Plant stuff Flashcards
5 key traits that appear in all plants but are absent from charophytes?
- Alternation of generations
- Multicellular, dependent embryos
- Walled spores produced in sporangia
- Multicellular gametangia –multicelluar organs
that produce gametes - Apical meristems
Alteration of generations
Unlike animals in plants after meiosis the
haploid cells can develop into independent
organisms rather than gametes
Alternation of generations:
- Gametophytes (haploid)
- Sporophytes (diploid)
Where does meiosis occur in plants?
In meiosis, sexual cell division, one diploid (2n) meiocyte (a.k.a. germline cell) divides to produce four haploid (n) daughter cells.
These are further processed to become sex cells (gametes).
In plants this occurs in the archegonia in females and in the antheridia in males.
In plants, walled spores are produced by sporangia
Plant spores are haploid reproductive cells that grow into gametophytes by mitosis.
Sporopollenin makes the walls of spores very tough and resistant to harsh environments.
Multicellular organs called sporangia are found on the sporophyte and produce spores.
○ Within sporangia, diploid cells called sporocytes undergo meiosis and generate haploid
spores.
The outer tissues of the sporangium protect the developing spores until they are ready to be released into the air
Plant gametophytes produce gametes within multicellular organs called gametangia.
A female gametangium, called an archegonium, produces a single egg cell in a vase-shaped
organ.
○ The egg is retained within the base.
Male gametangia, called antheridia, produce and release sperm into the environment.
In many major groups of living plants, the sperm have flagella and swim to the eggs though a
water film.
Each egg is fertilized within an archegonium, where the zygote develops into the embryo.
The gametophytes of seed plants are so reduced in size that archegonia and antheridia have
been lost in some lineages.
What is a gymnosperm?
Gymnosperms are called “naked seed” plants because their seeds are not enclosed in
chambers.
What is an angiosperm?
Angiosperm seeds develop inside chambers called ovaries, which originate within
flowers and mature into seeds.
Walled Spores Produced in Sporangia
The sporophyte produces spores in organs
called sporangia.
Spore walls contain sporopollenin, which makes
them resistant to harsh environments
Plant spores are haploid reproductive cells that
can grow into multicellular haploid gametophytes
by mitosis.
Multicellular Gametangia
Gametes are produced within organs called
gametangia
• Female gametangia, called archegonia, produce
eggs and are the site of fertilization
• Male gametangia, called antheridia, produce
and release sperm
Moss life cycle
A spore germinates into a gametophyte
composed of a protonema and gamete-producing gametophore
• The height of gametophytes is constrained by
lack of vascular tissues
• Rhizoids anchor gametophytes to substrate
• Mature gametophytes produce flagellated
sperm in antheridia and an egg in each
archegonium
• Sperm swim through a film of water to reach
and fertilize the egg
The Ecological and Economic
Importance of Mosses
Mosses are capable of inhabiting diverse and
sometimes extreme environments, but are
especially common in moist forests and wetlands
• Some mosses might help retain nitrogen in the
soil
• Many mosses can exist in very cold or dry
habitats because they are able to lose most of
their body water and then rehydrate and
reactivate their cells when moisture again
becomes available.
What are the characteristics of vascular plants?
Life cycles with dominant sporophytes
Vascular tissues called xylem and phloem
Well-developed roots and leaves
Transport in Xylem and Phloem
Vascular plants have two types of vascular tissue: xylem and
phloem
• Xylem conducts most of the water and minerals and includes
dead cells called tracheids
• Water-conducting cells are strengthened by lignin and
provide structural support
• Phloem consists of living cells and distributes sugars, amino
acids, and other organic products
• Vascular tissue allowed for increased height, which provided
an evolutionary advantage
Sporophylls and Spore Variations
Milestone in the evolution of plants was the
emergence of sporophylls – modified leaves that
bear sporangia
• Sori are clusters of sporangia on the undersides of
sporophylls
• Strobili are cone-like structures formed from groups
of sporophylls
Heterospory: The Rule Among Seed Plants
A heterosporous species produces two kinds of spores.
- megaspores, which develop into female gametophytes.
- microspores, which develop into male gametophytes.
Advantages of Reduced Gametophytes
The gametophytes of seed plants are microscopic
• The gametophytes of seed plants develop within the
walls of spores that are retained within tissues of the
parent sporophyte
• This arrangement protects the developing
gametophyte from environmental stress and enables
it to obtain nutrients from the sporophyte
Pollen and Production of Sperm
Microspores develop into pollen grains, which contain the male
gametophytes
• Pollination is the transfer of pollen to the part of a seed plant
containing the ovules – contrast with bryophytes and seedless v
plants
• Pollen eliminates the need for a film of water and can be
dispersed great distances by air or animals
• If a pollen grain germinates, it gives rise to a pollen tube that
discharges sperm into the female gametophyte within the ovule
Seeds provide some evolutionary advantages over
spores
Spores are single celled, seeds are multicelled.
– They may remain dormant for days to years, until
conditions are favorable for germination
– Seeds have a supply of stored food
– They may be transported long distances by wind or
animals
The Angiosperm Life Cycle
The flower of the sporophyte is composed of both male and
female structures
• Male gametophytes are contained within pollen grains
produced by the microsporangia of anthers
• The female gametophyte, or embryo sac, develops within an
ovule contained within an ovary at the base of a stigma
• Most flowers have mechanisms to ensure cross-pollination
between flowers from different plants of the same species
Development of Male Gametophytes
in Pollen Grains
Pollen develops from microspores within
the microsporangia, or pollen sacs, of anthers
• Each microspore undergoes mitosis to produce two cells: the
generative cell and the tube cell
• A pollen grain consists of the two-celled male gametophyte and
the spore wall
• If pollination succeeds, a pollen grain produces a pollen tube
that grows down into the ovary and discharges two sperm cells
near the embryo sac
Development of Female
Gametophytes (Embryo Sacs)
The embryo sac, or female gametophyte, develops within the
ovule
• Within an ovule, two integuments surround a megasporangium
• One cell in the megasporangium undergoes meiosis,
producing four megaspores, only one of which survives
• The megaspore divides, producing a cell partitioned into a
multicellular female gametophyte, the embryo sac
Double Fertilization
One sperm fertilizes the egg, while the other combines with two
nuclei in the central cell of the female gametophyte and initiates
development of food-storing endosperm
• The triploid endosperm nourishes the developing embryo
• Within a seed, the embryo consists of a root and two seed
leaves called cotyledons
Fruit Form and Function
A fruit develops from the ovary
• It protects the enclosed seeds and aids in seed
dispersal by wind or animals
• A fruit may be classified as dry, if the ovary dries
out at maturity, or fleshy, if the ovary becomes
thick, soft, and sweet at maturity
Fruits are also classified by their development
– Simple, a single or several fused carpels
– Aggregate, a single flower with multiple separate
carpels
– Multiple, a group of flowers called an
inflorescence
What are the 2 angiosperm groups?
