THE STOMATA ARE PRIMARILY MEANT FOR ABSORPTION OF co2 but these also helps in exchange of gases but at the same time water vapour also escape through stomata.thus transpiration is described as necessary evil because it is an envitable process but potentially harmfull.
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Transpiration  is described as a necessary evil because it involves loss of water which is an essential compound for life.Transpiration is the process of loss of water from the aerial parts of the body in the form of water droplets. It was Curtis who stated that Transpiration is a necessary evil. Transpiration is an effect which either opposes or favours the cause that produced it depending on the availability of water in the soil. It is just an emergent property which emerges out when there are pores on the surface of living organisms. They need this pores for the exchange of gases which is important for metabolic activities. The number of pores depend on the concentration of gas in the atmophere that has to be fixed. Animals have just two nostrils or pores because the concentration of oxygen was quite high when they appeared on land.