STEMS: FORM

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STEMS FORM FUNCTION

• Function
• External Anatomy
• Internal Anatomy
• Specialized Stems

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The Plant Body Stems

• FUNCTION OF STEMS
• Stems support leaves and branches.
• Stems transport water and solutes between roots
  and leaves.
• Stems in some plants are photosynthetic.
• Stems may store materials necessary for life
  (e.g., water, starch, sugar).
• In some plants, stems have become adapted for
  specialized functions.

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Stems support a display of leaves.
Stems orient the leaves toward the light with
minimal overlap among the leaves.
Asclepias - milkweed
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The stem supports a display of flowers
Cercis canadensis - redbud
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The stem does photosynthesis
and stores water.
Opuntia-prickly pear
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This stem does photosynthesis, stores water, but
also produces a defense chemical mescalinea
hallucinogen.
Lophophora williamsii - peyote
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EXTERNAL ANATOMY
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EXTERNAL ANATOMY
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STEM APICAL MERISTEM
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Apical Dominance

• Apical dominance refers to the suppression of
  growth by hormones produced in the apical
  meristem.  The Christmas tree pattern of pines
  indicates strong apical dominance.  Bushy plants
  have weak apical dominance.  If apical meristem
  is eaten or destroyed, plants may become bushy.
• Lateral branch growth are inhibited near the
  shoot apex, but less so farther from the tip.
• Apical dominance is disrupted in some plants by
  removing the shoot tip, causing the plant to
  become bushy.

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PRIMARY SECONDARY GROWTH
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Monocotyledonous Dicotyledonous Flowering Plants
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Monocot Stem cross section
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INTERNAL STEM ANATOMY
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Typical Stem Cross Section (Dicot Stem)
Helianthus annuus- sun flower annual
A ring of vascular bundles
Pith
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VIP Stem Provide both name and function labels
Vascular Bundle
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Vitis vinifera - grape
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Notice how the vascular cambia of adjacent
vascular bundles line up side by side.
Notice that cambium tissue differentiates between
the bundles, connecting the cambia together.
Remnants of the procambium Intrafasicular
cambium Interfasicular cambium
Vitis vinifera - grape
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• If you have ever been to Washington DC you will
  see how the early architects and artists for
  governmental buildings were impressed with
  Greco-Roman architecture and symbols.
• On each side of the seat occupied by the
  presiding officer of the Senate are two Fasces.
  The Romans had many symbols of Power.
• One of them was a bundle of sticks lashed
  together in a cylinder with a long axe in the
  center. This is a Fasces! Early Botanists noted
  that the vascular tissue in stems appeared in
  discrete bundles which they called Fascicles!!!
• Fasces gt Fascicles (bundle). 
• An area of Ground Tissue between the Fascicles
  was called Interfascicular! Remnants of the
  procambium between the primary phloem and xylem
  was called Intrafascicular.
• Information obtained from http//www.biologie.un
  i-hamburg.de/b-online/library/webb/BOT311/PrimSec/
  primarysecondary4.htm and http//en.wikipedia.org/
  wiki/Fasces

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Fasces

• Fasces (from the Latin word fascis, meaning
  bundle) symbolise summary power and jurisdiction,
  and/or "strength through unity.
• The traditional Roman fasces consisted of a
  bundle of birch rods tied together with a red
  ribbon as a cylinder around an axe.
• One interpretation of the symbolism suggests that
  despite the fragility of each independent single
  rod, as a bundle they exhibit strength.
• See http//en.wikipedia.org/wiki/Fasces

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The vascular cambium makes 2 tissues
Vitis vinifera - grape
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Basswood 1 2 years old
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Three years of Secondary Growth
Tilia - basswood
Secondary Phloem
cambium
Secondary Xylem
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A cork cambium differentiates and produces a
periderm.
cutin
suberin
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Over time, the epidermis dies. The cork cells
build up to for a thick layer for the bark of a
tree. We use this to make stoppers for wine
bottles and so on.
When suberin is fully developed, the cortex cells
will eventually be in the dark. So these
chloroplasts will lose their function!
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Bark epidermis periderm cortex phloem
vascular cambium
Wood secondary xylem only!
Pith a small percentage of tree diameter at
maturity
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Anatomy of a Woody Stem
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The trees pictured below have long lost their
epidermis on the woody portion of the stem
Sequoia sempervirens - giant sequoia
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The study of the growth rings in wood
Dendrochronology
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Each year the cambium produces a layer of
secondary xylem and a layer of secondary phloem.
This photo shows secondary xylem from parts of
three years in Pinus strobus (white pine).
spring of the next year
winter of that year
fall of that year
mid-summer of one year
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This tree is Pinus aristata (bristlecone pine).
One individual of this species shows more than
5000 growth rings!
Inner wood, harvested by boring, was used to
validate carbon-14 dating.
Imagine the stories that this California tree
could tellperhaps something of migration of
Asian peoples down the western coast of North
America! They were contemporaries of Pharaohs!
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Modified Specialized Stems
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Food Storage Stems
Prickly Pear Cactus
Bamboo Shoots
Kohlrabi
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Food Storage Stems - Sugarcane
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Food Storage Stems - Asparagus
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Rhizomes

• Rhizomes - horizontal stems that grow below the
  ground with adventitious roots
• Examples of plants that can produce rhizomes are
  irises, ferns, and grasses.

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Stolons

• Stolons or runners - horizontal stem that grows
  above the ground with long internodes
• Examples of plants that can produce stolons are
  strawberry and airplane plants

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Tuber

• Tubers - accumulation of food at the tips of
  underground stolons
• The "eyes" of a potato are the nodes of a
  starch-ladened stem

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History of the Potato
Potato first domesticated in region of modern day
Bolivia and Peru
Failure of the potato crop in 1845-49 led to the
Irish Potato Famine
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Taters and Spuds
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Rosette

• Rosette - stem with short internodes and leaves
  attached at nodes

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Wild Radish Rosette Bolt
A FLOWERING ANNUAL
YEAR ONE
YEAR ONE
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Common Mullen Rosette Bolt
A FLOWERING BIENNIAL
YEAR ONE
YEAR TWO
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Bulb

• Bulbs - large buds with a small stem at the lower
  end surrounded by numerous fleshy leaves,
  adventitious roots at base
• Examples include onion, tulip, and lily

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Corm

• Corms - resemble bulbs but composed entirely of
  stem tissue surrounded by a few papery scale like
  leaves, food storage organs with adventitious
  roots at the base of corms
• Examples include crocus and gladiolus.

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Cladophylls

• Cladophylls - leaf-like stems examples include
  butcher's broom, asparagus

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Photosynthetic Stems

• Cacti - stout fleshy stems that are modified for
  food and water storage and photosynthesis.

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Thorns

• Honey locust (modified stem)
• Black Locust (modified leaf stipules)

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Tendrils
Grape Tendrils
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Sugar Loading of Phloem and Bulk Flow
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Sugar Loading of Phloem and Bulk Flow
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Transpiration-Cohesion Hypothesis for Water
Movement