Usually, among land plants, there are more on the under side of the leaf, and in very dry places all are on the under side. The sun shining on the upper side would often cause too great evaporation, so the stomata are found underneath. In very hot, dry air there will be a little evaporation, even when the stomata are closed.
But when we come to look at leaves that lie on the surface of the water, like water lily leaves, of course the stomata are all on top, as that is the only part of the leaf the air can reach.
Many water plants have their stomata above, for you see there is no danger of their water supply running short.
It is very important for a plant to keep its pores open and it is quite ingenious in contriving ways to do this. Perhaps hairs are most frequently used.
They often cover the under side of the leaf where the stomata are thickest, or are found in lines along the leaf, when the stomata are distributed in this way.
But, you say, rain cannot get to the under side of the leaf. No, but dew can. Dew wets the under side of the leaf quite as much as the upper side, for dew does not fall, as some people think, but is deposited all over the surface of a cool object like a leaf, for dew is nothing but the vapor in the air which is deposited in the form of water at night.
To see better how the stomata work, here is a side view of one closed (a) and one open (b).
Stomata, you see, are the doors to the plant through which things pass in and out. Not only water goes out through them, but also other waste substances, such as oxygen and carbon dioxide.
You must not suppose because so many things go out at the doors that nothing goes in; for air passes in and also carbon dioxide.