A midrib is the thick central vein that runs down the middle of a leaf, from the stem to the tip. You can see it clearly on most leaves: it's usually raised on the underside and looks like a spine holding the whole leaf together.

The midrib does two important jobs. First, it provides structure, keeping the leaf flat and helping prevent it from folding or tearing easily. Second, it acts like a highway system, carrying water and nutrients from the stem out to all parts of the leaf, and bringing the sugars the leaf makes through photosynthesis back to the rest of the plant.

If you've ever pulled a leaf apart, you've probably noticed how the midrib is tougher than the rest of the leaf. That's because it contains special tubes called xylem and phloem that transport materials. Some leaves, like maple or oak leaves, have very obvious midribs, while others have more subtle ones.

When people talk about the “veins” in a leaf, the midrib is the main vein, with smaller veins branching off from it like roads branching off a highway.