Hypotheses about the origin of life. which is your favorite hypothesis and why
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PlantsandtheconquestoflandII28129.pptxPlants & The Conquest Of Land II
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Ancestral species gave rise to a vast diversity of modern plants
Figure 26.18-2 Highlights of plant evolution (part 2: art)
Extensive forests dominated by tree-sized lycophytes, pteridophytes, and early lignophytes occurred in widespread swampy regions during the warm, moist Carboniferous period (354–290 million years ago)
Ecological effects of vascular plants
First appear 420-429 mya – Coal Age
Carboniferous plants converted huge amounts of atmospheric CO2 into decay-resistant organic material
Removal of large amounts of the greenhouse gas CO2 from the atmosphere by plants had a cooling effect on the climate. Also became drier because cold air holds less moisture than warm air.
Carboniferous proliferation of vascular plants was correlated with a dramatic decrease in CO2 in the atmosphere. It reached the lowest known levels about 290 mya. During this period of very low CO2, atmospheric oxygen levels rose to the highest known levels.
Plants Evolution
Bryophytes produce decay-resistant body tissues. Could have begun process to reduce amount of greenhouse gas CO2 in the atmosphere. Helped to enrich the soils.
Extensive forests dominated by tree-sized lycophytes, pteridophytes, and early lignophytes in widespread swampy regions during the warm, moist Carboniferous period (354–290 mya). They had the effect of cooling and dry the climate. These gave an advantage to seed plants.
65 mya, the K/T event marking end of Cretaceous and beginning of Tertiary. Huge amounts of ash, smoke and haze dimmed sunlight long enough to kill many of the world’s plants
Dinosaurs were also doomed. Surviving flowering plants diversified. Today Angiosperms Angiosperms represent approximately 80 % of all the known green plants now living.
Transition from seedless vascular plants to seed vascular plants:
Lignin
Wood
Heterospory
Seed
from the Upper Devonian to Lower Carboniferous (383 to 323 million years ago)
Reproductive innovations in seed plants
Reduced Gametophytes
Seed Plants Cell Cycle
. Most seed plants, adult sporophytes develop two different kinds of sporangia: microsporangia & megasporangia (instead of only one type of sporangia)
Megasporangia (2n) produce megaspores(n) by meiosis.
Megaspores (n) undergo mitosis & produce female gametophytes (n) (mega-gametophytes)
The mega-gametophytes (n) remain within the tissues of the ovule and produce one or more egg cells (n) in the archegonium
An ovule consists of a megasporangium surrounded by one or two layers of tissue called integuments.
Microsporangia (2n) produce microspores (n) by meiosis
Microspores (n) undergo mitosis & develop into pollen grains (n), which are the young male gametophytes
Seed plants produce heterospores
Male gametophytes develop from small microspores
Microspores develop into pollen grains, which consist of a male gametophyte enclosed within the protective pollen wall
Pollination is the transfer of pollen to reproductive organs of the plant.
Pollen eliminates the need for a film of water and can be dispersed great distances by air or animals
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Winged pollen from pine
Pollen: the male gametophyte in seed plants
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Immature
ovulate cone
Integument (2n)
Megaspore (n)
Spore wall
Female
gametophyte (n)
Egg nucleus (n)
Discharged
sperm nucleus (n)
Pollen tube(n)
Seed coat
Spore wall
Food
Supply (n)
Embryo (2n)
Megasporangium(2n)
Micropyle
Pollen grain (n)
Male
gametophyte
(a) Unfertilized ovule
(b) Fertilized ovule
(c) Gymnosperm seed
After fertilization, ovules develop into seeds. Mature seeds are ready for dispersal of the diploid generation and contain:
embryonic sporophyte (2n)
stored food
protective coat
Figure 26.23-s3 From ovule to seed in a gymnosperm (step 3)
The Evolutionary Advantage of Seeds
A seed develops from the whole ovule
A seed is a sporophyte embryo, along with its food supply, packaged in a protective coat
Seeds provide some evolutionary advantages over spores:
Seeds are multicellular; spores are usually single-celled
They may remain dormant from days to years, until conditions are favorable for germination
Seeds have a supply of stored food
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Extant seed plants are divided into two clades:
Gymnosperms have “naked” seeds that are not enclosed in chambers
Angiosperms have seeds that develop inside chambers (vessels) called ovaries
Angiosperms also have flowers and fruits
Double fertilization and endosperm
Ovaries that develops in fruits
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Sperms: Greek for seeds
Gymno: Greek for unclothed, naked
Angio: Greek for vessel
flowers
ovaries- fruits
seed with endosperm
Wood, ovules, seeds, pollen, euphylls
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Ovule
Microsporangium
Megaspore
Egg (n)
Archegonium (n)
Pollen grain (n)
Megasporangium (2n)
Female gametophyte (n)
Integument
Male gametophyte (n)
Embryo (2n)
Sperm
Seed
Seedling
Seed
coat
Ovule
Microspores
Haploid
Diploid
KE
Y
Scale
Fertilization
Ovule
cone
Mature
sporophyte
(2n)
Pollen
cone
Section
of cone
Mitosis
Scale
Cone
scale
Meiosis
Mitosis
Megasporangium
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Pine life cycle
Gymnosperms
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Flowers
The flower is an angiosperm structure specialized for sexual reproduction
Many species are pollinated by insects or other animals, while some species are wind-pollinated
A flower is a specialized shoot with up to four types of modified leaves called floral organs
Sepals, which enclose the flower
Petals, which are brightly colored and attract pollinators
Stamens, which produce pollen
Carpels, which produce ovules
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Angiosperms
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Carpel anatomy
(The term pistil can be used to refer to a single carpel or two or more fused carpels)
Stigma – receives and recognizes pollen
Only appropriate pollen will be allowed to germinate
Style – connects stigma with ovary
Ovary – pollen tube delivers sperm to ovule
Ovaries develop into fruits
Stamen anatomy
Anther – sac called an anther where the pollen is produced
Filament- sustain (stalk)
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Diploid cell produces 4 megaspores by meiosis (3 die) and one is left.
