1. Stems and Plant Transport
Chapter 33

2. Stem Functions Support leaves and reproductive structures
Allow leaves to absorb sunlight needed for photosynthesis
Flowers and fruits are located in areas accessible to insects, birds, or air currents
Provide internal transport
Water and dissolved nutrients up from ground
Sugars down from leaves
Produce new living tissues
Buds  leaves, reproductive structures
In some plants additional functions:
Asexual reproduction
Photosynthesis
Starch storage

3. External Stem Structure
Woody stems (terms to know)
Terminal bud
Bud scales
Axillary (lateral) buds
Node and internode
Scars:
Bud scale scars
Leaf scars
Bundle scars
Lenticels

4. Types of Stem Growth Primary growth Secondary growth In all plants
Increase in length
Occurs at the apical meristems
Secondary growth
Only in woody plants
Increases in girth
Occurs at the lateral meristems

5. Internal structure – herbaceous dicots
Epidermis – outer protective layer
Cuticle – waxy coating on epidermis
Stomata – permit gas exchange
Cortex – may contain photosynthetic cells; also provides support
Vascular bundles – arranged in a circle around the central core
Xylem – located toward the stem’s interior
Phloem – toward the outside
Pith – in the center of the stem and functions mainly in storage

7. Internal structure – herbaceous monocots
Vascular bundles are scattered throughout
Do NOT have lateral meristems and do NOT produce wood or bark
Palm trees and bamboo – monocots that produce wood-like tissue from primary growth

9. Woody plants: secondary growth
The result of two lateral meristems:
Vascular cambium produces
- Secondary xylem (wood) and secondary phloem (inner bark)
Cork cambium produces
Cork cells and cork parenchyma  periderm
Heartwood – the older wood in the center of the tree; cells are plugged with pigments and other wastes
Sapwood – the younger, functional secondary xylem that is conducting water and minerals

11. Transportation of water
In general, water and dissolved minerals travel upwards from roots in one direction
Water moves by being pulled to the top of the plant
Water potential – the free energy of water
Pure water has a water potential of 0
Dissolved solutes decrease the motion of the water molecules and therefore decrease the free energy
More solutes = negative number
Water moves from a region of higher free energy (less negative number) to a region of lower free energy (more negative number)

12. Water Potential Water potential of soil varies:
Dry soil = very low potential (very negative)
Wet soil = higher potential (but still a negative number)
Water potential in root cells is also negative:
Due to the presence of dissolved solutes in the cells
Under normal conditions, the water potential of the root is more negative than the surrounding soil
This means water moves by osmosis from soil into the root

13. Water potential… Water potential of the air is extremely negative
Water potential gradient from least negative (soil) through the plant into the most negative (air)
Most scientists think that the tension-cohesion model explains the movement
Water sticks together and is pulled up the xylem similar to water moving up a straw

14. Translocation of food
Dissolved sugars move in all different directions in phloem cells
Sugar made during photosynthesis is converted to sucrose (table sugar – glucose + fructose) before being loaded into phloem cells
Sugar moves from source to sink
Pressure-flow hypothesis - movement in phloem is due to pressure gradient
High pressure at source  low pressure at sink