DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Plant vascular bundles. Light micrograph of a section through vascular bundles that have been dissected out of a ribwort plantain (Plantago lanceolata) petiole - the stalk of a leaf. The primary walls of the xylem vessels are made of cellulose and can stretch. The secondary walls are thick and lignified in patterns. The secondary walls are constructed of lignified annular, spiral or helical rings, which are capable of elongating. Magnification: x138 when printed 10 centimetres wide.
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