Venation is pinnate if all veins extend individually from the midrib outward, or palmate if principal veins branch from the petiole near the leaf base. Pinnate and palmate venation are forms of net-venation where veins subdivide, forming a complicated network of veinlets.
Venation classifies the vein distribution of a leaf blade. Typical venation schemes include parallel-veined and net-veined. Parallel-veined leaves have veins running alongside each other from the leaf base or the midrib towards the tip. Veinlets cut across and connect the veins.
Veins serve as vascular networks, transporting liquids between leaf blade cells and providing turgidity to maintain blade shape. They deliver water and nutrients and carry away waste and the products of photosynthesis.
Insects and weather damage leaves by severing the midrib, veins or veinlets. Midrib breaks near the base of the leaf are especially critical. A leaf with backup veins has a better chance of surviving such injuries. Palmate-veined leaves have more redundancy than pinnate-veined leaves and can continue vascular functions using alternative pathways.
The benefits of damage tolerance are offset by added cost. Palmate-venation uses more energy and requires more complex building material than pinnate-venation. The type of venation found in individual leaves supports leaf morphology. Delicate leaves benefit from palmate networks because they are more susceptible to damage. Substantial leaves take advantage of the lower metabolic cost of pinnate architecture because of the innate protection their thickness confers.