DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY
The cut surface of a branch of a laburnum tree, Laburnum vulgaris. Woody tissues grow thicker with age by cell production within a region of persistent cell division called the cambium. In a cross section of wood, as here, the cambium is an annular tissue sited at the junction between the (brown) central heartwood and the (pale cream) parenchyma beyond. Cambium generates new cells, with differing fates according to their position. To the centre, they become xylem, the water-conducting tissue of the plant. To the periphery, they develop into phloem, the tissue that transports the products of photosynthesis from the leaves. The seasonal growth and dormancy of the cambium produces the "annual rings" visible. The fine radial lines are ray cells which also divide as the branch grows. The cracks visible in the wood are the result of the differential shrinkage of the tissues due to drying.
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