Transpiration Class 10 ICSE Notes
ICSE Class 10 Biology Chapter 5 Transpiration Notes, Summary, Definition, Diagram. Transpiration Notes.
- Transpiration is a process of loss of water in the form of water vapour from the leaves and other aerial parts of the plants.
- It create a suction force in stem to enable the roots to absorb water and mineral nutrients, also cools plants in hot weather.
- The water vapour condenses only if the outside temperature is cool enough.
- Cobalt chloride paper is an indicator of moisture, blue when its dry and pink when exposed to moisture.
- In an experiment when plant is present and exposed to sunlight, moisture produced.
- But in absence of plant, there’s no moisture and hence no water vapour droplet seen.
Measurement of transpiration
- Weighing method – loss in weight of plant in sunlight (after fully covered plant) is due to loss of water by transpiration.
- Photometer method – a device that measures rate of water intake of plants.
Water intake almost equal to water loss through transpiration.
- Ganong’s photometer – also known as transpirometer, used to measure the transpiration rate in a laboratory.
Done by lifting the bent capillary tube above the coloured water so that air may be sucked is due to sucked is due to suction pull and is again dipped into the water.
Precautions in the use of photometer-
- Should be made compulsory water-tight.
- The twig should be cut obliquely and under water to avoid suction of air bubble into twig.
Limitations in the use of photometer –
- Twig may not remain fully alive for long time.
- Introducing the air bubbles is not very easy.
- Any changes in the outside air temperature may affect the position of air position of the bubble in the capillary tube.
I) Mechanism of Stomatal transpiration –
- After absorption by roots from soil, water vapour travel from stem and reaches the tissues of leaves through veins.
- Spongy mesophyll cells have surface of cells which gives out some of the water as thin film.
- The water from this film evaporates and water vapour formed.
- This vapour diffuses out from intercellular spaces and reaches into sub-stomatal space and this process of diffusing out of water from cell into atmosphere is diffusion.
- Molecules of water vapour (or gas) moves from higher concentration to the region of lower concentration.
- There is a turger pressure which forces water out of cell wall into the air spaces between the cells.
- Imbibitions –
- Transpiration pill thus created can draw water to about 50 metres or more in tall trees.
- There are stomatal opening on undersurface of dicot leaf therefore more transpiration occurs from the undersurface.
Stomatal regulation of transpiration
- Stomata are minute structures occuirring in large numbers on lower epidermis of leaf.
- Transpiration occurs as long as the stomata are open and stops when they are closed.
- Opening and closing mechanism of stomata is regulated by the amount of water and solutes present in guard cells.
- During day time, stomata open and diffusion of gases in and out starts fulfilling the need for photosynthesis as well as allowing transpiration.
- Water content of the leaf is falling short, the guard cells fail to remain turgid rather turn flaccid (lose turgidity) thereby closing the stomatal opening and transpiration stops.
- The rate of transpiration during midday inspite of the fact that there is plenty of water in the soil. Hence cell loses its turgidity.
- And during evening / night, the stomata are constricted and temperature is not high, no loss of water through transpiration and turgidity of leaves re-acquired.
II) Cuticular transpiration
- Cuticle is waxy layer secreted by the epidermis on the two surfaces of the leaf.
- Cuticle serves to prevent evaporation of water from the leaf surfaces.
- Greater the thickness of cuticle, lesser is the evaporation (transpiration).
III) Lenticular transpiration
- Special openings that develop on the barks of older stems in place of stomata.
- Allow diffusion of gases for respiration as well as for photosynthesis.
- Lenticels never close and remain open all time.
- Amount of transpiration from lenticels is certainly more than cuticular transpiration, but very much less than stomatal transpiration.
Factors that affect transpiration
- External factors
- Intensity of sunlight – more transpiration occurs during day
- Temperature – increase in temperature allows more water to evaporate. Warm air can hold more water than cold air.
- Velocity of wind – transpiration increases with the velocity of wind.
- Humidity – transpiration is reduced if the air outside is humid.
- Carbon dioxide – increase in CO2 level causes stomatal closure and decrease of transpiration.
- Atmospheric pressure – rate of transpiration increases with the decrease in atmospheric pressure. Thus, it enhances diffusion of water vapour.
- Internal factors
- Water content of leaves – water content of leaves decreases due to insufficient absorption of water by roots, the leaves wilt and transpiration is reduced.
Such reduction in transpiration is indirectly due to closure of stomata and also naturally conserves water within plants.
- Adaptations in plants to reduce excessive transpiration
- Sunken stomata
- Fewer stomata
- Narrow leaves
- Reduced exposed surfaces
- Loss of leave
- Thick cuticle
Significance/advantage of transpiration
i) Cooling effect – evaporation reduces temperature on leaf surface.
ii) Creating Suction force – helps in ascent of a sap by producing suction force acting from top of plant.
Osmotic pressure also takes part.
Suction force is produced at the top of plant drawing more water up through the stem and making the roots absorb more of it from soil.
iii) Distribution of water and mineral salts – higher the rate of transpiration, greater the rate of absorption of water and solutes from soil.
Transpiration affects climate –
- Transpiration increases moisture in the atmosphere and brings rain.
Difference between evaporation and transpiration
Evaporation |
Transpiration |
Loss of water from the surface of water bodies in vapour state | Loss of water in the form of vapour from aerial parts of the plant |
Physical change controlled by temperature and humidity of the atmosphere | Vital and partly a physical process controlled by both internal and external factors |
Fast process | Slow process |
Difference between guttation and bleeding
Guttation |
Bleeding |
Occurs from edges of leaves by hydathodes in injured plants | Occurs from any cut or injured part of plant |
Exudate is mainly in water with some dissolved mineral salts | Exudates is mainly plant sap and sugars |
Occurs early mornings or late nights | Occurs at the time of injury |
Ex- banana, strawberry, nasturtium | Ex- in all plants that have been cut or injured |
Difference between transpiration and guttation
transpiration | Guttation |
Water loss in vapour form | Water lost in water droplet form |
Pure water only changes into vapour | Water lost contains mineral salts in solution |
Water vapour lost through stomata, lenticels, cuticle | Water lost through pores (hydathodes) in ends of veins |
Loss of turgidity in leaf cells to cause wilting | Has no effect on turgidity of leaf |
Stomatal transpiration regulated by guard cells | Opening of hydathodes cannot be regulated |
Occurs in presence of sunlight | Occurs at night or early morning
|
Cools plant body | No cooling effect on plant |
Occurs during dry condition | In humid conditions |
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