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
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
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)
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
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
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