Transcript Plant Structure and Growth

P LAN T S
Mrs. Daniels
Advanced Biology
Revised Nov. 2006
Why Study Plants?
• Plants form the foundation on which most
terrestrial ecosystems are built
– Basis of food chain because they are producers
• Plants were first studied by humans who had to
distinguish btwn edible and poisonous plants
– These early humans later began to use plant products to
make useful tools and other items
– What uses do plants have today?
– Modern plant biology continues to center on how to use
plant and plant products to benefit humans
Alternation of Generation
• plants spend part of their life cycle as a
haploid and part as a diploid organism
• The haploid portion is the gametophyte
generation
• The diploid portion is called the sporophyte
generation
Alternation of Generation
• The gametophyte generation is dominant in
lower plants
• The sporophyte generation is dominant in more
advanced plants
• All plants produce spores by meiosis
• Algae and fungi produce spores by either
mitosis or meiosis
Figure 26-2
Page 500
Gametophyte
Spore
Egg
Sperm
HAPLOID (n)
GAMETOPHYTE
GENERATION
Meiosis
Fertilization
DIPLOID (2 n)
SPOROPHYTE
GENERATION
Zygote
Embryo
Sporophyte
Plants move onto land
• Environmental challenges of living on land
required adaptations over thousands of years
• These adaptations had to be:
• Anatomical
• Physiological
• Reproductive
Adaptations
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Alternation of generations
Reproductive structures
Seeds vs. spores
Stomata for gas exchange
Cuticle for prevention of water loss
Vascular tissues and more structural support
Etc.
Four major types of plants
• 1. Bryophytes: seedless, non-vascular
• 2. Tracheophytes: Seedless vascular plants
– 3.
– 4.
Gymnosperms: non-flowering, seeded,
vascular plants
Angiosperms: flowering, seeded,
vascular plants
Bryophytes
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Mosses, liverworts, and hornworts
Colonial plants
Rhizoids anchor them
No true roots, stems, or leaves
Lack vascular tissue
Sporophyte portion
of moss
• Small and easy to grow
• Used for experiments in genetics, plant hormones,
tropisms, etc.
haircap moss
(Polytrichum commune)
common hornwort
(Anthoceros natans)
common liverwort
(Marchantia polymorpha)
Tracheophytes
• Vascular, seedless plants
• Ferns (11,000 species)
• Whisk ferns, horsetails, club
mosses
• Dominant sporophyte generation
• Used to study apical meristems
• Are polyploidy, so used in gene
expression studies
Seeds Vs. Spores
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Both Bryophytes and Tracheophytes
(ferns) are seedless
• The next two groups of plants we’ll
discuss have produce seeds
• What are the advantages of seeds?
1. Further development than spores
2. Contain abundant food supply
3. Protected by a thick seed coat
Parts of a Seed
• We’ll examine this later in chapter 35
• For now, let’s see which plants produce
them
Gymnosperms
• Vascular, seeded plants
• “naked seed” plants
• Redwoods, Sequoias, Ginkgoes,
pines, spruce, firs, etc.
• Divided into 4 phyla: Pinophyta,
Cycadophyta, Ginkgophyta, and
Ginkgo Trees
Gnetophyta
• Tropical vines and ephedrine (shrub
resembling horsetails) are oddly enough
gymnosperms
• Phylum Pinophyta
– Conifers that produce
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Wood
Bark
Needles
Seeds in cones
– Most are monoecious: both male and female
reproductive structures are on the same plant
– Phylum Cycadophyta
• Look like palms or ferns
• Dioecious (male plant and female plant), but
reproduce with pollen and seeds in conelike
structures
• Once numerous, now few members left
Cycads
This female cycad in
South Africa has a
trunk that reaches a
height of about 9 m.
