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Stems-II
Primary thickening meristem occurs in many monocots
Primary thickening meristems
Primary Thickening Meristem
Banana “tree”
Foxglove vascular ring
Eustele in dicots
Alfalfa and ragweed VB
Clover fasicular cambium spreads to interfasicular region
Secondary growth restricted to VB
Ordinary secondary growth in dicots and gymnosperms secondary xylem and phloem develops from the vascular cambium
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
MonocotsYucca brevifolia Cocus nutifera
Cordyline (Ti plant)
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)
Secondary thickening meristem in Cordyline (Ti plant)
Palm trunk Royal palm
Palm trunk
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
Resin duct in poison ivy stem
Oil and mucilage cells
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)
Non-articulated laticifer in Euphorbia
Articulated laticifers