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Prepared by:Pooja KhanparaAPIP, Jamnagar
STEMS
STEMS• Origin• Functions • External Anatomy • Internal Anatomy • Specialized
Stems
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.
The stem supports a display of flowers
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
Two Types of Aerial Stems
Herbaceous Stems Woody Stems
Soft & greenLittle growth in diameterTissues chiefly primaryChiefly annualCovered by epidermisBuds mostly naked
Tough & not greenConsiderable growth in
diameterTissues chiefly secondaryChiefly perennialCovered by corky barkBuds chiefly covered by
scales
Herbaceous Stem woody stem
EXTERNAL ANATOMY
STEM APICAL MERISTEM
PRIMARY & SECONDARY GROWTH
Apical DominanceApical 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(thick).
INTERNAL STEM ANATOMY
Monocotyledonous & Dicotyledonous Flowering Plants
Monocot Stem – cross section
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
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 - grapeThe vascular cambium makes 2° tissues:
2° phloem2° xylem
Basswood – 1 & 2 years old
Three years of Secondary Growth
Tilia - basswood
SecondaryXylem
Secondary Phloemcambium
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
Anatomy of a Woody Stem
Sequoia sempervirens - giant sequoia
The trees pictured below have long lost their epidermis on the woody portion of the stem
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).
mid-summer of one year
fall of that yearwinter of that year
spring of the next year
StemsStems—the axes of plants—consist of nodes (where leaves and axillary buds are produced) separated by internodes.
Node - region of the stem where the leaf and bud are borne.
Internode - the part of the stem between two adjacent nodes.
Herbaceous - not woody; dying down at the end of the growing season.
Woody - hard in texture, containing secondary xylem, and persisting more than one growing season.
Acaulescent - having an inconspicuous stem.
Caulescent - having a distinct stem.
Modified & Specialized Stems
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.
Rhizomes
• Rhizomes - horizontal stems that grow below the ground with adventitious roots
• Eg. irises, ferns, and grasses.
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)
Food Storage StemsPrickly Pear Cactus
Bamboo Shoots Kohlrabi
Food Storage Stems - Sugarcane
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
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).
Rosette• Rosette - stem with short internodes and leaves
attached at nodes
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
Buds
Buds are short embryonic stems. In angiosperms they are found at the nodes, in the leaf axil (the angle formed by the stem and the petiole of the leaf).
Axillary bud - a bud located in the leaf axil.Terminal bud - a bud at the apex of a stem.
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
Transpiration-Cohesion Hypothesis for Water Movement
Sugar Loading of Phloem and Bulk Flow
Sugar Loading of Phloem and Bulk Flow