Objectives
Vascular Tissues:
Phloem
February 27, 2006
Key Functions of Phloem
• Translocation of assimilates &
PGR’s
• Support in young growth
• Protective fibers & secretions
• Storage – especially in conifers
• Seasonal differences in transport
(up & down)
Companion Cells
• Common mother cell origin with
sieve tube element
• Possess all organelles
• Regulates translocation
of organic materials
in sieve tube element
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Identify different cells in phloem tissue
Functional roles of phloem
Roles of callose & P-protein
Gymnosperm vs. angiosperm aspects
Structure & role of sieve plates, sieve
pores, sieve tubes & sieve areas
• Elucidate primary vs. secondary phloem
In Primary Phloem–
(Angiosperms)
• Protophloem
•Matures in areas of extension growth
•Narrow; enucleate with or without
companion cells
• Metaphloem
•Wider cells with companion cells
•Matures in areas of little or no extension
Aspects of Secondary Phloem
• Much less secondary phloem than
xylem (Much less secondary
phloem is needed)
• Often layered within a single season
• Seasonal differences: Early sieve
elements wider than later ones
which may have sclerenchyma
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Sieve Tube Elements
in Angiosperms
• Sieve plates
•Ends of cells only
•End walls inclined to transverse
•Variable pores (scalariform to simple)
• Nucleus degenerates
• Cell does not totally destruct–retains
differentially permeable plasma
membrane, mitochondria, plastids &
some ER
Sieve plate
Sieve Tube Elements
• Walls thicker than surrounding
parenchyma
•Nacreous cellulose & pectins
•Non-lignified
• Sieve areas with pores adjoining
sieve elements
• Pores
•Sieve plates on end walls
with larger pores
•Sieve areas with smaller
pores
Development of Sieve Elements
• Plasmodesmata mark sites of sieve
pores
• Plasmodesmata canals enlarge as
nucleus degenerates
• Sieve plate maturation occurs along
with semi-autolysis
• P-protein (slime) can plug pores in
sieve areas
Gymnosperms:
Long sieve cells with
over-lapping sieve
areas
Angiosperms:
Shorter sieve tube
elements
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Albuminous Cells
(Axial & Ray in Gymnosperms)
Albuminous Cells
(Gymnosperms)
• Not same origin as sieve cells
• Rich in proteins
• Die when sieve cells are
disorganized
• Adjacent to walls of sieve cells &
with prominent sieve areas
• Occur in rays at margins; erect
cells*
* Following slide
Sieve Plates–
(Angiosperm)
Sieve Areas–
from Pine (Pinus sp.)
Seen with blue callose
deposits (due to staining
with aniline blue.
Can you identify the
direction of movement in
the sieve tube elements?
Future of Secondary Phloem
Describe the Phloem
• Dilated rays due to circumference
expansion
• Fibers form after maturity
• Compression & collapse
• Gives way to periderm
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Phloem: Annular Cycle of Activity
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