Our objective is to compare the rate of transpiration between the upper and lower surfaces of a leaf.
Transpiration is the process of water movement through a plant and its evaporation into the atmosphere from its aerial parts. In leaves and in young shoots the epidermal layer contains minute microscopic pore like structures called stomata. Transpiration occurs chiefly through the stomata of the leaves. The stomata are mainly concerned with exchange of gases during the process of photosynthesis and respiration. Each stomata has a slit like opening called the stomatal pore, which is surrounded by two special cells called the guard cells. These special cells help to regulate the rate of transpiration by opening and closing the stomata.
In different plants, distribution, number, size and type of stomata vary. Even within a plant, the upper and lower surfaces of the leaf may have different distributions. In some plants a greater number of stomata is present on the on the lower surface than on the upper surface of the leaf. Therefore, the loss of water from the lower surface is greater than from the upper surface.
We can study the rate of transpiration from the two surfaces of a leaf by comparing the loss of water vapour from the two surfaces of the leaf.
Rate of transpiration can be easily demonstrated by cobalt chloride paper test. Dry cobalt chloride paper that is blue in colour turns pink when it comes in contact with water. Using this property of cobalt chloride paper we can demonstrate water loss during transpiration.
We can measure the rate of transpiration by using the time taken for the paper to change its colour from blue to pink.