Monocots (one cotyledon)
Eudicots (two dicots)
Different Whorl Combos
Whorl 1=sepal, A genes
Whorl 2=petal, A+B genes
Whorl 3=stamen, B+C genes
Whorl 4=carpel, C genes
Flowering time is regulated by coordinated
genetic changes that respond to:
Photoperiod
- Temperature
- Plant hormone signals
The 2 Processes of Photosynthesis
The light reactions (photo) convert solar energy to chemical energy.
○ The Calvin cycle (synthesis) uses energy from the light reactions to incorporate CO2 from the
atmosphere into sugar.
What is an Autotroph?
Autotrophs sustain themselves without
eating anything derived from other organisms
• Autotrophs are the producers of the
biosphere, producing organic molecules from
CO2 and other inorganic molecules
• Almost all plants are photoautotrophs, using
the energy of sunlight to make organic
molecules
What is the reaction for photosynthesis?
6CO2 + 12H2O –> C6H12O6 + 6O2 + 6H2O
The light reactions (in the thylakoids)
Split H2O – Release O2 – Reduce NADP+ to NADPH – Generate ATP from ADP by photophosphorylation
Info on Calvin cycle
The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH.
The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules
What is a photosystem?
A photosystem consists of a reaction -center complex (a type of protein complex) surrounded by light -harvesting complexes • The light -harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center
2 Types of photosystems
Photosystem II (PS II) functions first (the numbers
reflect order of discovery) and is best at absorbing
a wavelength of 680 nm
• The reaction-center chlorophyll a of PS II is called
P680
• Photosystem I (PS I) is best at absorbing a
wavelength of 700 nm
What is linear electron flow?
Linear electron flow, the primary pathway,
involves both photosystems and produces ATP
and NADPH using light energy
• This is the ‘canonical’ process of the lightdependent reactions
What is Cyclic electron flow?
Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH • No oxygen is released • Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle
What are C4 plants?
C4 plants minimize the cost of photorespiration by
incorporating CO2
into four-carbon compounds in
mesophyll cells
• This step requires the enzyme PEP carboxylase
• PEP carboxylase has a higher affinity for CO2
than
rubisco does; it can fix CO2 even when CO2
concentrations are low; O2 does not compete.
• These four-carbon compounds are exported to
bundle-sheath cells, where they release CO2
that is
then used in the Calvin cycle
C4 photosynthesis
Primary CO2
fixing enzyme: PEP carboxylase (phosphoenol pyruvate carboxylase) which has higher affinity for
CO2
(in fact, almost no affinity for O2)
• Needs bundle sheath anatomy
• Advantageous in high light, high temperature, high
evaporation conditions
But requires high energy
CAM Plants
Some plants, including succulents, use crassulacean
acid metabolism (CAM) to fix carbon
• CAM plants open their stomata at night, incorporating
CO2
into organic acids
• Stomata close during the day, and CO2 is released from
organic acids and used in the Calvin cycle
CAM Plants photosynthesis
Primary CO2
fixing enzyme:
PEP carboxylase as in C4
• Separates CO2 uptake and temporary storage (during
night) from final CO2
fixation (during day)
• Survival mechanism in arid regions (eg deserts)
• High ‘water use efficiency’ (ratio of CO2
fixed to water
lost)
What are the functions of a root?
– Anchoring the plant
– Absorbing minerals and water (from the soil)
– Storing carbohydrates (from the leaves)
Eudicots and gymnosperm root system
Most eudicots and gymnosperms have a taproot
system, which consists of:
– A taproot, the main vertical root
– Lateral roots, or branch roots, that arise from the
taproot
Monocot root system
Most monocots have a fibrous root system, which
consists of:
– Adventitious roots that arise from stems or
leaves
– Lateral roots that arise from the adventitious roots
What are the 5 types of modified roots?
- Prop roots
- Storage roots
- Strangling/Aerial roots
- Buttress roots
- Pneumatophores
What are the 3 Meristematic tissues of a plant?
Dermal: epidermis, periderm
Vascular: xylem, phloem
Ground: pith and cortex
What is Dermal tissue?
In nonwoody plants, the dermal tissue system
consists of the epidermis
• A waxy coating called the cuticle helps prevent
water loss from the epidermis
• Trichomes are outgrowths of the shoot epidermis
and can help with insect defense
Ground tissue
Tissues that are neither dermal nor vascular are
the ground tissue system
• Ground tissue internal to the vascular tissue is
pith; ground tissue external to the vascular tissue
is cortex
• Ground tissue includes cells specialized for
storage, photosynthesis, and support
Vascular tissue
The vascular tissue system carries out long-distance transport
of materials between roots and shoots
• The two vascular tissues are xylem and phloem
• Xylem conveys water and dissolved minerals upward from roots
into the shoots
• Phloem transports organic nutrients from where they are made to
where they are needed
Parenchyma Cells
– Have thin and flexible primary walls – Lack secondary walls – Large vacuole – Are the least specialized – Perform the most metabolic functions – Retain the ability to divide and differentiate – Can regenerate
Collenchyma Cells
Collenchyma cells are grouped in strands and
help support young parts of the plant shoot
• They have thicker and uneven cell walls
• They lack secondary walls
• These cells provide flexible support without
restraining growth
Sclerenchyma Cells
Sclerenchyma cells are rigid because of thick
secondary walls strengthened with lignin
• They are dead at functional maturity
• There are two types:
– Sclereids are short and irregular in shape and
have thick lignified secondary walls
– Fibers are long and slender and arranged in
threads
2 types of plant growth.
Primary for growth in height
Secondary for growth in diameter
Water loss from plants
Plants take up large quantities of water
• > 90% of the water taken in by roots is lost from the plant as water
vapour.
