The primary purpose of the veins in a leaf is to carry food and water throughout the leaf. The veins also have a secondary purpose, which is to help provide support for the rest of the leaf.
In many types of leaves, the veins form a large pattern that resembles a net. This pattern is made up of much larger, primary veins that connect to the leaf stem as well as smaller, secondary veins. The larger veins' main purpose is to carry water from the stem into the leaf, while the smaller veins spread it throughout every part of the leaf. The smaller network of veins also collects chlorophyll created in the leaf, which the larger veins then transport back to the main part of the plant.
Each species of plant has a unique pattern of veins in its leaves, varying in the density of the network and the distance between the larger veins. Some scientists have begun studying exactly what this pattern says about a plant and have made a number of conclusions. For instance, the density of the veins shows how much energy the plant has put into making the leaves. In addition, the number of loops in the pattern can help determine how long the leaf can live, as more loops allow it to circulate food and water through another path.