secondarygrowth213S - Secondary Growth in Stems: Wood, Bark...

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Secondary Growth in Stems: Wood, Bark and Surface Features - 1 Secondary Growth in Stems Secondary growth in plants is responsible for the increase in girth or diameter of the plant by the addition of secondary vascular tissue and periderm. All woody plants exhibit extensive secondary growth, but many herbaceous plants have some secondary growth. Secondary growth has commercial value (wood and wood products) for humans and dimensional value for plants, because secondary growth allows for much greater size and volume. Although we focus on secondary growth in stems, roots, too, have secondary growth patterns that parallel the secondary growth of stems. Leaves have minimal, if any, secondary growth, which is generally restricted to strengthening vein tissue. Secondary growth originates from two lateral meristems: vascular cambium, derived from procambium retained in vascular bundles during primary growth, and cork cambium, produced by dedifferentiation of cells in the outer cortex. Vascular cambium produces secondary xylem to the interior of the cambium layer, and secondary phloem exterior to the cambium layer. Cork cambium produces the secondary dermal tissue, called periderm, comprised of cork, cork parenchyma and cork cambium. The secondary dermal tissue and secondary phloem form bark. Wood is comprised of secondary xylem. Vascular cambium forms a layer between bark and wood. Initiation of Secondary Growth Secondary vascular tissue is initiated in the primary growth stem from cambium cells (initials) located in open vascular bundles. Most of the cambium produces cells that are vertically elongated and form the axial growth of the stem. Other cambium cells are oriented laterally and form rays that radiate out towards the surface of the stem. (Naturally these two types of vascular cambium have special names: fusiform initials and ray initials.) In addition, most cambium initials divide in a plane so that cell plate formation is parallel to the surface of the stem (periclinal divisions), producing xylem or phloem cells. Some cambium cells divide in a plane perpendicular to the cell surface, producing more cambium to accommodate to the increasing girth of the plant (anticlinal divisions).
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Secondary Growth in Stems: Wood, Bark and Surface Features - 2 Cambium initials Anticlinal (C ± C) and Periclinal (C ± X or P) divisions In the transition from primary to secondary growth in those stems that have discrete vascular bundles, the early vascular cambium produces meristematic cells both within the vascular bundle, called vascular bundle cambium (fascicular cambium), and in the pith rays between adjacent vascular bundles (inter-fascicular cambium) to produce a complete ring of vascular cambium. Interfasciular Cambium Present
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secondarygrowth213S - Secondary Growth in Stems: Wood, Bark...

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