The veins of the leaf are formed by the vascular bundles. These bundles contain specialized cells, such as xylem and phloem, which transport water, nutrients, and sugars throughout the plant. The xylem cells carry water and minerals from the roots to the leaves, while the phloem cells transport sugars produced during photosynthesis from the leaves to other parts of the plant. The arrangement and pattern of the veins can vary among different plant species and can also help to support the leaf structure.