Stems-II

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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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Monocots
Yucca 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
Ducts may contain resins, gums, mucilage produced
by parenchyma (epithelial) cells lining the duct
Resin ducts in Pinus
and in Artemesia
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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