What is the Primary Vein of a Leaf?

Leaves come in a variety of colors, textures, sizes and shapes, but in most of them, the primary vein is easy to spot. Look for the large vein running down the middle of the leaf blade. The other veins radiate out from the primary vein, which botanists also call the midvein or midrib.

The primary vein works with the other veins in the leaf to make up a plant's vascular system. Together they send water and other materials needed for important functions such as photosynthesis throughout the leaf. The unusual arrangement of a leaf's veins also helps botanists identify plant species.

The primary vein and other leaf veins connect the leaf blade to the petiole, or leaf stem, that attaches the leaf to the plant itself. The leaf's vein pattern, or venation, consists of lateral veins arranged on either side of the primary leaf vein. Veins contain xylem and phloem cells. These structures consist of strands capable of carrying fluids. Xylem cells transport water while phloem cells move mineral salts through the leaf.

The primary vein, along with any secondary and tertiary veins, are located within the tissue of the leaf blade, between the upper and lower epidermis, or skin, of the leaf. This middle layer -- known as the mesophyll -- is the area where photosynthesis takes place, allowing the leaf and the plant to produce the energy from sunlight that it needs to grow.

Leaf veins arrange themselves into netted, or reticulated, patterns or into parallel patterns. For example, a grape leaf's veins branch out from the primary vein and divide and subdivide into a complex, often delicate netted pattern. Maple leaves show a similar pattern. A rice or corn plant, in contrast, has a vein pattern in which the veins branch straight out from the midrib in the same direction. Plants in the grass family often have leaves with parallel veins.