Leaf

Veins penetrate the mesophyll layers of a leaf. Veins consist of vascular tissue, xylem, and phloem, and connect the vascular tissue of the stem to the photosynthetic cells of the mesophyll, via the petiole. Xylem cells mainly transport water and minerals from the roots to the leaves, and phloem cells mainly transport carbohydrates made by photosynthesis in the leaves to the rest of the plant. Typically, the xylem cells are on the upper side of the leaf vein, and the phloem cells are on the lower side.