Note immense seed
cones, to 0.8 m long
• Phylum Ginkgophyta
– Sole member is
Ginkgo biloba
Branch from a female ginkgo, showing
exposed seeds and distinctive leaves
– Deciduous
– Dioceious
– Female ginkgo produces seeds directly on
branches
– Gnetophytes
• Consist of three genera
–Gnetum
–Ephedra
–Welwitschia
• Unique among gymnosperms, sharing traits
with angiosperms
• Vessel elements in their xylem
• Cone clusters resemble flower clusters
• Life cycle details resemble those of
angiosperms
Leaves of Gnetum gnemon
resemble those of flowering
plants
Male Ephedra has pollen
cones clustered at the nodes
A specimen of Welwitschia mirabilis living in Namib
Desert, Namibia – survives on fog
Angiosperms
• Seeded, vascular plants that flower (and the seeds are
enclosed in a fruit)
• After fertilization, the ovules become seeds and the ovary
becomes fruit
• Monocots and dicots
• Monocots: herbaceous ex. Lilies, grasses, corn, palms,
orchids
• Dicots: herbaceous or woody
• Ex. Roses, oaks, potatoes
Monocots
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Flower parts in 3’s
One pore in pollen grain
Parallel venation in leaves
Scattered vascular bundles
Fibrous root system
One cotyledon
Lack secondary growth
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Dicots
Flower parts in 4’s & 5’s
3 pores in pollen grain
Netted venation in leaves
Vasc bundles in ring
Taproot system
Two cotyledons
Often have secondary
growth
Ch31
Plant Structure and Life Span
• Roots, stems, and leaves
• Herbaceous or woody
• Annuals, biennials, perennials
• Woody perennials can be either deciduous
or evergreen
The Root System
Fxn -anchor plants, absorb & conduct water and nutrients,store food
• Root systems (under ground)
• Tap root=
• One large, vertical root produces many smaller secondary
roots
• Provides firm anchorage
• Some taproots such as carrots, turnips, and sweet potatoes,
are modified to store a large amount of reserve food
• Fibrous root=found primarily in monocots (palms, bamboo,
grasses)
• A mat of threadlike roots spreads out below the soil surface
• Provides for extensive exposure to soil water and minerals
• Roots are concentrated in the upper few centimeters of soil,
preventing soil erosion
• Adventitious roots= roots arising above ground from stems
or leaves
• Form in addition to normal root system
• Some, such as prop roots of corn, help support the plant stem
The Shoot System
• “above ground”
• Stems, leaves, flowers, and fruits
Roots and Shoots
• Air is the source for photosynthesis and sunlight cannot
penetrate into the soil
– Soil provides water and dissolved minerals to the plant
• Each system depends upon the other for survival of the
whole plant
– Roots depend on shoots for sugar and other organic
nutrients
– Shoots depend on roots for minerals, water and support
• Materials are transported through the plant by vasc tissues
– Xylem conveys water and dissolved minerals to the
shoots
– Phloem conveys food from shoots to roots and other
nonphotosynthetic parts. And from storage roots to
actively growing shoots
• Xylem consists of two cell types, both with secondary walls
and both dead at functional maturity
• Tracheids= long, thin, tapered cells having lignin-hardened
secondary walls with pits(thinner regions where only
primary walls are present)
• Water flows from cell to cell through pits
– Also function in support
• Vessel Elements are wider, shorter, thinner-walled, and less
tapered than tracheids
– Vessel elements are aligned end to end
– The end walls are perforated, permitting the free flow of
water through long chains of vessel elements called
xylem vessels
• Sieve-tube members are chains of phloem cells
that transport sucrose, other organic compounds
and some minerals
– The cells are alive at functional maturity
• Companion cells also help load sugar produced
in the mesophyll into sieve-tubes of leaves of
some plants
The Three Tissue Systems of a Plant
Plant cells are arranged into three tissue systems.
• Dermal Tissue System
– Dermal tissue = epidermis = single layer of tightly packed cells covering
and protecting the young parts of the plant
• Functions in protection and has special characteristics consistent with the
function of the organ it covers
• Vascular Tissue System
– Vascular tissue = xylem and phloem that functions in transport and
support; is continuous throughout the plant
• Ground Tissue System
– Ground tissue = predominantly parenchyma, with some collenchyma and
sclerenchyma, that fills the space btwn dermal and vascular tissue
systems
– Functions in photosynthesis, storage and support
The many types of plant cells are organized into the three
major tissue systems
Each type of plant cell has structural adaptations that
make it possible to perform that cell’s function.
