The veins

In the middle of the leaf there is the midrib.
On each side of the midrib other veins branch off.
These are the primary veins.
They are smaller. These primary veins divide into many still smaller veins.

Perhaps you have seen a leaf that has been eaten by insects.
The leaf tissue has gone and only the veins are left. It is like a spider's web.

What are the veins for?

They carry the sap.
The sap passes along the stem vessels.
Then it enters the vein vessels.

Cut a palm frond and you will see the sap flow. The vessels of the palm frond carry the sap.

In a man's body, the vessels carry the blood.
In a plant, the vessels carry the sap.

Sap is the blood of plants.