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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 secondlargest 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 Tracheids Vessel elements Sieve tube elements Companion cells Underground Stems • Tubers Corms Rhizomes Stolons • Grow along the soil • For example: strawberries, mint