Some are coupled with specific characteristics of
the protoplast = the plant cell contents exclusive of
the cell wall
• Parenchyma
• Collenchyma
• Sclerenchyma
• Tracheids (already mentioned)
• Sieve-Tube members (already mentioned)
Parenchyma Cells
Parenchyma cells are the least specialized plant cells
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Primary cell walls are thin and flexible
Most lack secondary walls
The protoplast usually has a large central vacuole
Function in synthesizing and storing organic products
Most mature cells do not divide, but retain the ability to
divide and differentiate into other cell types under special
conditions - repair and replacement after injury
Collenchyma Cells
Collenchyma cells usually lack secondary walls
• They are usually grouped in strands or
cylinders to support young parts of plants
without restraining growth
• They are living cells which elongate as the
stems and leaves they support grow
Sclerenchyma Cells
Sclerenchyma cells function in support
• They have very rigid, thick secondary cell walls
strengthened by lignin
• Many lack protoplast at functional maturity
• they cannot elongate and may be dead, functioning
only as support
• There are two forms:
– Fibers= long slender, tapered cells occurring in
bundles
– Sclerids= shorter irregularly shaped cells
Plant Structure and Growth
Meristems generate cells for new organs throughout the
lifetime of a plant
-Apical meristems extend roots and shoots
-Lateral meristems add girth to stems and roots
Meristems generate cells for new organs throughout
the lifetime of a plant
Plant growth begins with germination of the seed and
continues for the lifespan of the plant
• Indeterminate growth= growth as long as the plant lives
– Most animals cease growing after reaching a certain
size(determinate growth)
– Certain plant organs, such as flower parts also show
determinate growth
Indeterminate growth made possible by Meristems
• Meristematic (perpetually embryonic tissues) are
unspecialized and divide to generate new cells near the
growing point
• Apical meristems are located in root tips and shoot
buds. They supply cells for plants to grow in length
– Primary growth(elongation) is initiated by apical
meristems and forms primary tissues organized into
the 3 tissue systems
• Secondary growth (increased girth) is the
thickening of roots and shoots which occurs in
woody plants due to development of lateral
meristems= cylinders of dividing cells extending
along the lengths of roots and shoots
– Cell division in the lateral meristems produces
secondary dermal tissues which are thicker and
tougher than the epidermis is replaces
– Adds new layers of vascular tissue
The primary plant body with its three tissue
systems is produced by primary growth
• Primary growth of roots is concentrated near the tip and
results in roots extending through the soil
– The root tip is covered by a root cap, which which protects
the meristem and secretes a polysaccharide coating that
lubricates the soil ahead of the growing root
– The root tip contains three zones of cells in successive
stages of primary growth. Although described separately,
these zones blend into a continuum
• Zone of Cell Division
• Zone of Cell Elongation
• Zone of Maturation
Primary Tissues of Stems
• The vascular tissue of the stem is organized into strands of
vascular bundles that run the length of the stem
– Converge at the transition zone (shoot to root)
– Each bundle is surrounded by ground tissue, including pith
and cortex
• In dicots, bundles are arranged in a ring with pith inside and
cortex outside
– Xylem faces the pith, phloem faces the cortex
– Pith and cortex are connected by pith-rays, thin layers of
ground tissue
• In monocots, bundles are scattered throughout the ground tissue
of the stem
– The stem ground tissue is mostly parenchyma, strengthened in
many plants by collenchyma located beneath the epidermis
Tissue Organization of Leaves
Leaves are cloaked by an epidermis of tightly interlocked cells
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Epidermis- tightly interlocked cells
– protect against physical damage, pathogens
– prevent excess water loss (waxy cuticle)
Stomata- pores flanked by guard cells
– regulate gas exchange with the surrounding air and photosynthetic cells inside the leaf
– Allows for transpiration (water loss from plant by evaporation)
– More numerous on the bottom surface of the leaf. This location minimizes water loss
Mesophyll=ground tissue of the leaf
– Consists mainly of parenchyma cells and equipped with chloroplasts for photosynthesis
– Dicots have two distinct regions of mesophyll:
• Palisade (mesophyll) parenchyma=one or more layers of culumnar cells of the upper
half of a leaf
• Spongy mesophyll=irregularly shaped cells surrounded by air spaces through which
oxygen and carbon dioxide circulate. Located in the lower half of the leaf
Lateral meristems add girth to stems and roots: a closer
look at secondary growth
• The secondary plant body is comprised of the secondary
tissues produced during growth in diameter
• Secondary growth results from two lateral meristems: the
vascular cambium and the cork cambium
– Vascular cambium produces secondary xylem and phloem
– Cork cambium produces a tough, thick covering for roots
and stems that replaces the epidermis
• Secondary growth occurs in all gymnosperms and most
dicot angiosperms
– It is rare in monocots
Secondary growth of stems
Vascular cambium forms when meristematic parenchyma cells
develop btwn the primary xylem and primary phloem of each
vascular bundle and in the rays of ground tissue btwn the bundles
Wood
• Accumulated layers of secondary xylem produces wood
which consists mostly of tracheids, vessel elements and
fiber
• The hardness and strength of wood results from the cells
which, while dead at maturity, have thick lignified walls
• Forms annual growth rings due to yearly activity: cambium
dormancy, spring wood production and summer wood
production
• The secondary phloem does not accumulate extensively.
