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The Plant Body: StemsFUNCTION 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
This stem does photosynthesis, stores water, but also produces a defense chemical: mescaline…a hallucinogen.
Lophophora williamsii - peyote
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.
Typical Stem Cross Section (Dicot Stem)Helianthus annuus-sun flower annual
Pith
A ring of vascular bundles
Epidermis
Cortex
Epidermis - window, reduce
water lossCortex Collenchyma- extensible support
Cortex Parenchyma- photosynthesis,
etc.Fibers- rigid support
Functional Phloem- conduct sugars etc.
away from leaf to rest of plantVascular Cambium
- adds 2° xylem and 2° phloemXylem
-conduct water and minerals
up from soilPith-water storage,
defense?
VIP Stem: Provide both name and function labels:
outside
to center
Epidermis: reduce evaporation, gas exchange
Cortex: photosynthesis, collenchyma support
Vascular Bundles: conduction
Pith: water storage? defense? disintegrate?
Vascular Bundle:
Phloem Fibers: supportFunctional Phloem: conduct CH2O away from
leafVascular Cambium:
add 2° Xylem and 2° PhloemXylem:
conduct minerals up from soil
outsi
de
to
center
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
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/b-online/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
A cork cambium differentiates and produces a periderm.
Epidermis
Cork Cambium
Phelloderm
cutin
suberin
Cork Cells
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
Sequoia sempervirens - giant sequoia
The trees pictured below have long lost their epidermis on the woody portion of the stem
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).
mid-summer of one year
fall of that yearwinter of that yearspring of the next 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!
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 PotatoPotato first domesticated in region of modern day Bolivia and Peru
Failure of the potato crop in 1845-49 led to the Irish Potato Famine
Wild Radish – Rosette & BoltWild Radish – Rosette & Bolt
YEAR ONEYEAR ONE YEAR ONEYEAR ONE
A FLOWERING ANNUALA FLOWERING ANNUAL
Common Mullen – Rosette & BoltCommon Mullen – Rosette & Bolt
YEAR ONEYEAR ONE
YEAR TWOYEAR TWO
A FLOWERING BIENNIALA FLOWERING BIENNIAL
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.
Photosynthetic Stems
• Cacti - stout fleshy stems that are modified for food and water storage and photosynthesis.