DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
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 (black ring) encloses a ring of phloem (brown-red) and scattered xylem cells with large cavities. There is no pith. Magnification: x103 when printed 10 centimetres wide.
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