STEMS: FORM & FUNCTION
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•
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• Function
External Anatomy
Internal Anatomy
Specialized Stems
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.
Stems support a display of
leaves.
Stems orient the leaves
toward the light with minimal
overlap among the leaves.
Asclepias - milkweed
The stem supports a display of flowers
Cercis canadensis - redbud
The stem does photosynthesis…and stores water.
Opuntia-prickly pear
This stem does
photosynthesis, stores
water, but also produces a
defense chemical:
mescaline…a hallucinogen.
Lophophora williamsii - peyote
EXTERNAL ANATOMY
EXTERNAL ANATOMY
STEM APICAL MERISTEM
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.
PRIMARY & SECONDARY GROWTH
Monocotyledonous &
Dicotyledonous Flowering Plants
Monocot Stem – cross section
INTERNAL STEM ANATOMY
Typical Stem Cross Section (Dicot Stem)
Epidermis
Cortex
A ring of vascular bundles
Pith
Helianthus annuussun flower annual
Epidermis
- window, reduce water loss
Cortex Collenchyma
- extensible support
Cortex Parenchyma
- photosynthesis, etc.
Fibers- rigid support
Functional Phloem
- conduct sugars etc. away
from leaf to rest of plant
Vascular Cambium
- adds 2° xylem and 2° phloem
Xylem
-conduct water and minerals
up from soil
Pith
-water storage, defense?
VIP Stem: Provide both name and function labels:
Epidermis: reduce evaporation, gas exchange
Cortex: photosynthesis, collenchyma support
Vascular Bundles: conduction
Pith: water storage? defense? disintegrate?
outside
Vascular Bundle:
to center
outside
Phloem Fibers: support
Functional Phloem:
conduct CH2O away from leaf
Vascular Cambium:
add 2° Xylem and 2° Phloem
Xylem:
to center
conduct minerals up from soil
Vitis vinifera - grape
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
 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 => 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.uni-hamburg.de/bonline/library/webb/BOT311/PrimSec/primarysecondary4.htm and
http://en.wikipedia.org/wiki/Fasces
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
The vascular cambium makes 2° tissues:
Vitis vinifera - grape
Basswood – 1 & 2 years old
Three years of Secondary Growth
Tilia - basswood
Secondary
Phloem
Secondary
Xylem
A cork cambium differentiates and produces a periderm.
Epidermis
cutin
suberin
Cork Cells
Cork Cambium
Phelloderm
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!
Bark =
epidermis +
periderm +
cortex +
phloem +
vascular
cambium
Wood =
secondary
xylem only!
Pith =
a small
percentage of
tree diameter
at maturity
Anatomy of a Woody Stem
The trees pictured below have long lost their
epidermis on the woody portion of the stem
Sequoia sempervirens - giant sequoia
The study of the growth rings in wood: Dendrochronology
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
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
tell…perhaps something of
migration of Asian peoples
down the western coast of
North America! They were
contemporaries of Pharaohs!
Modified & Specialized Stems
Food Storage Stems
Prickly Pear
Cactus
Bamboo Shoots
Kohlrabi
Food Storage Stems - Sugarcane
Food Storage Stems - Asparagus
Rhizomes
• Rhizomes - horizontal
stems that grow
below the ground with
adventitious roots
• Examples of plants
that can produce
rhizomes are irises,
ferns, and grasses.
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
Tuber
• Tubers accumulation of food
at the tips of
underground stolons
• The "eyes" of a potato
are the nodes of a
starch-ladened stem
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
Taters and Spuds
Rosette
• Rosette - stem with short internodes and leaves
attached at nodes
Wild Radish – Rosette & Bolt
A FLOWERING ANNUAL
YEAR ONE
YEAR ONE
Common Mullen – Rosette & Bolt
A FLOWERING BIENNIAL
YEAR ONE
YEAR TWO
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
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.
Cladophylls
•Cladophylls - leaflike stems; examples
include butcher's
broom, asparagus
Photosynthetic Stems
• Cacti - stout fleshy
stems that are
modified for food and
water storage and
photosynthesis.
Thorns
• Honey locust
(modified stem)
• Black Locust (modified
leaf stipules)
Tendrils
Grape Tendrils
Sugar Loading of Phloem and Bulk Flow
Sugar Loading of Phloem and Bulk Flow
Transpiration-Cohesion Hypothesis
for Water Movement