Fig. 4.2-4. Transverse section of leaf blade of oleander (Nerium oleander). When collenchyma is present in leaves, it is most often restricted to running parallel to the largest vascular bundles, but here there are two layers of collenchyma cells just interior to the upper epidermis of the leaf blade. The collenchyma is only weakly developed, with just a small amount of wall thickening in the corners of the cells.