See stomata are the openings on the leaves which function for the transpiration and exchange of material in and out of the leaf.<br />Therefore if the stomata were present in the upper part of the leaf then the stomata would have been blocked due to the dust particles. <br />And so due to the direct rays of the sun the transpiration would have been more.<br />Therefore more stomata are found in the lower part of the leaf.
Stomata in most plants are more in lower surface rather than on upper surface bcz on upper surface there would b more water loss