Homework to be emailed to biovcc@yahoo.com
Notes:
Plant Diversity I How Plants Colonized Land
Overview: The Greening of Earth
Looking at a lush landscape
It is difficult to imagine the land without any plants or other organisms
For more than the first 3 billion years of Earth’s history
The terrestrial surface was lifeless
Since colonizing land
Plants have diversified into roughly 290,000 living species
Land plants evolved from green algae
Researchers have identified green algae called charophyceans as the closest
relatives of land plants
Plants
Plants appeared on land about 425 million years ago, and the evolutionary
history of the plant kingdom reflects increasing adaptation to the terrestrial
environment. There are about 290,000 known plant species. (Food agriculture is
based on only about two dozen species.)
. The Invasion of the Land is really the Invasion of the Atmosphere!!!
The Protoplasm of Individual Plant Cells is surrounded by a Cellulose Wall.
While Cellulose is strong and prevents mechanical damage to the cell contents,
it is extremely hydrophilic and readily absorbs water.However, Cellulose easily
loses water via evaporation.
Land plants
Land plants
Land plants
Nontrcheophytes
Mosses
Mosses
Nontrcheophytes
Tracheophytes
Vascular system
Plants
A. General Characteristics of Plants
Plants are multicellular eukaryotes that are photosynthetic autotrophs. They
share the following characteristics with their green algal ancestors:
· Chloroplasts with the photosynthetic pigments: chlorophyll a, chlorophyll
b, and carotenoids.
· Cell walls containing cellulose.
Food reserve is starch that is stored in plastids.
Plants
As plants adapted to terrestrial life, they evolved complex bodies with cell
specialization for different functions.
· Aerial plant parts are coated with a waxy cuticle that helps prevent
desiccation.
Though gas exchange cannot occur across the waxy cuticle, CO2 and 02 can diffuse
between the leafs interior and the surrounding air through stomata, microscopic
pores on the leafs surface.
Plants
With the move from an aquatic to terrestrial environment, a new mode of
reproduction was necessary to solve two problems:
1. Gametes must be dispersed in a nonaquatic environment. Plants produce gametes
within gametangia, organs with protective jackets of sterile (nonreproductive)
cells that prevent gametes from drying out. The egg is fertilized within the
female organ.
Plants
2. Embryos must be protected against desiccation. The zygote develops into an
embryo that is retained for awhile within the female gametangia's jacket of
protective cells. Emphasizing this terrestrial adaptation, plants are often
referred to as embryophytes.
. Cellulose is like a sponge. If you drop a sponge in water, it saturates
instantaneously. A wet sponge readily loses water when it is placed on a dry
substrate. In order for an isolated plant cell, like a unicellular alga, to
survive, it must be in constant contact with water.
In order to withstand periodic dry spells, plant cells needed a water protective
coating.
. One of the most important plant adaptations is the Cuticle. It is a waxy
material that is secreted to the outside of the plasma membrane. It fills in the
spaces between cellulose fibrils and forms a continuous external waxy layer to
the outside of the cell wall. This makes the cell watertight!
. This cell can be called an "all purpose" cell because it Regulates its water
balance and performs Photosynthesis.
The Cuticle keeps water inside but it also prevents water uptake. The Cuticle is
usually thicker on the side of the cell facing the light. Consequently, water
could enter the bottom of the cell where the cuticle is thin and where water is
more
. abundant, and be retained within the cell by the thick cuticle on its upper
side. This could lead to the formation of colonies. The first multicellular
forms could be filaments. These might be followed by flat sheets.