• Very little is ‘used’ in any biochemical process
• The loss of water from the plant is termed transpiration
• Terrestrial plants lose water as an unavoidable consequence of
having to take up CO2
• Dehydration is always a risk
• But transpiration stream does distribute minerals
What are the 3 transport routes between cells?
Three transport routes for water and solutes are
– The apoplastic route, through cell walls and extracellular spaces
– The symplastic route, through the cytosol, on the inside
– The transmembrane route, across cell walls
Short-Distance Transport of Solutes
Across Plasma Membranes
Plasma membrane permeability controls short-distance
movement of substances
• Both active and passive transport occur in plants
• In plants, membrane potential is established through
pumping H by proton pumps
Bulk flow
Efficient long distance transport of fluid requires bulk
flow, the movement of a fluid driven by pressure
• Water and solutes move together through tracheids and
vessel elements
Water-Conducting Cells of the Xylem
The two types of water-conducting cells, tracheids and vessel
elements, are dead at maturity
• Tracheids are found in the xylem of all vascular plants
• Vessel elements are common to most angiosperms and a few
gymnosperms
• Vessel elements align end to end to form long micropipes called
vessels
Bulk Flow Transport via the Xylem
Xylem sap, water and dissolved minerals, is transported from
roots to leaves by bulk flow
• The transport of xylem sap involves transpiration, the
evaporation of water from a plant’s surface
• Transpired water is replaced as water travels up from the roots
Pulling Xylem Sap: The Cohesion-Tension
Hypothesis
According to the cohesion-tension hypothesis, transpiration
and water cohesion pull water from shoots to roots
• Xylem sap is normally under negative pressure, or tension
How does bulk flow differ from diffusion?
It is driven by differences in pressure potential, not solute potential
– It occurs in hollow dead cells, not across the membranes of living
cells
– It moves the entire solution, not just water or solutes
– It is much faster
Sugar-Conducting Cells of the Phloem
Sieve-tube elements are alive at functional
maturity, though they lack organelles
• Sieve plates are the porous end walls that allow
fluid to flow between cells along the sieve tube
• Each sieve-tube element has a companion cell
whose nucleus and ribosomes serve both cells
What is water potential?
Water potential is a measurement that combines the effects of
solute concentration and pressure
• Water potential determines the direction of movement of water
• Water flows from regions of higher water potential to regions of lower
water potential
Role of rhizobacteria
Rhizobacteria can play several roles
– Produce hormones that stimulate plant growth
– Produce antibiotics that protect roots from disease
– Absorb toxic metals or make nutrients more available to roots
Nitrogen fixing bacteria
Ammonifying bacteria produce NH3 by breaking down nitrogen in proteins and other organic compounds in humus. Nitrogen-fixing bacteria convert N2 into NH3 In the soil, NH3 picks up another H+ to form NH4 (which plants can absorb) • Plants acquire nitrogen mainly in the form of NO3 – • Soil NO3 - formed by two step processes called nitrification. – Nitrifying bacteria oxidize NH3 to nitrite (NO2 – ) then nitrite to nitrate (NO3 – ) (2 different bacteria) • Nitrogen is lost to the atmosphere when denitrifying bacteria convert NO3 – to N2
Fungi and Plant Nutrition
Mycorrhizae (fungus roots) are mutualistic associations of fungi
and roots
• The fungus benefits from a steady supply of sugar from the host
plant
• The host plant benefits because the fungus increases the
surface area for water uptake and mineral absorption
• Mycorrhizal fungi also secrete growth factors that stimulate root
growth and branching and produce antibiotics that protect the
root.
Ectomycorrhizae
In ectomycorrhizae, the mycelium of the fungus forms a dense sheath over the
surface of the root
• These hyphae form a network in the apoplast, but do not penetrate the root cells
• Ectomycorrhizae occur in about 10% of plant families including pine, spruce, oak,
walnut, birch, willow, and eucalyptus
Arbuscular mycorrhizae
in arbuscular mycorrhizae, microscopic fungal hyphae extend into the root
• These mycorrhizae penetrate the cell wall but not the plasma membrane to form branched
arbuscules within root cells
• Hyphae can form arbuscules within cells; these are important sites of nutrient transfer
• Arbuscular mycorrhizae occur in about 85% of plant species, including grains and legumes
What is an epiphyte?
An epiphyte grows on another plant and obtains water and minerals from rain • Epiphytes do not tap into hosts for sustenance
Macronutrients
Macronutrients
– need about 1-20 g kg-1 dry weight
– Nitrogen, Potassium Calcium Magnesium, Phopshorus
Sulphur
Micronutrients
Micronutrients
– need about 100 mg g kg-1 dry weight
– Chloride Iron Manganese Boron Zinc Copper Nickel
Molybdenum
– Often enzyme co-factors for occasional processes
WHAT IS A BIOME?
A biome is an area of the planet that can be classified according to the plants and animals that live in it. Temperature, soil, and the amount of light and water help determine what life exists in a biome.
Differs from an ecosystem; “interaction between
living and non-living things in the environment”
vs biome; a specific geographic area notable for
the species living there. A biome can consist of a
number of ecosystems.
GENERAL FEATURES OF TERRESTRIAL
BIOMES
major physical features
climatic (temp and ppt)
Vegetation (and adaptations)
Microorganisms, fungi and animals
adapted to that particular environment.
GENERAL FEATURES OF TERRESTRIAL BIOMES
VERTICAL LAYERING (STRATIFICATION)
- Determined by the shapes and sizes of
vegetation within the biome.
FORESTS
Upper canopy > low-tree layer > shrub layer >
ground layer of herbaceous plants > forest floor
(litter layer) > root layer.
Grasslands (similar but less pronounced)
Herbaceous layer of grasses and forbs > litter layer
> root layer.
r- strategists
‘Live fast, die young’ ¡ Quantity ¡ Unstable environments ¡ Earlier maturity ¡ Deciduous
K- strategists
‘grow slow, die old’ ¡ Quality ¡ Stable environments ¡ Later maturity ¡ Evergreen
3 Different types of dispersal?
Diffusion • gradual movement • several generations • Across suitable terrain
Jump • long distance • Short time scale • Across unsuitable terrain
Secular • Diffusion over evolutionary time • genetically modified species
What are biotic factors?
Biotic factors that affect the distribution
of organisms may include
¡ Disease + Parasitism
¡ Predation; inc herbivory
¡ Competition
¡ Mutualism
COMPETITION – ALLELOPATHY
The chemical inhibition of one plant (or other
organism) by another, due to the release into the
environment of substances acting as germination or
growth inhibitors.
