Transcript 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