Surviving megaspore generate female gametophyte by mitosis.
Female gametophyte consists of 7 cells, one of which is the egg cell
Doble fertilization
Pollen grains (immature male gametophytes) from the stamen are dispersed by the wind or transported by pollinators to the stigma of a flower.
Pollen germinates and delivers two sperms to the female gametophytes by means of a long pollen tube, that is enclosed, fed and guided by the style. Once the sperms reach an ovule in the ovary double fertilization occurs. Double fertilization involves two events:
True fertilization: one sperm nucleus fuses with the egg to produce a diploid zygote that develops later into a young sporophyte
Triple fusion: the other sperm nucleus fuses with the two nucleus of the central cell, generating the first cell of the nutritive endosperm tissue
Double fertilization and therefore endosperm are present in angiosperms, but not in gymnosperms.
Both are derived traits of angiosperms and the presence of endosperm is considered a critical innovation in angiosperms because provide a more efficient food storage to the seed.
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Fruits
Develop from ovary walls
Aid the dispersal of enclosed seeds
Dispersal prevents competition and aids in colonization
Fruits may be adapted to
Attract animals to eat them, wind dispersal, attach to animal fur or float in water
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Coevolution
Process by which two or more species of organisms influence each other’s evolutionary pathway
Explains the diverse forms of most flowers and many fruits, and how plants accomplish effective pollen and seed dispersal
In plants:
Pollination coevolution
Seed dispersal coevolution
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Pollination coevolution
Pollinators foster genetic variability and plant potential for evolutionary change
Constancy or fidelity – pollinators learn the flower characteristics and visit them preferentially
Precision pollen transfer
Attract appropriate pollinator using attractive colors, odors, shapes, sizes
Some flowers specialized for particular pollinators
Birds – odorless red flowers
Bees – blue, purple, yellow or white flowers with a sweet odor
If pollinator becomes extinct, plant may also face extinction (pollination syndrome)
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Seed-dispersal coevolution
Influenced both plant and animal seed-dispersal agent
Plant may have juicy, sweet fruit with small seed to readily pass through gut
Plants may signal fruit ripeness with color change
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Domestication – artificial selection for traits desirable to humans
Agriculture originated 10,000 – 5,000 years ago independently in at least 10 different locations
African bottle gourd, bread wheat, corn, rice
Shattering – wild fruit breaks apart and disperses seeds
Loss of this trait makes it easier for human harvest
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Human Influences on Angiosperm Diversification
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Video: Plant Time Lapse
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PLANT GROUP
Mosses and other
nonvascular plants
Gametophyte
Reduced, dependent on gametophyte for nutrition
Ferns and other
Seedless vascular plants
Reduced, independent
(photosynthetic and free- living)
Dominant
Seed plants (gymnosperms and angiosperms)
Reduced (usually microscopic), dependent on
surrounding sporophyte tissue for nutrition
Sporophyte
Dominant
Gymnosperm
Microscopic female
gametophytes (n) inside
ovulate cone
Angiosperm
Microscopic female
gametophytes
(n) inside these parts
of flowers
Microscopic
male
gametophytes
(n) inside
these parts
of flowers
Sporophyte
(2n)
Sporophyte
(2n)
Gametophyte
(n)
Example
Gametophyte
(n)
Microscopic male
gametophytes (n)
inside pollen cone
Sporophyte
(2n)
Sporophyte
(2n)
Dominant
pollen
Figure 26.21 Gametophyte-sporophyte relationships in different plant groups
