Download Stem

Plant Structure and Function
Stem
• Support leaves
• Transport materials
• Provide storage
Types of Stems
Types of Stems
• Woody or nonwoody (herbaceous)
• Stolons
– Grow along soil surface
– Produce new plants
• Tuber
– Store energy
• Cacti
– Green fleshy stems
– Store water and photosynthesis
• Modified for protection
Stem Structures
• Did you know that a sign nailed 2m (7ft) high
on a tree will remain at that height, even
though the tree may grow much taller?
Stem Structure
• Grow in length only at tip
• Apical meristems
– Primary tissues
• Lateral meristems
– Secondary growth (diameter)
Stem Features
• Node
– Leaf attaches to stem
• Internode
– Space between nodes
– Lateral bud
• Bud
– Stem or leaf
– Apical meristem enclosed by bud scales
Winter Identification of Trees
Primary Growth in Stems
• Apical meristem
– Dermal, protect plant
– Ground, cortex and pith
• Pith located at center of stem
– Vascular tissue, occurs in bundles
• Xylem (inside) and phloem (outside)
– Monocots v.b. scattered
– Dicots v.b. rings
Monocot and Dicot Vascular Bundles
Secondary Growth in Stems
• Stems increase in thickness
– Cell division in vascular cambium
– Dicot and gymnosperms
• Stem forms b/w xylem and phloem in vascular bundle
– Vascular cambium
• produces 20 xylem (inside) and phloem (outside)
– 20 xylem represents the wood of the plant
• New cambium cells increases stems diameter
Stem Growth pg581
• http://academic.kellogg.edu/herbrandsonc/bio111/stems.htm
• Older xylem stop producing water
– Darken wood called heartwood
• Will increase in diameter
– Lighter – colored wood called sapwood
• Stays about the same
• Phloem near outside of stem
– Bark
• Cork, cork cambium, and phloem
– Protective layer
– Cork cells die at maturaty
• Cannot elongate and will rupture as stem increases in
diameter
• Bark pattern (maples and oaks)
Cross Section of Woody Stem
Annual Rings
• Springwood
– Water plentiful
– Vas. Cambium form new xylem cells
– Wide and thin cell walls
• Summerwood
– Water scarce
– Vas. Cambium forms smaller thicker cells
• Difference causes a change in appearance
• One ring each year used to determine age
Stem Functions
• Transport nutrients and water
• Store these nutrients
• Support leaves
Translocation
• Movement of carbohydrates through a plant
• Source to sink
– Source (storage)
– Sink (usage)
• Pressure – flow hypothesis
– Actively transported into sieve tubes
– Water transported by osmosis
Pressure – Flow Hypothesis
Transport of Water
• Water and mineral nutrients occur in xylem
• Water is constantly evaporating from plant
– Stomata
– Transpiration
• Cohesion – tension theory
– Water is pulled up the stem xylem by the strong
attraction of water molecules to each other and
the wall of the xylem
– Cohesion and adhesion properties
– Tug – n – pull, from roots up the xylem
Cohesion – Tension Theory