Stems

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Chapter 9

• Stems

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Vascular bundles

• Bundles of xylem and phloem
• Conducting tissue

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Close-up of a vascular bundle

• Phloem
• Conducts photosynthate (dissolved sugars)
• Xylem
• Conducts water and dissolved minerals

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Bundle arrangement differs in monocots vs. dicots

• Monocot stems have vascular bundles scattered
  throughout the stem
• Dicot stems have bundles on the periphery of the
  stem

Young Dicot Stem
Monocot stem
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Dicot vs. monocot stems
Older Dicot stem (Basswood?)
Monocot stem (asparagus)
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Development of xylem and phloem

• In a dicot stem xylem to the inside, phloem to
  the outside
• Xylem is secondary tissue we call wood

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Typical dicot stem development

• Year one bundles on the periphery of the stem
• Year two bundles begin to coalesce vascular
  cambium in a continuous ring
• Year three rings are being laid down
• Later years pith gets crushed on continuous
  xylem is present

Year One
Year Three
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Xylem development (continued)
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Xylem

• Xylem conducts water, dissolved nutrients
• Xylem supports tree
• Made of three cell types
• Tracheids
• Vessel elements
• Fibers

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Xylem development

• As development progresses, secondary cell walls
  form
• Vessels are big
• Tracheids small
• Fibers add support

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Xylem cells tracheids and vessel elements

• Tracheidssmaller diameter less efficient means
  of conducting water
• Vessel elementslarger, only in flowering plants

Pine Tracheids
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Annual rings of wood (secondary xylem)
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Oldest trees

• Bristlecone pine in California and Nevada
• 10,000 years old!

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Phloem

• Phloemconducts dissolved sugars (photosynthate)
  through the plant
• Two cell types
• Sieve cells
• Companion cells

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Phloem (continued)

• Phloem is living at maturity
• No thick secondary cells walls (i.e. parenchyma)
• Sieve cells dont have nucleiwhy?
• Companion cells do have nuclei

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How does phloem work

• Sugar water is loaded from a source cell into
  phloem
• Source cells are usually photosynthetic cell
• Water diffuses by osmosis into phloem from xylem
• Sink cells (e.g. root) usually unload sugars, but
  this requires ATP (unlike water transport)