¡ Secondary metabolites that are not required for
the growth, development and reproduction of the
allelopathic plant.
e.g. English Laurel, garlic mustard weed, eucalyptus.
Applications;
¡ Agriculture – intercropping
- crop rotation
- insect biocontrol
Abiotic factors
¡ Temperature ¡ Water ¡ Oxygen ¡ Salinity ¡ Sunlight ¡ Soil Most abiotic factors vary in space and time.
ADAPTATIONS TO TEMPERATURE AND
MOISTURE
Species can extend their distributions by local
adaptations to limiting environmental variables.
¡ Ecotype – genetic varieties within a single species.
How?
All differences are phenotypic, seeds transplanted
from one place to another will all perform the same
as the resident individuals.
2) All differences are genotypic, if seeds are
transplanted between areas the seedlings will retain
physiology identical to their home site.
3) A combination of phenotypic and genotypic
variation causes an intermediate results.
RECAP: EVOLUTION AND ARTIFICIAL
SELECTION
Natural populations are shaped by natural selection
– Competition for survival and reproduction
– Individuals with most advantageous traits are
more likely to survive and reproduce
Allele frequencies change in population to become better adapted to the environment • Higher survival and reproduction of individuals with specific traits • Highly selective process increasing alleles associated with specific traits
What is acclimation?
individual plants respond to changes in the environment, by altering their physiology or morphology allowing them to survive the new environment.
Info on polyploid plants
Polyploid (multi-genome) plants are often bigger and so selected for propagation
What is natural food?
The food we eat comes from plants already extensively modified from their original form. Even heritage varieties are extensively genetically modified.
Phenotype: physical expression of traits
Conventional breeding based on phenotypes.
Cross plants,
Look in subsequent generations for “best”
Repeat
Often only one crop per year
LONG process
ISSUES WITH MODERN BREEDING
Crops were typically bred for high yield under optimal growth conditions
Loss of genetic variation in other important traits
Modern Agriculture involved planting large areas with a single crop (monoculture)
Pros: ease of planting, harvesting, and looking after
your crop
Cons: May lack resilience against disease and other
stresses
Older and wild varieties of crop plants can be excellent resources for increasing genetic variation (i.e. stress tolerance)
Why use GM methods sometimes and molecular breeding others?
Molecular breeding
- Desired trait must be present in population
- Genetic resources must be available
- Plant should be propagated sexually
Why use GM methods sometimes and molecular breeding others?
GM
- Gene can come from any source
- Genetic resources not required
- Plant can be propagated vegetatively
RECOMBINANT DNA TECHNOLOGY
To work directly with specific genes, scientists prepare well defined DNA segments in multiple identical copies by a process called DNA cloning
Plasmids are small, circular DNA molecules that replicate separately from the bacterial chromosome
Researchers can insert DNA into a plasmid to produce a recombinant DNA molecule, which contains DNA from two different sources
What are plasmids?
Small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently through their own origin of replication
Can be manipulated to include foreign genes into bacteria and from there into plants
STEPS OF PLANT TRANSFORMATION
Propagate plasmid vector in E. coli
Isolate plasmid vector from E. coli and introduce foreign gene
Amplify by reintroducing to E. coli
Isolate engineered binary vector and introduce into
Agrobacterium containing modified Ti plasmid
Infect plant tissues with engineered Agrobacterium
What are the 4 major physical components of climate?
Four major physical components of climate are:
Sunlight
Temperature
Precipitation
Wind.
HOW DOES C3 PHOTOSYNTHESIS RESPOND TO RISING [CO2]?
INCREASED CARBON GAIN FROM INCREASED RATES OF CARBOXYLATION AND DECREASED RATES OF OXYGENATION AND PHOTORESPIRATION
Heterospory: The Rule Among Seed Plants
Homosporous plants produce one kind of spore, which
usually produces a bisexual gametophyte
• Heterosporous plants produce two types of spores,
which develop into either male or female
gametophytes
• Ferns and other close relatives are homosporous;
seed plants are heterosporous
What adaptions do seeded plants share?
Have seeds (by definition)
• Reduction in gametophyte generation
– microscopic
• Heterospory
– Single large megaspore , many small microspores
• Ovules – contain, megaspore, megasporangium, and
integuments
• Pollen
– protected male gametophytes can travel & disperse widely
What is Speciation?
The process by which one species
splits into two or more species, is at the focal
point of evolutionary theory
What is micro and macroevolution?
Microevolution consists of changes in allele
frequency in a population over time
• Macroevolution refers to broad patterns of
evolutionary change above the species level
– when different habitats occur at the same time with
little competition, results in gradual refinement of
existing structures for new functions that lead to an
advantage in these different habitats
What are the 4 types of modified leaves?
A flower is a specialized shoot with up to four types of
modified leaves – floral organs
Sepals, which enclose the flower
Petals, which are brightly colored and attract pollinators
Stamens, which produce pollen
Carpels, which produce ovules
• A stamen consists of a stalk called a filament, with a sac
called an anther where the pollen is produced
• A carpel consists of an ovary at the base and a style
leading up to a stigma, where pollen is received
Development of Male Gametophytes
in Pollen Grains
Pollen develops from microspores within
the microsporangia, or pollen sacs, of anthers
• Each microspore undergoes mitosis to produce two cells: the
generative cell and the tube cell
• A pollen grain consists of the two-celled male gametophyte and
the spore wall
• If pollination succeeds, a pollen grain produces a pollen tube
that grows down into the ovary and discharges two sperm cells
near the embryo sac
Development of Female
Gametophytes (Embryo Sacs)
The embryo sac, or female gametophyte, develops within the
ovule
• Within an ovule, two integuments surround a megasporangium
• One cell in the megasporangium undergoes meiosis,
producing four megaspores, only one of which survives
• The megaspore divides, producing a cell partitioned into a
multicellular female gametophyte, the embryo sac
POOLS OF STEM CELLS RECEIVE SIGNALS THAT PROMOTE THE
DEVELOPMENT OF SPECIFIC TISSUES
Plants produce new organs via meristems • Meristems are pools of stem cells that can be activated to produce specific tissue types. • The shoot apical meristem produces vegetative growth • The inflorescence and floral meristems produce the reproductive structures that will ultimately become flowers. • These transitions are promoted by genes encoding transcription factors (TFs).
