Transcript Stems and Plant Transport

Stems and Plant Transport
Chapter 33
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
External Stem Structure
• Woody stems (terms to know)
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Terminal bud
Bud scales
Axillary (lateral) buds
Node and internode
Scars:
• Bud scale scars
• Leaf scars
• Bundle scars
– Lenticels
Types of Stem Growth
• Primary 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
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
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
Woody plants:
secondary growth
• The result of two lateral meristems:
1. Vascular cambium produces
- Secondary xylem (wood) and secondary phloem (inner
bark)
2. Cork cambium produces
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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
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)
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
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 tensioncohesion model explains the movement
– Water sticks together and is pulled up the
xylem similar to water moving up a straw
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