Overview

• support above ground structures
• transport water & nutrients from environment up to leaves
• transport nutrients from leaves down to rest of plant
• may store toxins away from transport system

Development

• embryo [in seed] has epicotyl and plumule
  • epicotyl with apical and small lateral meristems
  • plumule is usually undeveloped leaves
• on germination epicotyl becomes primary stem
• branch stems grow from primary stem
• adventitious stems may grow from roots
  • sometimes spontaneously
  • often in response to root injury
  • may be triggered by hormone
    [auxin (pg 194-197) or auxin and gibberellin (pg 197-198)]

Anatomy, tissue level

• indeterminate growth
• Apical Meristem increases length
• Lateral meristem increases girth [diameter]
  • divides toward axis: producing xylem
  • divides laterally: increasing circumference of meristem
  • divides toward epidermis: producing phloem
  • divides laterally: increasing circumference of meristem
  • repeats indefinitely
• seasonal differences in growth rate produce annual growth rings
• cork cambium outside of phloem
• herbaceous versus woody stems
  • produces bark and sometimes cork
• mature tissues
  • xylem conducts water & nutrient up stem
    • theoretical max at sea level = 32 feet above ground water
    • central region dies; provides support for above ground portions
    • xylem rays
    • toxins are deposited as crystals in non-living xylem
  • phloem conducts sugar down stem
  • bark protects plant from mechanical and fire damage
  • cork, when present, aborbs mechanical energy

Anatomy, gross

• terminal bud
  • with scales when dormant
  • exhibits apical dominance by secreting auxins [suppress lateral buds]
• internodal regions between leaves, branches
• leaves attached at nodes
  • alternate [including spiral]
  • opposite [including whorls]
  • lateral buds often associated with leaves
    • adaxial [normal]
    • abaxial [sometimes]
  • lateral buds produce branches
    • terminal buds produce vertical growth
    • lateral buds produce horizontal growth
    • pruning can determine future growth
    • trees at timberline: lateral buds produce krumholtz

Specialized stems

• Rhizomes [underground root-like stems usually with branches]
• Runners [above ground horizontal stems with long internodes]
• Stolons [below ground stems with long internodes and tubers]
• Bulbs [buds on small stems]
• Corms [bud-like stems]
• Cladophylls [leaf-like stems]

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