The intricate network of vein-like structures seen in leaves are called leaf veins or vascular bundles.
The main vein running through the center of the leaf is termed the midrib. Branching off the midrib are smaller lateral veins and even finer intermediate veins.
Leaf veins consist of xylem and phloem tissues that transport water, minerals, and sugars to different parts of the plant. The pattern of the veins is called venation and can be pinnate (feather-like) or palmate (hand-like).
Veins provide structural support for the leaf blade and allow the leaf to orient itself horizontally to maximize light absorption. They also enable efficient nutrient transport, especially in deciduous trees before autumn leaf drop.