The Chlorophyta (Green Algae) is algal group which probably gave rise to land
plants. The genus Coleochaete is regarded as the
Plant Evolution
Adaptations
Cuticle
alternation of generations
specialized tissues
Cuticle
Waxy coating on surfaces
resists drying out
stomata exist to allow necessary gas exchange
Morphological and Biochemical Evidence
Many characteristics of land plants
Also appear in a variety of algal clades
There are four key traits that land plants share only with charophyceans
Rose-shaped complexes for cellulose synthesis
Peroxisome enzymes
Structure of flagellated sperm
Formation of a phragmoplast*
Genetic Evidence
Comparisons of both nuclear and chloroplast genes
Point to charophyceans as the closest living relatives of land plants
Adaptations Enabling the Move to Land
In charophyceans
A layer of a durable polymer called sporopollenin prevents exposed zygotes from
drying out
The accumulation of traits that facilitated survival on land
May have opened the way to its colonization by plants
: Land plants possess a set of derived terrestrial adaptations
Many adaptations
Emerged after land plants diverged from their charophycean relatives
Defining the Plant Kingdom
Systematists
Are currently debating the boundaries of the plant kingdom
Some biologists think that the plant kingdom
Should be expanded to include some or all green algae
Until this debate is resolved
This textbook retains the embryophyte definition of kingdom Plantae
Derived Traits of Plants
Five key traits appear in nearly all land plants but are absent in the
charophyceans
Apical meristems
Alternation of generations
Walled spores produced in sporangia
Multicellular gametangia
Multicellular dependent embryos
Apical meristems and alternation of generations
Walled spores; multicellular gametangia; and multicellular, dependent embryos
Additional derived units
Such as a cuticle and secondary compounds, evolved in many plant species
The Origin and Diversification of Plants
Fossil evidence
Indicates that plants were on land at least 475 million years ago
Fossilized spores and tissues
Have been extracted from 475-million-year-old rocks
Whatever the age of the first land plants
Those ancestral species gave rise to a vast diversity of modern plants
Land plants can be informally grouped
Based on the presence or absence of vascular tissue
An overview of land plant evolution
: The life cycles of mosses and other bryophytes are dominated by the
gametophyte stage
Bryophytes are represented today by three phyla of small herbaceous (nonwoody)
plants
Liverworts, phylum Hepatophyta
Hornworts, phylum Anthocerophyta
Mosses, phylum Bryophyta
Debate continues over the sequence of bryophyte evolution
Mosses are most closely related to vascular plants
Bryophyte Gametophytes
In all three bryophyte phyla
Gametophytes are larger and longer-living than sporophytes
The life cycle of a moss
Mosses
Bryophyte gametophytes
Produce flagellated sperm in antheridia
Produce ova in archegonia
Generally form ground-hugging carpets and are at most only a few cells thick
Some mosses
Have conducting tissues in the center of their “stems” and may grow vertically
Bryophyte Sporophytes
Bryophyte sporophytes
Grow out of archegonia
Are the smallest and simplest of all extant plant groups
Consist of a foot, a seta, and a sporangium
Hornwort and moss sporophytes
Have stomata
Bryophyte diversity
Ecological and Economic Importance of Mosses
Sphagnum, or “peat moss”
Forms extensive deposits of partially decayed organic material known as peat
Plays an important role in the Earth’s carbon cycle
Plants
Bryophytes are embryophytes that generally lack vascular tissue and require
environmental water to reproduce
The bryophytes include plants found in three divisions:
· Bryophyta (mosses)
· Hepatophyta (liverworts)
Anthocerophyta (hornworts)
Plants
Bryophytes have two adaptations that made the move onto land possible:
A waxy cuticle that prevents desiccation.
Plants
Gametangia that protect developing gametes.
a. Antheridium, or male gametangium, produces flagellated sperm cells.
b. Archegonium, or female gametangium, produces a single egg; fertilization
occurs within the archegonium, and the zygote develops into an embryo within the
protective jacket of the female organ (embryophyte condition).
Plants
Bryophytes are not totally free from their ancestral aquatic habitat.
They need water to reproduce. Their flagellated sperm cells must swim from the
antheridium to the archegonium to fertilize the egg.
Most have no vascular tissue to carry water from the soil to aerial plant parts;
Plants
they imbibe water and distribute it throughout the plant by the relatively slow
processes of diffusion, capillary action, and cytoplasmic streaming.
