The lines on the leaf blade are called veins . Veins serve the purpose of transportation of water to the chlorophyll and the stomata, It also transports the food produced by stomata to the stem from where food is transported to the whole plant.

The lines on the broad green part of the leaf are called veins.

Veins are an essential part of the leaf structure and play a crucial role in the functioning of plants. They are responsible for transporting water, nutrients, and sugars throughout the leaf. Veins consist of two types: xylem and phloem.

Xylem:
Xylem is responsible for transporting water and minerals from the roots to the rest of the plant. It is made up of specialized cells called vessel elements and tracheids. These cells are long and tubular, forming a network of interconnected tubes that run through the leaf. Xylem tissues are also responsible for providing structural support to the leaf.

Phloem:
Phloem is responsible for transporting sugars and other organic compounds from the leaves to the rest of the plant. It is made up of specialized cells called sieve tubes and companion cells. Sieve tubes are long, cylindrical cells that form a network throughout the leaf. Companion cells are located alongside the sieve tubes and provide metabolic support to these cells.

Vein Arrangement:
The arrangement of veins in a leaf is known as venation. There are two main types of venation patterns:

1. Pinnate Venation: In pinnate venation, the primary vein called the midrib extends from the base to the tip of the leaf, and smaller veins branch out from it. These smaller veins are called secondary veins. Pinnate venation is commonly found in dicotyledonous plants.

2. Palmate Venation: In palmate venation, multiple primary veins originate from a single point near the base of the leaf, resembling the fingers of a hand. Smaller veins branch out from these primary veins. Palmate venation is commonly found in monocotyledonous plants.

Importance of Veins:
Veins are crucial for the survival and growth of plants as they provide a pathway for the transportation of essential substances. Water and minerals are absorbed by the roots and transported through the xylem to the leaves, where they are used for photosynthesis. The sugars produced during photosynthesis are then transported through the phloem to other parts of the plant for growth and energy storage.

In addition to transportation, veins also provide support to the leaf structure, allowing it to remain flat and capture sunlight efficiently. They also help in removing waste products from the leaf.

Overall, the veins in a leaf are a vital component of the plant's vascular system, ensuring the efficient distribution of water, nutrients, and sugars throughout the plant.