The dark layer is produced later in the growing season. Narrower cells with thicker walls are produced. Xylem that is actively conducting water is called sapwood. Bark is formed as a result of the production of cork by the cork cambium. Lenticels are spongy regions of the bark that allow gas exchange.
Air spaces allow carbon dioxide to diffuse to photosynthesizing cells.
Veins form a network of vascular tissue, so that mesophyll cells are well supplied with water and nutrients. Leaves export photosynthetic products to the rest of the plant body.