transpiration is low (due to high humidity). | Can happen at any time when there’s internal sap pressure and an injury. |
| The exuded fluid is primarily water, sometimes with dissolved minerals and sugars | The exuded fluid is nutrient-rich sap, containing water, sugars, hormones, and other organic compounds |
D. LONG ANSWER TYPE:
1) What is wilting? Some plants show the wilting of their leaves at noon even when the soil is well watered, Why is it so?
Ans: At noon, despite sufficient soil moisture, some plants wilt because intense midday heat and sunlight accelerate transpiration beyond the plant’s capacity for water absorption. This temporary water deficit causes cells to lose rigidity, leading to drooping. As temperatures cool later in the day, transpiration slows, allowing the plant to rehydrate and recover.
2) Why are the stomata in most plants more numerous on the lower surface of a leaf instead of being on the upper surface?
Ans: Stomata are more abundant on the lower surface of most leaves primarily to minimize water loss through transpiration. The lower surface is typically cooler and less exposed to direct sunlight and wind, all of which reduce the rate of evaporation. If stomata were predominantly on the upper surface, they would be more susceptible to the direct dehydrating effects of the sun and air currents, leading to excessive water loss and a higher risk of desiccation for the plant.
3) Suppose you have a small rose plant growing in a pot. How would you demonstrate transpiration in it?
Ans: To show transpiration in a potted rose plant, water it well the day before. Then, cover a few leaves and a small branch with a clear plastic bag, sealing it around the stem. Place the plant in a well-lit spot, avoiding harsh sun, for a few hours. You’ll see tiny water droplets inside the bag. This happens because the leaves release water vapor, which then condenses into visible droplets on the cooler bag surface.
4) What is a photometer?
Ans: A photometer is an instrument used to measure the intensity, or strength, of light. It can measure various aspects of light, including visible light, ultraviolet, and infrared radiation. Many photometers work by converting light into an electrical current, which then provides a quantifiable reading. They are commonly used in fields like photography, science, and industry to assess brightness, absorption, reflection, and other light-related properties.
5) What is lenticular transpiration? Mention one major difference between lenticular transpiration and stomatal transpiration.
Ans: Lenticular transpiration is the passive release of water vapor through lenticels, which are permanent openings on woody stems and some fruits. Unlike stomatal transpiration, which is precisely controlled by the plant through the opening and closing of stomata on leaves in response to environmental factors, lenticular transpiration is largely unregulated, leading to continuous water loss
6) List any three major factors that accelerate the rate of transpiration.
Ans: Three primary factors boost transpiration rates. Firstly, higher temperatures supply the necessary energy for water to evaporate from leaf surfaces more readily. Secondly, low humidity in the air creates a strong pull, as the drier atmosphere encourages more water to move out of the leaf due to a steeper water potential difference. Lastly, wind plays a role by continuously removing the water vapor that accumulates around the leaf, which helps maintain that critical water potential gradient and keeps evaporation going.
7) There is a general belief that forests tend to bring more frequent rains. Can you explain it scientifically?
Ans: Forests significantly influence local and regional rainfall through several interconnected mechanisms. Trees release substantial water vapor into the atmosphere via evapotranspiration, enriching the air with moisture essential for cloud formation. Additionally, forests emit volatile organic compounds and aerosols, which serve as cloud condensation nuclei, providing surfaces for water vapor to condense and form cloud droplets. The rough, uneven forest canopy also creates friction, forcing air to rise and cool, which can lead to condensation and precipitation. Locally, temperature differences between shaded forest areas and warmer open land can generate convection currents, drawing in moist air and promoting rain cloud development. Finally, forests are highly efficient at recycling moisture; water transpired by trees forms clouds that release rain back over the forest, perpetuating a cycle that sustains rainfall within the ecosystem and influences downwind precipitation patterns.
8) List the four advantages of transpiration to the plants.
Ans: Transpiration offers several key benefits to plants. Firstly, it acts as a crucial pump, pulling water and vital minerals upwards from the roots to the furthest leaves. Secondly, the evaporation of water from the leaf surface provides a cooling effect, preventing the plant from overheating, particularly in warm environments. Thirdly, this continuous water movement is essential for maintaining turgor pressure within plant cells, which in turn supports the plant’s overall structure. Finally, the necessary opening of stomata for transpiration also facilitates the vital exchange of gases, allowing the plant to take in carbon dioxide for photosynthesis and release oxygen.