What are Phytochrome
Photoreceptors?
Phytochromes are pigments that regulate many of a plant’s responses to light throughout its life • These responses include seed germination and flowering
Long and short day plants
Some processes, including flowering in many species, require a certain
photoperiod
• Plants that flower when a light period is shorter than a critical length are called
short-day plants
• Plants that flower when a light period is longer than a certain number of hours
are called long-day plants
• Flowering in day-neutral plants is controlled by plant maturity, not photoperiod
How is flowering time regulated?
Flowering time is regulated by coordinated genetic changes that respond to: - Photoperiod - Temperature - Plant hormone signals
Linear Electron Flow
During the light reactions, there are two possible
routes for electron flow: cyclic and linear
• Linear electron flow, the primary pathway,
involves both photosystems and produces ATP
and NADPH using light energy
• This is the ‘canonical’ process of the lightdependent reactions
Carbon reactions in the stroma
Carbon enters the cycle as CO2 and leaves as a sugar
named glyceraldehyde 3-phospate (G3P)
• For net synthesis of 1 G3P, the cycle must take place
three times, fixing 3 molecules of CO2
• The Calvin cycle has three phases
– Carboxylation/ Carbon fixation (catalyzed by rubisco)
– Reduction
– Regeneration of the CO2 acceptor (RuBP)
C4 Plants
C4 plants minimize the cost of photorespiration by
incorporating CO2
into four-carbon compounds in
mesophyll cells
• This step requires the enzyme PEP carboxylase
• PEP carboxylase has a higher affinity for CO2
than
rubisco does; it can fix CO2 even when CO2
concentrations are low; O2 does not compete.
• These four-carbon compounds are exported to
bundle-sheath cells, where they release CO2
that is
then used in the Calvin cycle
Dermal tissue
In nonwoody plants, the dermal tissue system
consists of the epidermis
• A waxy coating called the cuticle helps prevent
water loss from the epidermis
• Trichomes are outgrowths of the shoot epidermis
and can help with insect defense
Ground tissue
Tissues that are neither dermal nor vascular are
the ground tissue system
• Ground tissue internal to the vascular tissue is
pith; ground tissue external to the vascular tissue
is cortex
• Ground tissue includes cells specialized for
storage, photosynthesis, and support
Water loss from plants
Plants take up large quantities of water
• > 90% of the water taken in by roots is lost from the plant as water
vapour.
• Very little is ‘used’ in any biochemical process
• The loss of water from the plant is termed transpiration
• Terrestrial plants lose water as an unavoidable consequence of
having to take up CO2
• Dehydration is always a risk
• But transpiration stream does distribute minerals
What are the 3 transport roots for water and solutes in plant cells?
Three transport routes for water and solutes are
– The apoplastic route, through cell walls and extracellular spaces
– The symplastic route, through the cytosol
– The transmembrane route, across cell walls
Short-Distance Transport of Solutes
Across Plasma Membranes
Plasma membrane permeability controls short-distance
movement of substances
• Both active and passive transport occur in plants
• In plants, membrane potential is established through
pumping H by proton pumps
Short-Distance Transport of Water Across
Plasma Membranes
• To survive, plants must balance water uptake and loss
• Osmosis determines the net uptake or water loss by a cell and is
affected by solute concentration and pressure
• Water potential is a measurement that combines the effects of
solute concentration and pressure
• Water potential determines the direction of movement of water
• Water flows from regions of higher water potential to regions of lower
water potential
Water Movement Across Plant Cell Membranes
Water potential affects uptake and loss of water by plant
cells
• If a flaccid (limp) cell is placed in an environment with a
higher solute concentration, the cell will lose water and
undergo plasmolysis
• Plasmolysis occurs when the protoplast shrinks and pulls
away from the cell wall
how does Bulk flow differ from diffusion?
It is driven by differences in pressure potential, not solute potential
– It occurs in hollow dead cells, not across the membranes of living
cells
– It moves the entire solution, not just water or solutes
– It is much faster
Sugar-Conducting Cells of the Phloem
Sieve-tube elements are alive at functional
maturity, though they lack organelles
• Sieve plates are the porous end walls that allow
fluid to flow between cells along the sieve tube
• Each sieve-tube element has a companion cell
whose nucleus and ribosomes serve both cells
Plants adapted to deserts
A xerophyte is a species of plant that has adaptations to survive in an environment with
little liquid water. Desert or an ice- or snow-covered region.
Types:
¡ 1) Succulents; store water in their stems or leaves (enlargement of vacuoles) e.g.
(cacti&euphorbs)
¡ 2) Non-succulent ‘drought endurers’ perrenials‘true xerophytes’. Lose 70% water.
Effects growth processes such as cell elongation; usually small and weak.
¡ 3) Ephemerals ‘drought escaping’ – rain > seeds germinate > r-growth > flower and set
seed before soil dries.
What causes temperature differentials?
1) Incoming solar radiation.
2) The distribution of land and sea.
Water heats and cools slowly
due to high specific heat.
Land heats and cools quickly so
land controlled (continental)
climates have large daily and
seasonal variation
THE EFFECT OF TOPOGRAPHY ON ABIOTIC
FACTORS
High latitudes/elevation ¡ Treeline – above which no trees can grow. 1. Lack of soil 2. Desiccation of leaves in cold weather 3. Short growing season 4. Lack of snow, exposing plants to winter drying 5. Excessive snow through the summer 6. Strong winds 7. Rapid heat loss at night 8. Excessive soil temp during the day 9. Drought
SIGNAL TRANSDUCTION
Protein kinase add phosphates to proteins in a process called phosphorylation,
¡ Protein phosphatases rapidly remove the phosphates from proteins, a process
called dephosphorylation
¡ This phosphorylation and dephosphorylation system acts as a molecular switch,
turning activities on and off or up or down, as required
¡ Many signaling pathways involve second messengers
¡ These are small, nonprotein, water-soluble molecules or ions that spread
throughout a cell by diffusion (Calcium ions (Ca2+), cyclic GMP (cGMP),
inositol triphosphate (IP3) and diacylglycerol (DAG))
Genetic Variation makes evolution
possible
Genetic variation among individuals is caused by differences in genes or other DNA segments
• Phenotype is the product of inherited
genotype and environmental influences
• Natural selection can only act on variation
with a genetic component
• Artificial selection can only act on variation with a genetic component
Why do we sequence genomes?
