DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Stem of old man's beard. Light micrograph through the cross section of the stem of old man beard (Clematis vitalba, family Ranunculaceae), showing the large wood vessels called the xylem (inner ring) through which water is transported. Medullary rays are continuous through the xylemeven after secondary thickening.
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