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