Rate of Transpiration

Transpiration is a process by which plants lose water through the leaves. Plants transpire the most from their leaves. In the plant kingdom, there are different species of plants. The angiosperms are divided into monocots and dicots. In monocots, the distribution of stomata is equal on both surfaces so the rate of transpiration is the same whereas, in dicots, the number of stomata is more on the lower surface and less on the upper surface so the rate of transpiration is low on the upper surface and high on the lower surface.  

We will study in detail leaves, stomata, a structure involved in photosynthesis, and the rates of transpiration in the upper and lower surfaces of leaves. This all will help us to make a comparative study of the rates of transpiration in the upper and lower surfaces of leaves.  

The Leaf

It is a lateral and flat structure that is born on the stem. It bears a bud in its axil and develops at the node. The leaf is generated from shoot apical meristem. It is arranged in the acropetal order of the stem. As we read above that they are the most important vegetative organs of the plant for photosynthesis. 

A Leaf Structure is Made of Three Parts: 

• Leaf Base: 

It is the lowermost part of the leaf and is attached to the node of the stem. They bear stipules. Stipules are small leaf-like structures. In monocots, this leaf base covers the stem partially or completely. It is called pulvinus when it swells in some leguminous plants. 

• Petiole: 

The leaf base and the leaf blade are joined by the petiole. The petiole help in holding the leaf above so that it gets proper sunlight and air. As the petiole is flexible in nature, it helps the leaf to swirl in the winds. This helps in providing a cooling effect on the leaves. 

• Lamina: 

It is also called leaf blade and it is an extended part of the leaf. Veins and veinlets are present on it. The midrib is the prominent vein that is present in the middle of the lamina. The veins are responsible for providing support and strength to the leaf and also they help in carrying out the transport of water and minerals across the leaf. Thus they act as a channel of transport. The shape, surface, and margin vary in different leaves. 

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Transpiration

The process of transpiration will provide us with a fair idea in making a comparative study of the rates of transpiration in the upper and lower surface of leaves. Transpiration is a process by which water is lost from the plants in the form of water vapour. As the loss of water takes place, it can sometimes also lead to the death of the plant. Though this process can affect the plant badly, still it is very important for the plants. It helps in the water and mineral absorption, ascent of sap, and also helps in lowering the temperature of the plant. Stomata in the leaves are mainly responsible for it. This process is almost similar to evaporation. There are 4 types of transpiration:

• Stomatal Transpiration: 

As the name suggests, it occurs through the stomata. The epidermis portion of the leaf has a very high amount of stomata. This method is responsible for 60-90% of transpiration in plants.

• Bark Transpiration: 

It occurs from the bark of the tree. Only 1% of transpiration takes place in this process. 

• Lenticular Transpiration: 

This process occurs through the lenticels. The lenticels are small pores in woody stems. Very less, almost less than 1% water is lost through this process. 

• Cuticular Transpiration: 

It occurs through the cuticles of the epidermis. The cuticle covers the epidermis. It is a waxy covering. It may help in reducing the water loss but gives out water through small pores. 

Factors Affecting Transpiration

There are Five Factors that Affect the Rate of Transpiration. They Are: 

• Light: 

The maximum opening of stomata is done by blue light. When it is absent, the stomata remain closed. Light also increases the temperature thus affecting the rate of transpiration. Blue and red light are the most effective in the process of transpiration. 

• Wind: 

Water vapours can accumulate on the surface of the leaf and then lower the transpiration. Thus, wind helps in increasing the rate of transpiration. The wind removes the humidity from the leaves and thus brings fresh air. 

• Soil Water: 

If the amount of water in the soil is not enough, then the rate of transpiration is also lowered. If more salts are present in the soil then also the rate of transpiration is decreased. 

• Relative Humidity: 

The rate of transpiration increases when the level of humidity is low. High humidity decreases the rate of transpiration. 

• Temperature: 

The rate of transpiration is increased by an increase in temperature. 

Stomata

Stomata are responsible for carrying out the process of transpiration. The more the amount of stomata, the more transpiration occurs. We will study now that where is their amount more. This will help in making a comparative study of the rates of transpiration in the upper and lower surface of leaves.

The stomata are tiny apertures that are found on the epidermis of the leaves. Guards cells are the two specialized epidermal cells that surround each stoma. They differ in their shape from the epidermal cells. The outer wall of the guard cells is thin while the inner wall is thick and elastic. The turgidity in the guard cells is responsible for the opening and closing of the stomata. The guard cells are further bordered by subsidiary cells. In monocots, the number of stomata is equally distributed on the surface of the leaf. Whereas in the dicot plants the number of stomata is more on the lower surface and less on the upper surface. In monocots, the stomata are dumbbell-shaped. 

Anatomy of Leaf

This topic will give us a clear understanding that why rates of transpiration in the upper and lower surface of leaves are different. As we read above the leaf is the main photosynthetic organ of the plant. The epidermis is present on both the sides of the leaves that is the upper and the lower side. In dicotyledon plants, dorsiventral leaves are present. It means that they remain horizontal and sunlight falls upon them horizontally. The upper surface is called the ventral surface or adaxial surface. The lower surface is called the abaxial surface or dorsal surface. As its name implies, both the surfaces differ in colour and also in anatomical features. The upper surface is darker in colour. Monocotyledons have isobilateral leaves. This means that these leaves are aligned vertically in the direction of sunlight. Due to this, both surfaces receive an equal amount of sunlight. In dicots, the stomata are lesser in number on the adaxial surface as compared to the abaxial surface. As the number of stomata is less on the upper surface, therefore, the rate of transpiration from the upper surface is also less. This is because if the number of stomata is more on the upper surface then it will be more in contact with the sunlight which will thus result in more transpiration and hence more amount of water loss will take place. So the rate of transpiration is lower on the upper surface of the leaf due to less amount of stomata and more on the lower side of the leaf due to more number of stomata. But in monocots, as both sides of the leaf have an equal amount of stomata, so the rate of transpiration is equal on both the upper as well as lower surfaces of the leaf.