Bryophytes lack woody tissue and cannot support tall plants on land; they may
sprawl horizontally as mats, but always have a low profile.
Plants
Mosses (Division Bryophyta)
A tight pack of many moss plants forms a spongy mat that can absorb and retain
water.
Each plant grips the substratum with rhizoids, elongate cells or cellular
filaments.
Plants
·a
. Meiosis marks the start of the Haploid Gametophyte Generation
The Meiospores are Green.
Plants
·a
Plants
·a
Plants
Photosynthesis occurs mostly in the small stemlike and leaflike structures found
in upper parts of the plant; these structures are not homologous with stems and
leaves in vascular plants.
Plants
There is an alternation of haploid and diploid generations in the moss life
cycle.
The sporophyte (2n) produces haploid spores by meiosis in a sporangium; the
spores divide by mitosis to form new gametophytes.
Plants
· Contrary to the life cycles of vascular plants, the haploid gametophyte
is the dominant
generation in mosses and other bryophytes. Sporophytes are generally smaller and
depend on the gametophyte for water and nutrients.
Plants
2. Liverworts (Division Hepatophyta) Less conspicuous than mosses, liverworts:
· Sometimes have bodies divided into lobes.
·Have a life cycle similar to mosses. Their sporangia have elaters, coil-shaped
cells that spring out of the capsule and disperse spores.
Plants
Can also reproduce asexually from gemmae (small bundles of cells that can bounce
out of cups on the surface of the gametophyte when hit by rainwater).
Plants
3. Hornworts (Division Anthocerophyta)
Hornworts:
· Resemble liverworts, but sporophytes are horn-shaped, elongated capsules
that grow from the mat-like gametophyte.
Their photosynthetic cells have only one large chloroplast, unlike the many
smaller ones of other plants.
: Ferns and other seedless vascular plants formed the first forests
Bryophytes and bryophyte-like plants
Were the prevalent vegetation during the first 100 million years of plant
evolution
Vascular plants
Began to evolve during the Carboniferous period
Origins and Traits of Vascular Plants
Fossils of the forerunners of vascular plants
Date back about 420 million years
These early tiny plants
Had independent, branching sporophytes
Lacked other derived traits of vascular plants
Life Cycles with Dominant Sporophytes
In contrast with bryophytes
Sporophytes of seedless vascular plants are the larger generation, as in the
familiar leafy fern
The gametophytes are tiny plants that grow on or below the soil surface
The life cycle of a fern
Transport in Xylem and Phloem
Vascular plants have two types of vascular tissue
Xylem and phloem
Xylem
Conducts most of the water and minerals
Includes dead cells called tracheids
Phloem
Distributes sugars, amino acids, and other organic products
Consists of living cells
Evolution of Roots
Roots
Are organs that anchor vascular plants
Enable vascular plants to absorb water and nutrients from the soil
May have evolved from subterranean stems
Evolution of Leaves
Leaves
Are organs that increase the surface area of vascular plants, thereby capturing
more solar energy for photosynthesis
Leaves are categorized by two types
Microphylls, leaves with a single vein
Megaphylls, leaves with a highly branched vascular system
According to one model of evolution
Microphylls evolved first, as outgrowths of stems
Sporophylls and Spore Variations
Sporophylls
Are modified leaves with sporangia
Most seedless vascular plants
Are homosporous, producing one type of spore that develops into a bisexual
gametophyte
All seed plants and some seedless vascular plants
Are heterosporous, having two types of spores that give rise to male and female
gametophytes
Classification of Seedless Vascular Plants
Seedless vascular plants form two phyla
Lycophyta, including club mosses, spike mosses, and quillworts
Pterophyta, including ferns, horsetails, and whisk ferns and their relatives
The general groups of seedless vascular plants
Phylum Lycophyta: Club Mosses, Spike Mosses, and Quillworts
Modern species of lycophytes
Are relics from a far more eminent past
Are small herbaceous plants
. The species to the right grows locally on disturbed sites and could be a
candidate for soil stabilization research. It is a complex plant which has
horizontal Stolons which have Isotomous Branching. These produce the Roots which
anchor the plant to the substrate.
