Transpiration is the loss of water by leaves into the atmosphere.

it is useful in that it enables the plant to be able to take in water again through its roots.

It also enables this water to rise up the stem, by what is refered to as the transpiration pull.

Transpiration is the loss of water vapour from aerial parts of plants into the atmosphere. Plants can loose water from their leaves as a liquid through pores called hydathodes, it is not transpiration but rather this process is called guttation.

Types of transpiration;
Most of the transpiration is through stomata an estimated 80%.
1:Stomatal (takes place in leaves and green stems).
2:Lenticular: Parts of the stem, which are not green, are called lenticels. Some transpiration takes place through these.
3:Cuticular: Some transpiration takes place through the cuticle though it has no stomata.

Experiment:

Aim: To prove that plants transpire
Apparatus; bell jar, potted plants polythene bag.
Procedure:
-Make sure that the bottom is smooth and add Vaseline to ensure that it is airtight.
-Tie the potted plant with a polythene bag to ensure that the moisture is from non other than the plant.
Method2
Apparatus polythene bag
-Tie a polythene bag tightly on a branch with leaves. Likewise tie another polythene on a branch with out leaves as a control experiment.
Observation
Droplets of colourless liquid droplets are seen more in A than in B
Conclusion
Green plants transpire

To find which side of the leaf loses more water.
-We shall need square of cobalt paper.
-We use adhesive tape to stick a small piece of cobalt chloride paper on the upper and lower surfaces of the leaf.
Precautions;
Avoid the main veins to ensure a tight seal.
Observation:
The lower part turns the cobalt chloride paper pink faster than the upper one
Deduction
The lower surface loses more water than the upper one. This is because there are more stomata on the lower surface than the upper one. This is to reduce transpiration.

A potometer is used to compare the rates of shoot in different situations that is various environmental conditions for example in a cool shade outdoors, a shade in the laboratory, in sunshine in a sheltered place, In sunshine in a droughty place and in a warm humid place such as a fume cupboard in which a beaker of water is boiling.
Setting up a potometer.
A leafy shoot is cut under water and fixed to the potometer. This is done to prevent air from entering the xylem vessels. When you get the potometer out of the water, you flood the apparatus to sweep out air bubbles by opening the tap of the air funnel. The end of the capillary tube must not stick out below the bung or air will be trapped there. When all the air has been expelled from the potometer, close the tap. Air will gradually enter the capillary tube as water is sucked along it. If a ruler is placed behind the capillary tube the time taken for the column of water to move along the tube may be noted. The same bubble can be used over and over again because it can be sent back when the reservoir tank is opened.
A potometer really measures the rate of water intake rather than transpiration. Here we assume that the rate of water intake is equal to rate of water loss in that case transpiration.
Environmental Factors that affect the rate of transpiration.
Temperature;
A high temperature increases the rate of evaporation of water from the mesophyll cells. So when the temperature is high the rate of transpiration will be high. A high temperature also increases the capacity of the atmosphere for water vapour.
Humidity
A high humidity will lead to a low transpiration rate because the surrounding of the plant will already be circulated with water vapour.
Wind
When there is wind, the rate of transpiration is high because wind blows away water vapour from around the leaves creating more room for evaporation. When there is still air then the surroundings of the leaf will be saturated with water
Light Intensity
Light affects stomatal opening and closing called stomatal aperture.
Atmospheric Pressure
When the pressure is low, the rate of transpiration is high.
Control of Transpiration;
1:Leaf fall. Some plants during the day season shade their leaves in the tropics to reduce loss of water by transpiration. In winter some plants also shade their leaves for the same reason. This is because there is less supply of water during winter.
2:Plants have a waxy cuticle, which is water proof and this reduces water loss by evaporation.
3:Plants have more stomata on the lower side than on the upper side. This also helps to reduce transpiration because the upper side is directly affected by heat.
4:The shape of leaves. Some plants have leaves with a small surface area. This reduces transpiration. In some plants it so reduced that leaves appear as thorns.
5:There are plants with sunken stomata and this reduces the rate of transpiration.
6:Some plants have hairy stomata, which shed off some heat thus reducing transpiration.
Evaporation of water from the leaf.
Mesophyll cells cause their turgor to fall and the concentration of their cell sap to rise therefore increasing their osmotic pressure. Cells in this conditions will absorb water from their neighbours deeper in the leaf and eventually from the xylem vessels in the leaf. Withdrawal of water from the xylem produces a tension that is the water is submitted to pressures below atmospheric. This tension draws water up the vessels of the stem from the roots. The flow of water is called transpiration stream and is dependent on the rate of evaporation from the leaves. This is possible because of the fairly strong adhesive forces between the water molecules.