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