. While extant Lycopods are small plants with little ecological significance.
Forests of tree-sized lycopods once dominated certain habitats. The most famous
of these is Lepidodendron which reached heights up to 30 meters. They had
secondary growth. The stems were coated with leaf bases and there appeared to be
little internodal elongation.
Phylum Pterophyta: Ferns, Horsetails, and Whisk Ferns and Relatives
Ferns
Are the most diverse seedless vascular plants
Ferns
The Significance of Seedless Vascular Plants
The ancestors of modern lycophytes, horsetails, and ferns
Grew to great heights during the Carboniferous, forming the first forests
The growth of these early forests
May have helped produce the major global cooling that characterized the end of
the Carboniferous period
Decayed and eventually became coal
Plants
A. The Earliest Vascular Plants
Oldest fossilized vascular plant is Cooksonia (late Silurian):
· Discovered in both European and North American Silurian rocks; North
America and Europe were probably connected during the late Silurian, about 408
million
years ago.
Plants
· Simple plant with dichotomous branching and bulbous terminal sporangia on
some stems.
· True roots and leaves were absent; the largest species was about 50 cm
tall.
· Grew in dense stands around marshes.
As vascular plants became more widespread, new species appeared.
Plants
Ferns and other seedless vascular plants dominated the Carboniferous "coal
forests"
The earliest vascular plants were seedless and they dominated the Carboniferous
forests. Modern flora includes four divisions of seedless vascular plants.
Plants
A. Division Psilophyta
Psilophyta consists of only two genera: Psilotum (whiskferns) and Tmesipteris.
Whiskferns are the most well known and share the following characteristics:
· True roots and leaves are absent, subterranean rhizomes are covered with
hair-like rhizoids, and shoots have scales which lack vascular tissue.
Plants
· Dichotomous branching.
· The gametophytes are subterranean and lack chlorophyll, depending on
symbiotic soil fungi for food.
· Flagellated sperm swim through the soil from antheridia to the
archegonium of the gametophyte.
• The sporophyte emerges from the gametophyte, which then dies.
Plants
. B. Division Lycophyta (Lycopods)
The Division Lycophyta include the club mosses and ground pines.
· Survived through the Devonian period and dominated land during the
Carboniferous Period (340-280 million years ago).
Some are temperate, low-growing plants with rhizomes and true leaves.
Plants
· Some species of Lycopodium are epiphytes, plants that use another
organism as a substratum but are not parasites. The sporangia of Lycopodium are
borne on sporophylls, leaves specialized for reproduction. In some, sporophylls
are clustered at branch tips into club-shaped strobili - hence the name club
moss.
Plants
Spores develop into inconspicuous gametophytes. The non-photosynthetic
gametophytes are nurtured by symbiotic fungi.
Most are homosporous (making only one type of spore which develops into a
bisexual gametophyte).
Plants
Genus Selaginella is heterosporous, having megaspores which develop into
gametophytes bearing archegonia, and microspores which develop into gametophytes
with antheridia. The gametophytes are unisexual, either male or female.
Plants
C. Division Sphenophyta (Horsetails)
The division Sphenophyta includes the horsetails; it survived through the
Devonian and reached its zenith during the Carboniferous period.
Plants
The only existing genus is Equisetum, which:
· Lives in damp locations and has flagellated sperm.
· Is homosporous.
· Has a conspicuous sporophyte generation.
Has photosynthetic, free-living gametophytes (not dependent on the sporophyte
for food).
Plants
Division Pterophyta (Ferns)
Appearing in the Devonian, ferns radiated into diverse species that coexisted
with tree lycopods and horsetails in the great Carboniferous forests.
Most diverse in the tropics, ferns are the most well represented seedless plants
in modern floras; there are more than 12,000 existing species of ferns.
