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12.3 Stems
Pages 552-557
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Canadian Economy
• The forest sector’s contribution to the Canadian economy (GDP) was $23.5 billion, or 1.9 percent, in 2010.
• In 2010, Canada was the world’s second-largest forest product exporter (10.2 percent).
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Functions of Stems
• Connect roots to leaves• Raise leaves to the sun and flowers to
pollinators• Store water or carbohydrates• Photosynthesis• Protection
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Types of stems
• Herbaceous-do not contain wood
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Vascular bundle
• Long continuous strand of vascular tissue (xylem and phloem)
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Herbaceous Stems
• Eudicot Monocot
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Difference
• In monocots the vascular bundles are found throughout the ground tissue of the stem.
• In eudicots the vascular bundles are found in a ring around the stem.
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Eudicot-herbaceous
• Xylem is closer to the centre and phloem is closer to the outside of the stem
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Woody stems
• Contain wood and bark• All gymnosperms • Most woody angiosperms are eudicots• Monocots do not produce wood
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Cross-section of woody stem
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Vascular cambium
• Woody stems contain vascular cambium meristematic tissue.
• Divides to form new xylem tissue on the inside and new phloem tissue on the outside.
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Wood
• Sapwood-young xylem transports water and minerals
• Heartwood-cells fill up with resins and oils -provide rigidity and support
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Typical Woody Stem
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Bark
• Tissues found outside the vascular cambium (phloem, cork cambium, and cork)
• Cork cambium-meristematic tissue produces cork
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Bark
• Protects the tree from predators and fires
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Growth rings
• Spring…rapid growth, large xylem cells, thin walls, lighter in colour
• Summer…slow growth, smaller xylem cells, thick walls, darker in colour
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Xylem cells
• Thick-walled• Dead at maturity• Cell walls with lignin for strength
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Tracheids
• Tracheids-long cylindrical cell with tapered ends with pits, holes that allow water to move to adjoining cells
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Vessel Elements
• Shorter, wider, less tapered• Sides have pits• Ends have perforation plates
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Phloem
• -living at maturity• -contain cytoplasm
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Types of phloem
• Sieve cells• Sieve tube elements• Companion cells
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Phloem
• Sieve cells- pores, organelles • Sieve tube elements have cytoplasm but lack a
nucleus, ends are called sieve plates• Companion cells have nucleus and organelles
associated with sieve tube element
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Pictures
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Table 1. Xylem and phloem in gymnosperms and angiosperms
Plant Group Xylem tissue cell types Phloem tissue cell types
Gymnosperms Tracheids Sieve cells
Angiosperms TracheidsVessel elements
Sieve tube elementsCompanion cells
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Underground Stems
• Tubers Corms Rhizomes
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Stolons
• Grow along the soil• For example: strawberries, mint