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
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
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
Underground Stems • Tubers Corms Rhizomes
Stolons • Grow along the soil • For example: strawberries, mint