Many leaves are composed of three parts: the blade, the petiole, and stipules. The blade is the flat, wide, photosynthetic part of the leaf. The petiole is the thin stalk that holds the blade and attaches the leaf to the stem. Stipules look like tiny leaves that are found at the base of the petiole; they are not always present. An oak leaf has a clear example of blade and petiole, but grass leaves have blades only. Both are example of simple leaves, because the blade is entire and not split/sectioned into leaflets. A leaf that is segmented into leaflets is called a compound leaf. Discerning a leaf from a leaflet, and thus a simple from a compound leaf, can be challenging. The simplest way is to look at the intersection between the “stem” and the base of the “leaf”. If there is a bud at this location, then you are observing the attachment of a leaf to a stem. If the bud is missing, then you are looking at a leaflet attached to a larger, compound leaf. All leaves have stomates, which are pores in the leaf, with guard cells that regulate the exchange of gases and water vapor with the environment. These stomates can be opened and closed to regulate the physiology of the plant. Oaks, like many trees, have stomates only on the bottom of their leaves; grass leaves have stomates on both sides. Almost all people know what a leaf looks like, but contemporary leaves can range from microscopic (water weed: Wolffia) up to 83 feet long (Raphia palm).