Plants
· Fern leaves are generally much larger than those of lycopods and probably
evolved in a different way.
Lycopods have microphylls, small leaves that probably evolved as emergences from
the stem that contained a single strand of vascular tissue.
Ferns have megaphylls, leaves with a branched system of veins. Megaphylls
probably evolved from webbing formed between separate branches growing close
together.
Plants
·a
Plants
·a
Plants
Most ferns have fronds, compound leaves that are divided into several leaflets.
· The emerging frond is coiled into a fiddlehead that unfurls as it grows.
Leaves may sprout directly from a prostrate stem (bracken and sword ferns) or
from upright stems many meters tall (tropical tree ferns).
Plants
Ferns are homosporous and the conspicuous leafy fern plant is the sporophyte.
Specialized sporophylls bear sporangia on their undersides; many ferns have
sporangia arranged in clusters called sori and are equipped with springlike
devices that catapult spores into the air, where they can be blown by the wind
far from their origin.
Spore Release From a Fern Sporangium
Plants
·a
Plants
·a
Plants
· The spore is the dispersal stage.
The free-living gametophyte is small and fragile, requiring a moist habitat.
· Water is necessary for fertilization, since flagellated sperm cells must
swim from the antheridium to the archegonium, where fertilization takes place.
The sporophyte embryo develops protected within the archegonium.
129—Ferns are seedless vascular plants
130—Spore production by ferns
131—”Standard” fern gametophyte
Plants
E. The Coal Forests
During the Carboniferous period, much of the land was covered in shallow seas
and swamps.
· Organic rubble of the plants above accumulated as peat.
When later covered by the sea and sediments, heat and pressure transformed the
peat into coal.
Enough for now!
Plant Diversity II: The Evolution of Seed Plants
Plant Evolution
Bryophytes - no roots, leaves or stems, no vascular system, simple reproduction
relying on water, gametophyte (haploid) dominant generation
Ferns - first vascular system, rhizomes (horizontal stems), fronds, sporophyte
(diploid) dominant generation
Plant Evolution
Gymnosperms - first leaves (needles), vascular system, stems and roots, naked
seeds
Angiosperms - vascular system more organized, leaves, ability to shed leaves,
seed provided with nutritive tissues, flowers, more sophisticated reproductive
methods
Angiosperms
Flowering, vascular plants
most successful plants
fertilization in flowering plants called double fertilization
2 sperm involved - 1 fertilizes the egg, the other fuses with 2 cells in female
gametophyte to form endosperm
Overview: Feeding the World
Seeds changed the course of plant evolution
Enabling their bearers to become the dominant producers in most terrestrial
ecosystems
The reduced gametophytes of seed plants are protected in ovules and pollen
grains
In addition to seeds, the following are common to all seed plants
Reduced gametophytes
Heterospory
Ovules
Pollen
Advantages of Reduced Gametophytes
The gametophytes of seed plants
Develop within the walls of spores retained within tissues of the parent
sporophyte
Gametophyte/sporophyte relationships
Heterospory: The Rule Among Seed Plants
Seed plants evolved from plants that had megasporangia
Which produce megaspores that give rise to female gametophytes
Seed plants evolved from plants that had microsporangia
Which produce microspores that give rise to male gametophytes
Ovules and Production of Eggs
An ovule consists of
A megasporangium, megaspore, and protective integuments
Pollen and Production of Sperm
Microspores develop into pollen grains
Which contain the male gametophytes of plants
Pollination
Is the transfer of pollen to the part of a seed plant containing the ovules
If a pollen grain germinates
It gives rise to a pollen tube that discharges two sperm into the female
gametophyte within the ovule
Pollen, which can be dispersed by air or animals
Eliminated the water requirement for fertilization
The Evolutionary Advantage of Seeds
A seed
Develops from the whole ovule
Is a sporophyte embryo, along with its food supply, packaged in a protective
coat
: Gymnosperms bear “naked” seeds, typically on cones
Among the gymnosperms are many well-known conifers
Or cone-bearing trees, including pine, fir, and redwood
The gymnosperms include four plant phyla
Cycadophyta
Gingkophyta
Gnetophyta
Coniferophyta
Exploring Gymnosperm Diversity
Exploring Gymnosperm Diversity
Gymnosperm Evolution
Fossil evidence reveals that by the late Devonian
Some plants, called progymnosperms, had begun to acquire some adaptations that
characterize seed plants
Gymnosperms appear early in the fossil record
And dominated the Mesozoic terrestrial ecosystems
Living seed plants
Can be divided into two groups: gymnosperms and angiosperms
Plants
Three life cycle modifications contributed to seed plant success:
1. Gametophytes became reduced and were retained in the moist reproductive
tissue of the sporophyte generation (not independent).
