The stomata must be open during the daylight hours to let oxygen and carbon dioxide pass through. While they are open, water vapor escapes into the atmosphere (transpiration). ... Transpiration is the loss of water through stomata, so, more stomata are found on the lower surface to prevent excessive loss of water.
In a lotus plant, stomata is found on the upper epidermis of the leaf.
Stomata are responsible for the exchange of gas with the surrounding atmosphere of the leaf. Their guard cells stay taut during the day to keep the stoma open.
Guard cells can only stay open if they are saturated with a certain quantity of water.
From this information we can infer that stomata are typically found on the lower epidermis to prevent water damage when it rains.
The rain would not only cover the openings, it would also ruin the osmotic balance of the guard cells causing inefficiency and damage to the cycle of gaseous exchange.
Natural selection probably selected against such plants giving rise to those with stomata on the underside.
