Pondweed stem. Light micrograph of a transverse section through the stem of a pondweed (Potamogeton sp.) plant. All aquatic plants (hydrophytes) have a similar stem structure. The epidermis of the petiole has a thin cuticle (outer layer) and is permeable to water. The cortex consists of parenchyma cells with large air spaces (lacunae, white circles). The vascular cylinder (dark patch, centre) acts as a cable to resist the movement of water. The outer endodermis encloses a ring of phloem and scattered xylem cells with large cavities. There is no pith. Magnification: x30 when printed 10 centimetres wide.