Genome sequencing provides:
complete gene catalogue for a species,
• regulatory elements that control their function
• framework for understanding genomic variation
Genome sequence is a prerequisite resource for: Understand the roles of genes in plant development and adaptation
• Exploit the natural genetic diversity of an organism for plant breeding (artificial selection)
Types of polyploidy
AUTOPOLYPLOIDS: duplicate genome of same Species
Autotetraploid: duplicate genome of same diploid species
ALLOPOLYPLOIDS: duplicate genome of different species
Allohexaploid: six complete sets of chromosomes derived from different species.
Seed Dormancy: An Adaptation for Tough Times
Seed dormancy increases the chances that
germination will occur at a time and place most advantageous to the seedling
• The breaking of seed dormancy often requires environmental cues, such as temperature or lighting changes
• Germination depends on imbibition, the uptake of water due to low water potential of the dry seed
• The radicle (embryonic root) emerges first
• Next, the shoot tip breaks through the soil surface
Development is promoted
by different types of
meristems
• During transition to reproductive development, the inflorescence meristem is formed. In turn, it leads to production of floral meristems • Both will form the organs of an individual flower. • The inflorescence meristem is indeterminate, meaning that it can keep producing new tissue. • The floral meristem is determinate, meaning that it makes only one set of the specified whorls.
POOLS OF STEM CELLS RECEIVE SIGNALS THAT PROMOTE THE
DEVELOPMENT OF SPECIFIC TISSUES
Plants produce new organs via meristems • Meristems are pools of stem cells that can be activated to produce specific tissue types. • The shoot apical meristem produces vegetative growth • The inflorescence and floral meristems produce the reproductive structures that will ultimately become flowers. • These transitions are promoted by genes encoding transcription factors (TFs).
Why is it important to time
flowering?
• It’s critical that plants turn on floral
development genes at the right time to
initiate flower formation.
• It is also critical that plants do NOT turn
on those genes before the plant is ready to
flower.
• If a plant flowers at the wrong time, it may
lack the appropriate environmental
conditions for successful pollination and
seed development.
Pfr info
Responses to Pfr: seed germination, inhibition of vertical growth and stimulation of branching, setting internal clocks and control of flowering.
Phytochromes exist in two photo reversible states, with conversion of Pr to Pfr triggering
many developmental responses
• Red light triggers the conversion of Pr to Pfr
• Far-red light triggers the conversion of Pfr to Pr
• The conversion of Pr to Pfr is faster than the reverse process
BIOLOGICAL CLOCKS -
CIRCADIAN RHYTHMS
Circadian rhythms are cycles that are about 24 hours long and are governed by an internal “clock” • These cycles can be free-running, varying from 21 to 27 hours, when organisms are kept in a constant environment • The 24-hour period arises from the transcription of “clock genes” regulated through negative-feedback loops
What is a photoperiod?
Photoperiod is detected by leaves, which cue buds to develop as flowers • The flowering signal molecule is called florigen • Florigen may be a protein governed by the FLOWERING LOCUS T (FT) gene • This is a mobile protein moving from the leaf to the meristem
Chloroplasts: The Sites of Photosynthesis in Plants
CO2 enters and O2 exits the leaf through
microscopic pores called stomata
• The chlorophyll is in the membranes of thylakoids
(connected sacs in the chloroplast); thylakoids may
be stacked in columns called grana
• Chloroplasts also contain stroma, a dense interior
fluid
Carbon reactions in the stroma
Carbon enters the cycle as CO2 and leaves as a sugar
named glyceraldehyde 3-phospate (G3P)
• For net synthesis of 1 G3P, the cycle must take place
three times, fixing 3 molecules of CO2
• The Calvin cycle has three phases
– Carboxylation/ Carbon fixation (catalyzed by rubisco)
– Reduction
– Regeneration of the CO2 acceptor (RuBP)
Alternative mechanisms of carbon
fixation
On hot, dry days, plants close stomata, which
conserves H2O but also limits photosynthesis
• The closing of stomata reduces access to CO2
and causes O2 to build up
• These conditions favor an apparently wasteful
process called photorespiration
What is Photorespiration?
In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (3- phosphoglycerate)
• In photorespiration, rubisco adds O2
instead of CO2 in the Calvin cycle, producing a two-carbon compound
• Photorespiration consumes O2 and organic ‘fuel’ and releases CO2 without producing ATP or sugar
What does a stem consist of?
A stem is an organ consisting of – An alternating system of nodes, the points at which leaves are attached – Internodes, the stem segments between nodes
Different modified stems
Bulbs: leaf bases and short stem
Tubers: swollen underground stems
Rhizomes: underground stems
Stolons: aboveground horizontal shoots
Basic organs of a plants
Leaves:
– main photosynthetic organ
– Leaf blade and petiole
Roots:
– main organ for water and nutrient uptake
– Taproots with laterals in eudicots and gymnosperms
– Adventitious roots in monocots (from the stem)
– Root hairs for absorption
– Also anchorage
Stem: – Nodes and internodes – Terminal bud with apical meristem – support and transport tissues – Storage
Secondary growth thickens roots and
shoots
Lateral meristems add thickness to woody plants,
a process called secondary growth
• There are two lateral meristems: the vascular
cambium and the cork cambium
• The vascular cambium adds layers of vascular
tissue called secondary xylem (wood) and
secondary phloem
• The cork cambium replaces the epidermis with
periderm, which is thicker and tougher
Secondary growth is characteristic of gymnosperms
and many eudicots, but not monocots
Short-Distance Transport of Solutes
Across Plasma Membranes
Plasma membrane permeability controls short-distance
movement of substances
• Both active and passive transport occur in plants
• In plants, membrane potential is established through
pumping H by proton pumps
Long-Distance Transport: The Role of Bulk
Flow
Efficient long distance transport of fluid requires bulk
flow, the movement of a fluid driven by pressure
• Water and solutes move together through tracheids and
vessel elements of xylem, and sieve-tube elements of
phloem
– Water primarily through Xylem
– Solutes primarily through Phloem
Water-Conducting Cells of the Xylem
The two types of water-conducting cells, tracheids and vessel
elements, are dead at maturity
• Tracheids are found in the xylem of all vascular plants
• Vessel elements are common to most angiosperms and a few
gymnosperms
• Vessel elements align end to end to form long micropipes called
vessels
Sugar-Conducting Cells of the Phloem
Sieve-tube elements are alive at functional
maturity, though they lack organelles
• Sieve plates are the porous end walls that allow
fluid to flow between cells along the sieve tube
• Each sieve-tube element has a companion cell
whose nucleus and ribosomes serve both cells
Depending on the species, sugar may move by
symplastic (passing through plasmodesmata)or
both symplastic and apoplastic pathways
• Companion cells enhance solute movement
between the apoplast and symplast
• In many plants, phloem loading requires active transport
• Proton pumping and cotransport of sucrose and
H+ enable the cells to accumulate sucrose
• At the sink, sugar molecules diffuse from the phloem to sink tissues and are followed by water
GENERAL FEATURES OF TERRESTRIAL
BIOMES
1) major physical features
2) climatic (temp and ppt)
3) Vegetation (and adaptations)
4) Microorganisms, fungi and animals
adapted to that particular environment.
