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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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12.3 Stems
Pages 552-557
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).
Functions of Stems
• Connect roots to leaves• Raise leaves to the sun and flowers to
pollinators• Store water or carbohydrates• Photosynthesis• Protection
Types of stems
• Herbaceous-do not contain wood
Vascular bundle
• Long continuous strand of vascular tissue (xylem and phloem)
Herbaceous Stems
• Eudicot Monocot
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.
Eudicot-herbaceous
• Xylem is closer to the centre and phloem is closer to the outside of the stem
Woody stems
• Contain wood and bark• All gymnosperms • Most woody angiosperms are eudicots• Monocots do not produce wood
Cross-section of woody stem
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.
Wood
• Sapwood-young xylem transports water and minerals
• Heartwood-cells fill up with resins and oils -provide rigidity and support
Typical Woody Stem
Bark
• Tissues found outside the vascular cambium (phloem, cork cambium, and cork)
• Cork cambium-meristematic tissue produces cork
Bark
• Protects the tree from predators and fires
Growth rings
• Spring…rapid growth, large xylem cells, thin walls, lighter in colour
• Summer…slow growth, smaller xylem cells, thick walls, darker in colour
Xylem cells
• Thick-walled• Dead at maturity• Cell walls with lignin for strength
Tracheids
• Tracheids-long cylindrical cell with tapered ends with pits, holes that allow water to move to adjoining cells
Vessel Elements
• Shorter, wider, less tapered• Sides have pits• Ends have perforation plates
Phloem
• -living at maturity• -contain cytoplasm
Types of phloem
• Sieve cells• Sieve tube elements• Companion cells
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
Pictures
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
Underground Stems
• Tubers Corms Rhizomes
Stolons
• Grow along the soil• For example: strawberries, mint