Question

Why is it said that stomata is more on the lower side

Answer 1

Because their function is to regulate the amount of water in the leaf and being on the bottom prevents it from absorbing too much water in the rain or transpirating too much in the sun. Not all plants have them on the bottoms of leaves, namely grasses and aquatic plants as the grass leaves don't have a fixed ‘up’ side and the aquatic plant needs them for gas exchange.