What is convergent evolution?
organisms share similar
characteristics as they undergo
natural selection independently,
but in similar environments
r and K strategies in plants
r- strategists ¡ ‘Live fast, die young’ ¡ Quantity ¡ Unstable environments ¡ Earlier maturity ¡ Deciduous
K- strategists ¡ ‘grow slow, die old’ ¡ Quality ¡ Stable environments ¡ Later maturity ¡ Evergreen
Role of nutrients
- Structural components
- Metabolic Components
- Enzyme activators
- Osmosis
- Cell permeability/membrane functioning
Mineral nutrient availability
Depends on soil characteristics: pH, the size and composition of soil
particles
• Most soil particles are negatively charged
• Rainfall leaches out the –ve anions, such as NO3 -, PO4 - , SO4 2-
• Cations held more strongly: K+
, Ca2+, Mg2+
Plant nutrition often involves relationships
with other organisms
Plants and soil microbes have a mutualistic
relationship
– Dead plants provide energy needed by soildwelling microorganisms
– Secretions from living roots support a wide variety
of microbes in the near-root environment
• The layer of soil bound to the plant’s roots is the
rhizosphere
• The rhizosphere contains bacteria (Rhizobacteria)
that act as decomposers and nitrogen-fixers
Rhizobacteria
Free-living rhizobacteria thrive in the rhizosphere, and
some can enter roots
• The rhizosphere has high microbial activity because of
sugars, amino acids, and organic acids secreted by roots
• Rhizobacteria can play several roles
– Produce hormones that stimulate plant growth
– Produce antibiotics that protect roots from disease
– Absorb toxic metals or make nutrients more available to roots
Nitrogen metabolism
Ammonifying bacteria produce NH3 by breaking down nitrogen
in proteins and other organic compounds in humus.
Nitrogen-fixing bacteria convert N2
into NH3
In the soil, NH3 picks up another H+ to form NH4+ (which plants can absorb)
• Plants acquire nitrogen mainly in the form of NO3 –
• Soil NO3 - formed by two step processes called nitrification. – Nitrifying bacteria oxidize NH3 to nitrite (NO2 –) then nitrite to nitrate (NO3 –) (2 different bacteria)
• Nitrogen is lost to the atmosphere when denitrifying bacteria convert NO3 – to N2
Mycorrhizae info
Mycorrhizae (fungus roots) are mutualistic associations of fungi
and roots
• The fungus benefits from a steady supply of sugar from the host
plant
• The host plant benefits because the fungus increases the
surface area for water uptake and mineral absorption
• Mycorrhizal fungi also secrete growth factors that stimulate root
growth and branching and produce antibiotics that protect the
root.
Biotic factors that affect the distribution
of organisms
may include: Disease + Parasitism Predation; inc herbivory Competition Mutualism
Adaptation (domestication, breeding) vs acclimation
Adaptation: genetic changes in the entire population fixed by natural or artificial selection over many generations.
Acclimation: individual plants respond to changes in the environment, by altering their physiology or morphology allowing them to survive the new environment.
SIGNAL TRANSDUCTION
Protein kinase add phosphates to proteins in a process called phosphorylation,
¡ Protein phosphatases rapidly remove the phosphates from proteins, a process
called dephosphorylation
¡ This phosphorylation and dephosphorylation system acts as a molecular switch,
turning activities on and off or up or down, as required
¡ Many signaling pathways involve second messengers
¡ These are small, nonprotein, water-soluble molecules or ions that spread
throughout a cell by diffusion
ISSUES WITH MODERN BREEDING
Crops were typically bred for high yield under optimal growth conditions
Loss of genetic variation in other important traits
Modern Agriculture involved planting large areas with a single crop (monoculture)
Pros: ease of planting, harvesting, and looking after your crop
Cons: May lack resilience against disease and other stresses
Older and wild varieties of crop plants can be excellent resources for increasing genetic variation (i.e. stress tolerance)
HOW DO I GET DNA INTO PLASMID?
Use restriction enzymes to cut plasmid DNA at a specific site
Use restriction enzymes to cut plasmid DNA at a specific site
Amplify “target” DNA sequence with PCR from other organism, cut with same RE, and ligate
Use restriction enzymes to cut plasmid DNA at a specific site
Amplify “target” DNA sequence with PCR from other organism, cut with same RE, and ligate
Plasmid has maintained ori, will replicate in bacteria
Exploring global climate patterns
First thing is the warming of the atmosphere to allow for survival of life. Solar radiation hits the earths surface, some is absorbed, some is reflected. And some is trapped in the greenhouse gases that makeup the ozone. Roughly half is absorbed by the earths surface.
Ozone largely depleted by CFC’s. But huge prohibition of these in 1987 Montreal Protocol. Phase out use of CFC’s internationally. Hole over Antarctica is decreasing in size. The hole is over Antarctica because conditions must be cool enough to allow for formation of Polar Stratospheric Clouds which requires much cooler temperatures. CFC’s and halons (containing bromine) are produced mostly in the N hemisphere, and are distributed polewards via wind circulation. Formation of PSC promote production of chemically active chlorine and bromine. Leads to rapid ozone loss when sunlight returns in sept/oct.
Sun strikes the earth at oblquie angles in the upper and lower regions, but at 90 degree angles at the equator. More diffuse at higher latitudes.
Atmospheric circulation
At the equator; low pressure, warm moisture rich air rises and then forms clouds as it cools, falls as rain and high storm risk.
At 300 N/S; high pressure, dry cool air draws moisture from the soil.
Ocean currents
Winds - atmospheric circulation.
