To demonstrate unequal transpiration from the two surfaces of the leaf.
To do the experimenet, soak small pieces of filter paper in 5% solution of cobalt chloride or cobalt nitrate and dry them over a flame. The cobalt paper is deep blue when dried, but in contact with moisture they turn pink. Place two dried cobalt papers, one on the upper and the other on the lower surface of a thick, healthy leaf. Cover them completely with glass slides or with a Jeaf-clasp and clamp them properly to the leaf. Then quickly seal the sides with vaseline to prevent atmospheric moisture from coming in contact with the papers.
It will be seen that the cobalt paper on the lower surface of the leaf turns pink sooner than the one on the upper surface and this change in colour takes place within a few minutes.
The leaf transpires water more vigorously from the lower surface than from the upper. This is due to the larger number of stomata on the lower surface than on the upper.