Rhubarb stem. Coloured scanning electron micrograph (SEM) of a transverse section through a rhubarb stem, Rheum rhaponticum. Xylem vessels are seen on the right (brown) and thin-walled parenchyma cells are on the left. Xylem vessels are responsible for the upward transport of water and solutes in the plant, from the roots into the stem and leaves. Here, these vessels are reinforced and strengthened with spiral bands of lignin. Spiral bands allow xylem vessels to elongate and grow lengthwise. Parenchyma cells form a ground tissue in which other tissues, such as xylem, are embedded. Magnification: x300 at 5x7cm size. x1050 at 10x8ins

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