Earth’s rotation – Coriolis force.
Changes in water density – temperature and salinity
This can be confounded by:
Topography of the ocean floor – this can modify the speed and direction of ocean currents.
Two types of currents:
Surface currents – 10%. At the shore these are affected by wind and tide. In the ocean wind is the major driving force.
Deep ocean currents – 90%
Local moderations of climate
Strong influence of heating and cooling of coastal regions by ocean currents.
During a hot day (when the land is warmer than the water), air over the land heats up and rises, drawing cooler air from across the ocean.
By contrast, land loses heat faster (such as during cool nights), this causes air above the ocean to rise (because it is now warmer), drawing cooler air from over the land across the ocean.
Describe three strategies plants can use to conserve water, and explain how they
work. Name the ecosystem or biome that each strategy is found in.
Explain whether these are the product of plant acclimation or adaptation and justify
your choice with reasoning.
Plants in the desert can have a waxy cuticle which stops water molecules from dissipating or being absorbed into the air. Furthermore, stomata cycles are also another important feature for desert plants. This is because they can open their stomata during the night to obtain CO2 for photosynthesis and can then close their stomata during the day to reduces water loss through transpiration. A third and final strategy is that leaves are reduced to spines which reduce the surface area for transpiration. Stomata are a result of acclimation because they are temporarily closed and opened. However, a waxy cuticle and spines are a result of adaptation because they are long term/ permanent and they are a change in physical composition.
Explain long distance transport of water in plants from the soil to the leaves,
including the different tissues and compartment involved. Explain how
stomata control the process of water loss through transpiration
Long distance transport of water from the soil to the leaves happens due to root hair cells and roots absorbing the water due to water potential. Moreover, the water is absorbed via the apoplastic and the symplastic route. The water is then taken up in the xylem, where Water and solutes move together through tracheid and
vessel elements of xylem. Water is brought up through the xylem because of bulk flow where there is a movement in fluid due to pressure. Some water is lost due to transpiration. Stomata can help reduce water loss by closing during certain conditions such as in arid regions of hot or cold climates where the stomata can open at night to let in CO2, but close during the day to minimize water loss.
Describe the movement of electrons in photosynthetic electron transport and
explain how this process results in the production of ATP and NADPH. Name
the three phase of the Calvin cycle and what happens in each phase. Explain the
process of photorespiration and under what conditions it might occur.
During the light reactions, there is one possible route for electron flow to produce ATP and NADPH which is linear flow. In linear electron flow, the primary pathway which involves both photosystems and produces ATP and NADPH using light energy. This happens due to a primary electron acceptor in the reaction centre accepting a excited electron from chlorophyll a which takes place at both reaction centres. ATP and NADPH are produced on the side
facing the stroma, where the Calvin cycle takes place. The 3 phases of the Calvin cycle are: Carbon fixation, reduction and regeneration. During carbon fixation a CO2 molecule combines with five-carbon acceptor molecules and is catalysed by rubisco. Next is reduction, where NADPH donates electrons to, or reduces, a three-carbon intermediate to make G3P. Finally is regeneration. where Some G3P molecules go to make glucose, while others must be recycled to regenerate the RuBP acceptor. Photorespiration is the process of light-dependent uptake of molecular oxygen (O2) concomitant with release of carbon dioxide (CO2) from organic compounds. The gas exchange resembles respiration and is the reverse of photosynthesis where CO2 is fixed and O2 released. Furthermore, Photorespiration generally occurs on hot, dry, sunny days causing plants to close their stomata and the oxygen (O2) concentration in the leaf to be higher than the carbon dioxide (CO2) concentration.
What are the 4 main groups of plants and features.
Bryophytes: are considered the simplest of the four types, because they lack a full vascular system, true roots and true leaves. These include mosses and other small plants that grow in wet areas and use spores to reproduce.
Pteridophytes: Like ferns, are plants that also use spores to reproduce, however they have evolved a vascular system and so are considered to be more sophisticated than bryophytes. These were the earliest trees on earth; as soon as plants gained vascular systems they were able to grow quite tall as fern trees. They do not have a tap root but have roots that adventitiously sprout from the stem.
Gymnosperms: Include plants like conifers and pines. These plants use seeds to reproduce instead of spores, which is an improvement upon the pteridophyte model because seeds are much more durable than spores. This means the genetic information is safer and more likely to be successful.
The seeds of gymnosperms are not enclosed in an ovule; they require both male and female cones to reproduce. Male cones are smaller and softer than their female counterparts, and pollination occurs via insects, animals or the wind.
Angiosperms are the most sophisticated plants on earth, and include all flowering plants. They can be broken down further into two groups: monocots and dicots.
Extra info on alteration of generation
Plants alternate between two multicellular stages, a reproductive cycle called alternation of generations
• The gametophyte (“gamate-producing plant”) is haploid and produces haploid gametes by mitosis (egg and sperm)
• Fusion of the gametes gives rise to the diploid sporophyte, which produces haploid spores by meiosis
Pollen and Production of Sperm
Microspores develop into pollen grains, which contain the male
gametophytes
• Pollination is the transfer of pollen to the part of a seed plant containing the ovules – contrast with bryophytes and seedless v
plants
• Pollen eliminates the need for a film of water and can be dispersed great distances by air or animals
• If a pollen grain germinates, it gives rise to a pollen tube that discharges sperm into the female gametophyte within the ovule
The Life Cycle of a Pine:
Three key features of the gymnosperm life cycle are
– Dominance of the sporophyte generation
• Miniaturization of their gametophytes
– Production/development of seeds from fertilized ovules
– The transfer of sperm to ovules by pollen
The pine tree is the sporophyte that produces sporangia in male and
female cones
• Pollen cones are small and consist of modified leaves (microsporophylls)
that bear microsporangia
• Cells called microsporocytes undergo meiosis to produce haploid
microspores inside the microsporangia
Ovulate cones are larger and consist of both modified leaves
(megasporophylls bearing megasporangia) and modified stem
tissue
• Within each ovule, megasporocytes undergo meiosis to produce
haploid megaspores
While pollen tube develops –
megasporocytes undergoes
meiosis – produces 4 haploid cells
– one survives as a megaspore.
The megaspore develops into a
female gametophyte that contains
two or three archegonia – each will
form an egg.
By the time eggs are mature,
sperm cells have developed into
pollen tube and fertilization occurs
when sperm and egg unite.
