The lines on a maple leaf are called veins. These veins are part of the plant's vascular system and serve important functions during the leaf's life. They transport water and nutrients from the roots to the leaf and distribute the products of photosynthesis throughout the plant. In maple leaves, the arrangement of veins contributes to their characteristic shape and structure, enhancing their ability to capture sunlight for photosynthesis. The veins are also crucial for the leaf's overall health and function, playing a significant role in supporting the leaf's weight and structure.