Two equal pieces of cobalt chloride paper are dried and placed one on the upper surface and the other on the lower surface of dorsiventral leaves. They are firmly held in place by pieces of cellotapes or glass slides. After a few minutes the cobalt chloride paper shows change in colour from blue to pink. The time taken for the cobalt chloride paper on the upper surface of the leaf to turn pink is much longer than for the under surface. Thus, it proves that the rate of transpiration is greater from the under surface of dorsiventral leaves.
