The petiole is the stem beneath the blade of a leaf that attaches to the branch. A simple leaf may or may not have a petiole. In a simple peltate leaf, the petiole joins to the vein system of a single blade. Other simple leaves, called sessile, do not have petioles, and these blades grow directly from the branch. Compound leaves always have long, twig like petioles from which multiple leaflets sprout.
The midrib is the central, main vein of a leaf blade. If the leaf is peltate, the midrib is contiguous with the petiole, and it is usually most prominent on the leaf's underside. The midrib provides the blade with structural integrity, holding it upright to better collect sunlight. The stomata, tiny openings through which the plant takes in carbon dioxide and eliminates waste, are located on the midrib. A simple leaf has a single, undivided midrib.
The veins of a leaf carry water and nutrients from the soil to the blade, transport waste out to the stomata and transport sugars back to the rest of the plant. There are two main vein patterns: feather-like pinnate and the hand-like palmate. Pinnately patterned veins branch out from the midrib and run to the edge of the blade, branching off into smaller veins. In a palmate pattern, several large veins branch out from the petiole on either side of the midrib.
Leaves come in a wide variety of sizes and shapes, but the defining characteristic of a simple leaf is that it consists of a single, undivided blade. A simple leaf may be lobed, as long as the lobes do not extend all the way to the midrib. A simple leaf blade may have a smooth or toothed edge, be rounded, tapered or hand-shaped. Trees with simple leaves include oak, apple, birch and maple.