This is a cross section of a three year-old American basswood tree (Tilia Americana). The inner pith (top) contains very large vessels (green) that store water and nutrients. Outside the pith are three rings of xylem vessels, hollow cells lined end to end that function primarily to transport water. Outside the xylem rings is a layer of phloem that contains cells specialized for transport of nutrients and proteins (clear circles) surrounded by fiber cells (green) that provide structural support and intervening triangular wedges of parenchymal cells (pink) that provide metabolic support. The cambium (brown), between the xylem and phloem, contains stem cells that give rise to both xylem and phloem. Amazingly, plants exhibit some of the same stem cell differentiation and growth mechanisms seen in animals and can provide insight into animal stem cell biology.
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