Sycamore stem, light micrograph. Longitudinal section through a leaf junction on a mature sycamore (Acer sp.) stem. The dark brown outermost layer is cork, an impermeable layer of cells that forms part of the bark of the tree. It is produced by the cork cambium (phellogen) a layer of meristematic tissue (orange). In autumn, the layer of cork begins to extend across the base of the leaf junction (as seen here), separating it from the stem. This layer is known as the abscission layer because it causes the leaf to fall off.