Plants
2. Pollination evolved, so plants were no longer tied to water for
fertilization.
3. The evolution of the seed.
· Zygote develops into an embryo packaged with a food supply within a protective
seed coat.
Seeds replace spores as main means of dispersal.
Plants
Gymnosperms began to dominate landscapes as climates became drier at the end of
the Paleozoic era
Plants
Gymnosperms appear in the fossil record much earlier than flowering plants, and
they:
· Lack enclosed chambers in which seeds develop.
Are grouped into four divisions: Cycadophpyta, Ginkgophyta, Gnetophyta and
Coniferophyta.
pictures
Plants
·a
Plants
Division Coniferophyta (Conifers)
Division Coniferophyta is the largest division of gymnosperms:
Most are evergreens and include pines, firs, spruces, larches, yews, junipers,
cedars, cypresses, and redwoods
Plants
Includes some of the tallest (redwoods and some eucalyptus); largest (giant
sequoias); and oldest (bristle cone pine) living organisms.
•Most lumber and paper pulp is from conifer wood. Needle-shaped conifer leaves
are adapted to dry conditions
Plants
· Thick cuticle covers the leaf.
· Stomata are in pits, reducing water loss.
Despite the shape, needles are megaphylls, as are leaves of all seed plants.
Plants
. The Life History of a Pine
The life cycle of pine, a representative conifer, is characterized by the
following:
The multicellular sporophyte is the most conspicuous stage; the pine tree is a
sporophyte, with its sporangia located on cones.
Plants
· The multicellular gametophyte generation is reduced and develops from
haploid spores that are retained within sporangia.
The male gametophyte is the pollen grain; note that there is no antheridium.
The female gametophyte consists of multicellular nutritive tissue and an
archegonium that develops within an ovule.
Plants
Conifer life cycles are heterosporous; male and female gametophytes develop from
different types of spores produced by separate cones.
· Trees of most pine species bear both pollen cones and ovulate cones,
which develop on different branches.
Plants
Pollen cones have microsporangia; cells in these sporangia undergo meiosis
producing haploid microspores, small spores that develop into pollen grains -
the male gametophytes.
Plants
Ovulate cones have megasporangia; cells in these sporangia undergo meiosis
producing large megaspores that develop into the female gametophyte. Each ovule
initially includes a sporangium (nucellus) enclosed in protective integuments
with a single opening, the micropyle.
Plants
It takes nearly three years to complete the pine life cycle, which progresses
through a complicated series of events to produce mature seeds.
Windblown pollen falls onto the ovulate cone and is drawn into the ovule through
the micropyle.
The pollen grain germinates in the ovule, forming a pollen tube that begins to
digest its way through the nucellus.
Plants
· A megaspore mother cell in the nucellus undergoes meiosis producing four
haploid megaspores, one of which will survive; it divides repeatedly by mitosis
producing the immature female gametophyte.
· Two or three archegonia, each with an egg, then develop within the
multicellular gametophyte.
Plants
More than a year after pollination, the eggs are ready to be fertilized; two
sperm cells have developed and the pollen tube has grown through the nucellus to
the female gametophyte.