The secondary phloem, and all tissues external to it,
develop into bark which eventually sloughs off the tree
trunk
Cork
• Cork cambium is a cylinder of meristematic tissue that forms
protective layers of the secondary plant body
• Forms first in the outer cortex of the stem
• Cork cells form to the outside of cork cambium
• As cork cells mature, they deposit suberin (a waxy material) in
their walls and die
• The dead cork tissues protect the stem from damage and
pathogens while reducing water loss
• The combination of cork cambium and layers of cork form the
periderm (protective coat of the secondary plant body that
replaces primary epidermis)
• The term bark refers to all the tissues external of the vascular
cambium (phloem, cork cambium and cork)
Take time to stop and smell the
roses…
Well, at least draw one.
• You had to draw a flower in your question
packet…so we won’t actually do that again
• But…
• What are the male reproductive parts?
• Stamen (made up of anther and filament)
• The female parts?
• Pistil or carpel (made up of stigma, style, and
ovary)
Plant Sex
• Just kidding…sexual reproduction in plants
involves the parts we just mentioned
• How does it work?
• There is a process called double fertilization
Double Fertilization
• Pollen is formed in the anther and is
released
• It lands on the stigma at the tip of the carpel
• A pollen tube grows from the pollen grain
down the carpel into the embryo sac
• Two sperm are discharged and travel
through the tube to the egg
• One sperm fertilizes the egg to become a
diploid zygote (develops into the embryo)
Double Fertilization
• The second sperm combines with the polar
nuclei to form a triploid endosperm, which
will become the food for the developing
embryo
• The fertilized egg, endosperm, and ovule
now make up a seed
• The ovary becomes the fruit, which can
contain several seeds
Seeds
• Some of the major parts of a seed that you
should know:
• Plumule: shoot tip
• Radicle: embryonic root
• Endosperm: provides nourishment in
monocots
• Cotyledon:provides nourishment in dicots
• Seed coat:provides protection
Plant Hormones
• What drives all of this growing and
reproducing?
• HORMONES!
• Yes, plants have them too.
• Quickly, before we look at the chemical
causes of growth let’s look at what plants
are growing in response to
Tropic Responses
• Tropism: directional growth movement to
directional stimuli
• Phototropism - light
• Gravitropism - gravity
• Thigmotropism - touch
• Skototropism - shadow
Plant Hormones
• Hormone:
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Organic chemical
Transported through the organism
Goes to a target site where action is required
Active in very small quantities
Plant Hormones
1. Auxins:
• Generally causes softening of the cell wall, which
allows cell expansion (of vacuole)
• Active in fruit development and stem elongation
• Role in abscission control
• Synthesized in areas of cell division
• Circulation in xylem and phloem through polar
transport (in only one direction - away from synthesis
site)
Plant Hormones
2. Gibberelins
• help to control stem elongation, seed germination,
flowering, etc.
• Moves very freely through xylem and phloem (no
polar transport)
• Found in shoot, young leaves, root tips, and seeds
• Effects membrane permeability and loosens cell walls
(may promote softening)
Plant Hormones
3. Cytokinins:
• Cell division factors
• Synthesized in root tips, developing endosperm,
embryo, and possible young leaves
• Found in phloem sap
• Initiates buds, promotes nutrient mobility, and
prevents senescence ( loss of chlorophyll and
break down of photosynthetic apparatus)
Plant Hormones
4. Absisic Acid:
• Plant growth inhibitor by antagonizing other
hormones, inhibiting DNA/RNA synthesis, and
changing membrane permeability
• Transported through xylem and phloem
• Prepares plant for winter, induces dormancy of
buds, and regulates water loss by controlling
stomata opening and closing
Plant Hormones
5. Ethylene:
• Any tissue can produce it under right conditions
• Environmental conditions (stress, pathogens,
mechanical action) determine production
• Controls fruit ripening, abscision, flower
senescence, and stem elongation
• Is counteracted by carbon dioxide
And that’s all I have to say
about that…