stems-ii. primary thickening meristem occurs in many monocots
TRANSCRIPT
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Stems-II
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Primary thickening meristem occurs in many monocots
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Primary thickening meristems
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Primary Thickening Meristem
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Banana “tree”
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Foxglove vascular ring
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Eustele in dicots
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Alfalfa and ragweed VB
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Clover fasicular cambium spreads to interfasicular region
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Secondary growth restricted to VB
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Ordinary secondary growth in dicots and gymnosperms secondary xylem and phloem develops from the vascular cambium
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Anomalous Secondary Growth
• Localized cambial activity because some regions of cambium die
• Uneven amounts of xylem and phloem
• Multiple cambia develop
• Concentric (or successive) layers of vascular cambium
• Monocots
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MonocotsYucca brevifolia Cocus nutifera
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Cordyline (Ti plant)
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Secondary Growth in Monocots
• Meristem (cambium) becomes distinct outside the outmost VB in the stem
• Secondary Thickening Meristem produces – Cells outward that differentiate into
parenchyma becoming part of a secondary cortex
– Cells produced inward are either • Conjunctive parenchyma that become sclerified• Parenchyma that differentiates into fibrovascular
bundles (called secondary bundles)
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Secondary thickening meristem in Cordyline (Ti plant)
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Palm trunk Royal palm
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Palm trunk
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Stems (gymnosperms, dicots, and monocots) also contain a variety secretory structures
Resin ducts in Pinus
and in Artemesia
Ducts may contain resins, gums, mucilage produced by parenchyma (epithelial) cells lining the duct
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Resin duct in poison ivy stem
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Oil and mucilage cells
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Laticifers
• Secretory structures (of one or more cells) that produce latex (latex is a milky secretion produced by many plants with varied composition– Non-articulated laticifers consist of extremely
elongated single cells (cell growth keeps pace with the growth of the stem)
– Articulated laticifers are composed of many individual cells that are usually interconnected by holes in their common walls (like vessels)
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Non-articulated laticifer in Euphorbia
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Articulated laticifers