· Fertilization occurs when one of the sperm nuclei unites with the egg
nucleus. All eggs in an ovule may be fertilized, but usually only one zygote
develops into an embryo.
Plants
The pine embryo, or new sporophyte, has a rudimentary root and several embryonic
leaves. It is embedded in the female gametophyte, which nourishes the embryo
until it is capable of photosynthesis. The ovule has developed into a pine seed,
which consists of an embryo (2n), its food source (n), and a surrounding seed
coat (2n) derived from the parent tree.
Plants
Scales of the ovulate cone separate, and the winged seeds are carried by the
wind to new locations. Note, that with the seed plants, the seed has replaced
the spore as the mode of dispersal.
A seed that lands in a habitable place germinates, its embryo emerging as a pine
seedling.
Plants
The History of Gymnosperms
Gymnosperms descended from Devonian progymnosperms.
· Adaptive radiation during the Carboniferous and Permian periods led to
today's divisions.
During the Permian, Earth became warmer and drier; therefore, lycopods,
horsetails and ferns (previously dominant) were largely replaced by conifers and
their relatives, the cycads
Plants
This large change marks the end of the Paleozoic era and the beginning of the
Mesozoic era.
A Closer Look at the Life Cycle of a Pine
Key features of the gymnosperm life cycle include
Dominance of the sporophyte generation, the pine tree
The development of seeds from fertilized ovules
The role of pollen in transferring sperm to ovules
The life cycle of a pine
: The reproductive adaptations of angiosperms include flowers and fruits
Angiosperms
Are commonly known as flowering plants
Are seed plants that produce the reproductive structures called flowers and
fruits
Are the most widespread and diverse of all plants
Characteristics of Angiosperms
The key adaptations in the evolution of angiosperms
Are flowers and fruits
Flowers
The flower
Is an angiosperm structure specialized for sexual reproduction
A flower is a specialized shoot with modified leaves
Sepals, which enclose the flower
Petals, which are brightly colored and attract pollinators
Stamens, which produce pollen
Carpels, which produce ovules
Fruits
Fruits
Typically consist of a mature ovary
Can be carried by wind, water, or animals to new locations, enhancing seed
dispersal
The Angiosperm Life Cycle
In the angiosperm life cycle
Double fertilization occurs when a pollen tube discharges two sperm into the
female gametophyte within an ovule
One sperm fertilizes the egg, while the other combines with two nuclei in the
center cell of the female gametophyte and initiates development of food-storing
endosperm
The endosperm
Nourishes the developing embryo
The life cycle of an angiosperm
Angiosperm Evolution
Clarifying the origin and diversification of angiosperms
Poses fascinating challenges to evolutionary biologists
Angiosperms originated at least 140 million years ago
And during the late Mesozoic, the major branches of the clade diverged from
their common ancestor
Fossil Angiosperms
Primitive fossils of 125-million-year-old angiosperms
Display both derived and primitive traits
An “Evo-Devo” Hypothesis of Flower Origins
In hypothesizing how pollen-producing and ovule-producing structures were
combined into a single flower
Scientist Michael Frohlich proposed that the ancestor of angiosperms had
separate pollen-producing and ovule-producing structures
Angiosperm Diversity
The two main groups of angiosperms
Are monocots and eudicots
Basal angiosperms
Are less derived and include the flowering plants belonging to the oldest
lineages
Magnoliids
Share some traits with basal angiosperms but are more closely related to
monocots and eudicots
Exploring Angiosperm Diversity
Exploring Angiosperm Diversity
Evolutionary Links Between Angiosperms and Animals
Pollination of flowers by animals and transport of seeds by animals
Are two important relationships in terrestrial ecosystems
Evolutionary Links Between Angiosperms and Animals
: Human welfare depends greatly on seed plants
No group is more important to human survival than seed plants
Products from Seed Plants
Humans depend on seed plants for
Food
Wood
Many medicines
Threats to Plant Diversity
Destruction of habitat
Is causing extinction of many plant species and the animal species they support