This is a thin section of the lower stem of water milfoil Myriophyllum sp. , stained with the fluorochrome calcofluor which binds to the cellulose of the cell walls and is fluorescing brightly in ultraviolet light. Marsh plants tend to be rooted in anaerobic mud and so have air channels (aerenchyma) that conduct oxygen down to the roots. Working from the outside inwards in this section, there is a well defined single outer layer of very small cells forming the epidermis, then inside that lies the stem cortex with 17 air channels arranged around the central stele, which contains the phloem (brightest flourescence) and the xylem. The stem and leaves of water milfoil. The small white structures in the leaf axils are the stigmas of the female flowers.