Common water-plantain (Alisma plantago-aquatica petiole) petiole. Light micrograph of a section through a petiole (leaf stalk) from a water-plantain plant. Seen here are the: epidermis with a thin cuticle; outer cortex consisting of three layers of parenchyma cells; middle cortex with large air space (lacuna) and radial strands of parenchyma; inner cortex. Five large vascular bundles can also be seen (dark patches). The outer ring of each is the endodermis, beneath this is a ring of phloem, then a ring of xylem, with the pith at the centre. Magnification: x6 when printed 10 centimetres wide.