what is Transpiration

Transpiration is defined as the removal of excess water from plants into the atmosphere in form of water vapour.
Plants are capable of loosing excess water through
(i) The stomata in the leaves and this is called stomata transpiration
(ii) Through the lenticels in the stem and this is called lenticular transpiration.
(iii) Through the cuticle of the leaf surface in what is called cuticular transpiration.

Conditions Affecting the Rate of Transpiration

The rate at which water vapour is lost by a plant depends on a number of factors which include:
(1) The size of the stomata pores: when stomata opens due to turgidity of the guard cells, transpiration takes place while flaccidity of the cells causes the guard cells to close and prevent transpiration from taking place.
(2) Humidity: the higher the humidity of the atmosphere the slower the rate of transpiration while the lower the humidity the higher the rate of transpiration.

(3) Temperature: increase in temperature give rise to high rate of transpiration while low temperature gives rise to low rate of transpiration.
(4) Light: high light intensity results in high rate of photosynthesis and consequently leads to increase in temperature, thereby giving rise to high rate of transpiration and vice versa.
(5) Wind: the higher the rate or speed of wind the higher the rate of transpiration and vice versa.
(6) Soil water: the higher the level of soil water the higher the rate of absorption and consequently the higher the rate of transpiration and vice versa.

Importance or Advantage of Transpiration to plants

Transpiration has the following importance or advantages to plants:
(i) It enables plants to absorb water and mineral salts from the soil.
(ii) It facilitates the movement of soil water.
(iii) The evaporation of water due to transpiration from the plants cools the plants.
(iv) It helps to remove excess water from the plants.

Experiment II

Aim: to demonstrate transpiration in plants.
Materials required: bell jar, leafy plant or twig, polythene bag, plastic sheet, oil layer, droplet of liquid, pot or beaker, glass sheet and Vaseline paste.
Method: the experiment is set up as shown in the picture below. When a potted plant is used, the soil surface should be covered with plastic sheet but when a leafy twig dipped in a beaker of water is used, the water surface should be covered with oil to prevent evaporation of water into the bell jar.
All joints in the setup must be air tight. The set up should be placed on a glass sheet with a bell jar inverted over the plant. All joints are greased with Vaseline paste to prevent gaseous water entry into the bell jar. The whole setup is placed in the sunlight or near the window for about 2—5 hours. In the control experiment, plant without leaves is used.
Observation: it is observed that droplets of liquid are seen on the inner surface of the bell jar in the main experiment while no liquid droplet is seen in the control experiment. He liquid is then collected and tested with blue cobalt chloride paper which turn pink, indicating that the liquid droplet is water.
Conclusion: green plants transpire.

Experiment III

Aim: to demonstrate the rate of transpiration in plant using potometre.
Materials required: potometre, water, leafy shoot cut under water, graduated scale and capillary tube.
Method: the experiment is setup as shown in the figure above. A leafy shoot is cut under water so that no air bubbles enter the xylem cells and it is inserted as shown in the diagram. As water is lost from the leaves by transpiration, it is drawn up from the end that is cut, causing air to be drawn into the open end of the capillary tube. All joints should be air tight and sealed with Vaseline paste.
Observation: the air bubble moves along the capillary tube. Increase in the speed of wind or high temperature in the environment tends to increase the rate of movement of air bubble along the capillary tube.
Conclusion: the presence and movement of air bubble in the capillary tube can be used to compare the transpiration rate under different environmental condition.

Experiment IV

Aim: to determine the tissue that conducts water.
Materials required: young seedling, e.g. water leaf plant (talinium triangulae) or amaranthus, knife, red ink, (eosin solution), water, beaker or microscope or hand lens.
Method: collect the young seedling and wash the roots under water to remove soil (do not pull the roots to avoid damage). The seedling is then placed in a beaker with roots completely covered by a solution of red ink or eosin solution. The seedling is allowed to stand for at least two hours. Then cut a transverse section of the stem above the solution and examine under the hand lens or microscope.
Observation: the red ink will be seen only in the xylem tissues.
Conclusion: this shows that the xylem vessels are responsible for the conduction of water from roots to leaves of plants.

