What Is a Plant Vein?

Some plant veins, such as the ones in maple leaves, are easily visible.

A plant vein is a vascular structure on the blade part of a leaf that transports food and water necessary for photosynthesis. It is one of the major tissues of the leaf, along with the mesophyll and the epidermis. Veins are composed of xylem and phloem surrounded by a bundle sheath.

Structure

Veins are composed of the xylem located towards the upper epidermis of the leaf and contain vessels and fibers. The phloem, in the lower epidermis, has sieve tubes, companion cells and transfer cells. Because of their proximity to the mesophyll, the xylem and phloem are essential to photosynthesis, by providing water necessary for the process, and then distributing manufactured food. The bundle sheath, or parenchyma, surrounds the xylem and phloem. Schlerenchyma cells sometimes surround plant veins to provide a framework of support in the soft tissues of leaf blades. The leaf petiole connects the leaf vein to the stem.

Xylem

Xylem cells transport water and minerals from the roots to the leaves. Lignin gives strength to xylem cell walls. Vessel elements and tracheids are the main xylem components in dicots. Sap and water moves through from foot to leaves through bulk flow. Three factors cause water to flow through the xylem. These are osmosis, capillary action and transpirational pull.

Phloem

Phloem cells transport carbohydrates from the leaves to the rest of the plant. Sieve-tube members and companion cells constitute phloem structure. The former are large, cylindrical cells with large pores in the cell wall. The much smaller companion cells carry out the sieve-tube functions. Phloem sap moves by using the sieve-tubes as pipes. This process is called phloem loading and unloading, which is activated by the sugars produced by photosynthesis.

Monocots

Plants such as suck grasses or corn plants are monocots, which have vein structures that run parallel to each other and to the perimeter of the leaves, and are uniform in structure and size. Some monocots have bulliform cells along the midvein to help the leaf fold or roll inward when water is scarce to help minimize evaporation.

Dicots

Dicot leaves have either pinnate or palmate venation, with smaller veins generally emanating for a larger midrib vein. Pinnate venation features a network of veins that emanate evenly throughout the leaf while palmate venation radiates from a point at the base of the leaf. The large midrib, as well as larger branching veins, often have bundle sheath extensions to provide additional support. Larger branching veins may also have bundle sheath extensions.

References

• Enchanted Learning: Botany and Paleobotany Dictionary
• Bellevue College: Leaves
• Science Clarified: Leaf
• Fact Index: Xylem
• Fact Index: Phloem

Resources

• Food For Everyone: The Parts of a Plant and Their Function
• PlantPhys: Plant Structure and Function