STOMATA:

The carbon dioxide needed for photosynthesis enters the citrus leaf through openings called stomata (Greek: stoma, meaning mouth). Stomata most often are located in the leaf's lower surface. The location of stomata in the lower epidermis keeps the stomata from becoming plugged with dust that normally collects on upper leaf surfaces, and also reduces the entry of harmful fungal spores.. To get some idea of how many of the tiny pores perforate a leaf's surface, consider the following; There are approximately 45,000 stomata per square centimeter (1cm is about 3/8 inch) on an orange leaf. - Millet