Running through the leaf are bundles of vascular tissue containing cells called Xylem and Phloem that act as plumbing pipes, carrying moisture and nutrients back and forth in the plant. Xylem carries water from the roots up and phloem transport sugars from the areas where they're made to parts of the plant that need them. We see this "plumbing" as the veins on the leaves.
Not all plants have veins. Those that do are termed "vascular" plants; mosses, for instance do not have them and are not grouped in lists of vascular plants.
Veins in monocot leaves, like grasses, are parallel, rather than branching - one way you can tell whether a plant is a grass.
Leaves, like people, have skin (the epidermis) which protects the working bits inside. This has a waxy coating (cuticle) that keeps moisture in and protects the leaf. On some leaves, this is quite evident and on others it is not, but it's always there. The cells that perform photosynthesis are part of the middle leaf or mesophyll. They're called palisade parenchyma. In dicots, they are located on the top side of the leaf, just under the upper epidermis, where they are closest to the light.
Leaves, unlike animals, can manufacture their own food through the process of photosynthesis, which actually means 'put together with light' because green pigmented chemicals inside leaf cells, called chlorophyll, enable plants to turn carbon dioxide and water into a simple sugar using the energy from sunlight.
From this simple sugar they make starch that is stored throughout the plant. Using compounds from soil and soil bacteria, plants make the vitamins and proteins they need. The next time that you handle a green leaf, think about what's going on inside of it...rather marvelous.
Some leaves also have hairs (trichomes) which are extensions of epidermal cells that provide protection to the leaf from ultra violet radiation, drying winds and munching critters. Pictured is a section of a hardy geranium leaf.
Stachys (lamb's ears) are covered with these hairs, making them great plants for hot, sunny sites and contributing to their rotting tendencies in humid climates, where the hairs tend to hold moisture.
Also part of the middle leaf are spongy cells, found in both dicots and monocots. These have large air spaces between them and are used for storing carbon dioxide and oxygen.
Environment
One of the factors that influences leaf shape is the environment in which the plant grows. On the simplest level, plants - like other living things - evolved over millennia in various environments. Those who adapted survived and are with us today. Those who did not have vanished like the dinosaurs.
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