Absorption of Water by Roots of Plants

The young root hairs of flowering plants have direct contact with water in the soil. The cell sap in the root hairs is more concentrated than the soil water, hence water is able to pass from the soil into the root hairs by osmosis. The water passes through the thin layer of cytoplasm or cell membrane which is selectively permeable into the vacuole of root hairs. The extra water raises the tugor pressure of the vacuole or reduces the osmotic pressure and forces water out into the cell walls towards the cortex. The cell next to the root hair cell on the inside has a lower tugor or higher osmotic pressure, hence water will pass into it by osmosis. In this way, the water absorbed will get to the xylem vessels.

Transport of Water in the Xylem Tissue

Transport of water in the xylem tissue is due to the following processes:
(i) Root pressure and suction pressure: root pressure is usually created as a result of differences in osmotic pressure between the cell sap and the concentration of cell nutrients. The cell sap being more concentrated, tends to draw up the nutrients. On the other hand, suction pressure is the total force by which the cell absorbs water from its surroundings. The suction pressure is normally created when water is lost in form of transpiration through the stomata of the leaves. By these pressure, the movement of water from the soil to the xylem tissues through the root hairs is achieved.
(ii) Capillary action: the upward movement of water through the xylem is mainly achieved through capillary action. The xylem vessels that extend from the roots to the leaves form a very fine capillary tubes. Water then rises up such tubes as a result of capillary action. Capillary action is due to the attraction between the water molecules and the walls of the xylem vessels.
(iii) Transpiration pull: the continuous flow of water from the roots to the leaves forms the transpiration stream. As water evaporates from the leaf cells and as photosynthesis produces more sugar, the osmotic pressure in the leaf cells increases with respect to that in the xylem cells. This causes more water to flow into the leaf cells from the xylem vessels. So, there is a pull on the water columns in the xylem vessels and water is drawn up in the plant.

Experiment V

Aim: to demonstrate root pressure.
Materials required: manometer, a young plant, knife, rubber tubing
Method: the experiment is setup as shown in the figure below. Cut the stem of the young plant about 9-12 cm from the ground with the knife. Attach the stump with the aid of rubber tubing into the manometer and allow the experiment to remain for few hours.
Observation: the watery sap is seen to force the mercury upward. This is due to root pressure. The amount of root pressure can be determined by measuring the difference in the level of mercury in the two limbs of the manometer.
Conclusion: he mercury difference in the two limbs shows that root pressure has taken place.

Experiment VI

Aim: to demonstrate transpiration pull
Materials required: a young plant cut under water, retort stand, clamps and a mercury manometer
Method: the experiment is setup as shown in the figure below. Cut a young shoot under water and connect the shoot by means of rubber tubing to a glass tube filled with water. The opposite end of the glass tube must be closed with the thumb. The lower end below the mercury surface is placed in a trough. Support the setup with clamps and retort stand. Allow the plant to carry out transpiration.
Observation: the mercury level is seen to have risen due to transpiration pull since water is lost through transpiration.
Conclusion: the force created as a result of the rise in mercury level is known as transpiration pull.

Similarities between transpiration and sweating

Both involve loss of water from the body of the organisms. Both processes result in cooling. Water is lost through pores.

Differences between transpiration and sweating

1. Occurs in plants through stomata or lenticels
2. Transpiration involves only loss of water
3. Water is lost in the form of vapour
4. Occurs during the day
1. Occurs in mammals/skin/through sweat pores
2. Loss of water, salts and nitrogenous
3. Water loss is liquid in form
4. Occurs both day and night


1. Tubular or cylindrical vessels are necessary in plants and animals.
2. Liquid medium is required for transportation in plants and animals.
3. Materials or food nutrients and hormones are transported in dissolved or fluid form.
4. Diffusion plays a major role in transportation in both plant and animals.


1. Cell sap is the medium of transportation
2. Root pressure or transpiration generates forces for pull
3. Water/mineral salts manufactured and food are transported through different vessels (xylem and phloem)
4. The transport medium is not tissue.

1. Blood is the medium of transportation
2. Heart generates forces for transport of nutrients
3. Water food substances and mineral nutrients are transported in the same vessels.
4. The transport medium is made up of cells of different types or tissues.



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