1. Skeletal System of the Plant World
Stems
Skeletal System of the Plant World

2. Stem Functions Support leaves
Transportation of water, nutrients, carbohydrates
Storage – water or starch in parenchyma of cortex
Protection – thorns, thick bark
Photosynthesis – green stems
Differences between species are adaptations to environment.

3. Stem Structure Nodes – points of 1 or more leaf attachment
Internodes – segments between nodes
Node usually also has 1 or more lateral buds
Tip of stem has terminal bud
Bud – contains apical meristem covered by bud scales that leave scars when bud opens
Stem Structure

4. Specialized Stems
Stolons – horizontal, above ground (“runners” of strawberry plants) – spread to form new plants
Tubers – storage of starch (potatoes)
Rhizomes – horizontal underground stems – also spread to form new plants
Cacti – storage of water, photosynthesis

5. Stem Growth Gymnosperms & Dicots
Primary – from apical meristems when bud opens.
Secondary – from lateral meristems
In woody plants:
Vascular cambium in vascular bundles eventually unites to form vascular cylinder.
Secondary xylem (wood) forms toward inside, secondary phloem toward outside.
Stem Growth Gymnosperms & Dicots

6. Woody Stem in Cross-section
Older xylem gets clogged, darker = heartwood
Functional light-colored wood = sapwood
Bark = cork, cork cambium, & phloem
Woody Stem in Cross-section

7. Woody Stem in Cross-section
Springwood = wide, thin-walled xylem produced in spring when water is plentiful.
Summerwood = narrower, thick-walled xylem produced in summer when less water is available. Production ceases when photosynthesis slows/stops for winter.
Annual ring/growth ring = 1 year’s xylem consisting of a layer of springwood and a layer of summerwood. (absent where water is abundant year-round).

8. Transportation within Stem
Translocation = Carbohydrates moved from source (where made or have been stored) to sink (where used or to be stored).
Pressure-flow hypothesis – carbohydrate movement
Cohesion-tension theory – water movement

9. Pressure-Flow Hypothesis
Carbs move into sieve tubes of phloem
Water follows via osmosis (why?)
Pressure builds at source
Process reversed at sink
Dissolved carbs move from high pressure to low pressure.

10. Cohesion-tension Theory
Transpiration - Water constantly evaporating through leaves.
Roots drawing in water create high pressure.
Transpiration creates low pressure at leaves, tension on water column.
Water pulled through xylem by polar water molecules’ attraction to each other (cohesion)
Attraction of water molecules to xylem walls = adhesion, keeps water column from breaking.
Combination of these results in upward movement
Cohesion-tension Theory