STEMS Origin Functions External Anatomy Internal Anatomy Specialized Stems Physiology.
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Transcript of STEMS Origin Functions External Anatomy Internal Anatomy Specialized Stems Physiology.
Origin
• First stem of a plant develops from part of a seed embryo called epicotyl, w/c is a continuation of the hypocotyl
The Plant Body: StemsFUNCTIONS OF STEMS• Produces & support appendages
of plant (leaves, flowers, fruits) • transport water and solutes
between roots and leaves. • Stems in some plants are
photosynthetic.• Produce & 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.
Two Types of Aerial Stems
Herbaceous Stems• Soft & green• Little growth in diameter• Tissues chiefly primary• Chiefly annual• Covered by epidermis• Buds mostly naked
Woody Stems• Tough & not green• Considerable growth in
diameter• Tissues chiefly secondary• Chiefly perennial• Covered by corky bark• Buds chiefly covered by
scales
Apical Dominance Apical dominance refers to the
suppression of growth by hormones produced in the apical meristem.
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 loss
Cortex 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° phloem
Xylem-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: support
Functional Phloem: conduct CH2O away from leaf
Vascular Cambium: add 2° Xylem and 2° Phloem
Xylem: conduct minerals up from soil
outs
ide
to c
ente
r
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
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 year
spring of the next year
Stolons
• Stolons or runners - horizontal stem that grow above the ground with long internodes
Eg. Bermuda grass
(Cynodon dactylon)
Spider plant (Chlorophytum)
Fern (Nephrolepis)
Bermuda grass (Cynodon dactylon).
Rhizomes
• Rhizomes - horizontal stems that grow below the ground with adventitious roots
• Eg. irises, ferns, and grasses.
Tubers
• Tubers – swollen regions of stems that store food for subsequent growth
• The "eyes" of a potato (irish potatoes Solanum tuberosum) are the nodes of a starch-ladened stem
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
Bulbs• Bulbs - large buds
with a small stem at the lower end surrounded by numerous fleshy leaves that store nutrients; adventitious roots at base
• Eg. onion, tulip, hyacinth, daffodil and lily
Corms• 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
• Eg. crocus and gladiolus.
Cladophylls
•Cladophylls or cladodes - leaf-like stems modified for photosynthesis
•Eg. butcher's broom, asparagus, orchids (eg. Epidendrum)
Succulent Stems
• Succulent stems - stout fleshy stems that are modified for water and food storage
Eg. cacti
Thorns- for protection from grazing animals
• Bougainvillea
• Honey locust (modified stem)
• Black Locust (modified leaf stipules)
PHYSIOLOGY OF STEMS
• Conduction of Materials by Xylem1) Root pressure – powered by transpiration of water from the leaves
2) Transpiration pull and water cohesion – water is pulled up from the roots due to adhesion of water to the xylem walls & tension generated by the water-potential gradient bet. leaves & xylem
Other contributing factors:3) Atmospheric pressure
4) Action of Living cells 5) Imbibition in cell walls of xylem 6) Capillary attraction
PHYSIOLOGY OF STEMS
• Conduction of Materials by Phloem - nutrient-rich fluid in the phloem moves from
areas of high solute concentration & water pressure to areas of low solute concentration & water pressure
• Hypotheses of phloem function are:1) cytoplasmic streaming2) movement through interface3) pressure flow or mass flow