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secondarygrowth213-page12

secondarygrowth213-page12 - The phloem rays often connect...

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Secondary Growth in Stems: Wood, Bark and Surface Features - 12 Phloem Secondary phloem originates from vascular cambium cells that divide and specialize outward. As secondary growth progresses, functioning phloem sieve tubes, companion cells and phloem fibers are spread out in patches interspersed with dilated phloem rays of parenchyma cells. Expansion nearer the exterior of the stem requires a greater surface area. The phloem rays are large parenchyma cells that can fill more space so that the phloem region can keep up with the expanding circumference of the stem.
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Unformatted text preview: The phloem rays often connect to xylem rays. Since the phloem consists of thin-walled sieve tubes and their companion cells, the older phloem gets crushed and compacted with the lateral expansion of the stem. New cork cambium tissue is formed from non-functioning phloem parenchyma and forms new periderm layers to separate older phloem from newer phloem. The older phloem and periderm layers are eventually sloughed off. Dilated phloem rays Phloem Older Phloem and Cork...
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