9) Mention any three methods by which the plants tend to reduce transpiration.
Ans: Plants are remarkably resourceful in conserving water. Their primary defense is regulating their stomata, the microscopic pores on their leaves, by closing them during periods of drought or intense heat. Additionally, many plants sport a thick, waxy cuticle on their leaves, forming a protective shield against evaporation. To further minimize water loss, some species have evolved smaller, needle-like leaves or even shed them entirely in dry conditions, effectively reducing the surface area available for transpiration.
10) Droplets of water may sometimes be seen along the margins of the leaves of a banana plant, growing in wet soil, in the mornings. Are these dew drops? Comment upon your answer.
Ans: No, these are most likely not dew drops.
These droplets are a result of guttation. Since the banana plant is growing in wet soil, it absorbs a lot of water. When transpiration rates are low (as they often are in the morning, especially if the air is humid), the plant’s roots continue to absorb water, creating root pressure. This pressure forces water out through specialized pores called hydathodes, located along the leaf margins, forming visible droplets. Dew, on the other hand, is water vapor from the air that condenses onto surfaces when they cool below the dew point.
11) Briefly explain how the rate of transpiration is affected by (a) Intensity of light (b) Humidity of the atmosphere
Ans:(a) Light Intensity: Brighter light leads to more transpiration. This is because light causes stomata to open wider, increasing water vapor release, and also warms the leaves, speeding up evaporation.
(b) Humidity: Low humidity increases transpiration. Dry air creates a steeper gradient for water vapor to move from the leaf to the atmosphere, drawing water out faster. High humidity, conversely, slows this process by reducing the difference in water vapor concentration.
E. STRUCTURED / APPLICATION / SKILL TYPE:
1) In an experiment, four freshly plucked leaves (A-D) of a plant, such as those of china – rose, were treated as follows:
(a) Coated with Vaseline on its upper surface.
(b) coated on the lower surface.
(c) coated on both surface
(d) left uncoated.
All the four leaves A, B, C & D were left in a room for about 24 hours.
(i) which leaf would become the most limp? Why?
(ii) which leaf would show least limping? Why?
Ans:(i) Leaf D (uncoated) would become the most limp. This is because it has stomata on both its upper and lower surfaces, allowing for maximum water loss through transpiration.
(ii) Leaf C (coated on both surfaces) would show the least limping. Coating both surfaces with Vaseline blocks almost all stomata, severely restricting water loss through transpiration and thus helping the leaf retain turgor.
2) Given below is a diagrammatic sketch (surface view) of a stomatal apparatus from a dicot leaf.
(a) Label the parts numbers 1-3
(b) Is this state, open or closed?
(c) Is this stoma, of a dicot leaf or a monocot leaf?
(d) Redraw a sketch of the stomatal apparatus in the state opposite to the one shown here.
Ans: (a) Labeling:
- Guard Cell: Bean-shaped cells regulating the pore.
- Nucleus: Controls guard cell activities.
- Stomatal Pore: Central opening for gas exchange and transpiration.
(b) Stoma State: Open, indicated by turgid, curved guard cells creating a gap.
(c) Leaf Type: Dicot leaf, characterized by kidney-shaped guard cells.
(d) Closed Stoma (Description): Guard cells would appear flaccid, straightened, and close together, reducing or eliminating the stomatal pore.
3) Given alongside is the diagram of an experimental set-up to demonstrate a certain phenomenon in plants.
(a) Name the phenomenon being demonstrated.
(b) What is the purpose of putting oil in the test tube?
(c) Would it make a difference if the experimental set – up is kept in bright sunshine?
(d) what is the purpose of the spring balance in the set-up?
Ans:(a) Phenomenon: Transpiration.
(b) Purpose of Oil: To prevent water evaporation from the test tube’s surface, ensuring that any measured weight loss is solely due to transpiration from the leafy shoot.
(c) Effect of Sunshine: Sunshine increases the rate of transpiration, causing the spring balance reading to drop more quickly due to greater water loss from the plant.
(d) Purpose of Spring Balance: To measure the decrease in the weight of the test tube and its contents, which directly indicates the amount of water lost by the leafy shoot through transpiration.
