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.
Aim: To prove that plants transpire
Apparatus; bell jar, potted plants polythene bag.
-Make sure that the bottom is smooth and add Vaseline to ensure that it
-Tie the potted plant with a polythene bag to ensure that the moisture
is from non other than the plant.
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.
Droplets of colourless liquid droplets are seen more in A than in B
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.
Avoid the main veins to ensure a tight seal.
The lower part turns the cobalt chloride paper pink faster than the upper
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.
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.
A high humidity will lead to a low transpiration rate because the surrounding
of the plant will already be circulated with water vapour.
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
Light affects stomatal opening and closing called stomatal aperture.
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
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
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