CHAPTER 30Plant Diversity II: The Evolution of Seed Plants

TRANSFORMING THE WORLD

Origin of Seed Plants 360 million years ago

Seeds consist of an embryo, its food supply and a surrounding protective coating

Fun Fact: seeds are analogous to a detachable and mobile version of a pregnant woman’s womb

Matured seeds dispersed from their parent by wind or other means.

Gymnosperms and angiosperms have come to dominate modern landscapes and make up the great majority of plant biodiversity.

The cultivation and harvest of plants, especially angiosperms, began 13,000 years ago.

Near East, East Asia, Africa, and the Americas is where humans began the cultivation of plants independently.

This was an important cultural change in the history of humanity.

In addition, it made possible the transition from hunter-gatherer societies to permanent settlements.

CHAPTER 30.1Seeds and Pollen Grains are key adaptations for life on land

ALL SEED PLANTS HAVE:

Reduced Gametophytes Heterospory Ovules Pollen

ADVANTAGES OF REDUCED GAMETOPHYTES Develop from spores located on the sporangia of

the parental sporophyte Protects female gametophytes from

environmental stresses Moist reproductive tissues shield against UV

Radiation and protect from drying out Dependent gametophytes ones obtain food from

sporophytes Free-living ones defend for

themselves

HETEROSPORY: THE RULE AMONG SEED PLANTS

Megasporangia produce megaspores (female gametophytes) and have only one function

Microsporangia produce vast numbers of microspores (male gametophytes)

Closest relatives of seed plants are all homosporous (single type of spore that is bisexual)

OVULES AND PRODUCTION OF EGGS

Integument: layer of sporophyte tissue that envelopes and protects the megasporangium (gymnosperm have 1 & angiosperm have 2)

Ovule: megasporangium, megaspore and their integuments

Ovules are where females produce one or more eggs

POLLEN AND PRODUCTION OF SPERM

Pollen grains consist of a male gametophyte enclosed within a pollen wall

Polymer Sporopollenin (pollen walls) protect pollen

Pollination is the transfer of pollen to part of a seed plant that contains ovules

If a pollen grain germinates it gives rise to a pollen tube

Flagellated sperm only travel a few centimeters, but sperm producing male gametophytes can be carried long distances

Living gymnosperms provide evidence of evolutionary transition to nonmotile sperm

THE EVOLUTIONARY ADVANTAGES OF SEEDS

Spores were the only protective stage in any plant life cycle before the advent of seeds

Moss spores can withstand many different conditions and are easily dispersed

Spores were the main way plants spread over Earth for the first 100 million years of plant life on land

Spores are single-celled and seeds are multi-cellular

Unlike spores, seeds have a supply of stored food

Spores have shorter lifetimes

CONTINUED…

Favorable conditions lead to the germination of seeds whose sporophyte embryos emerge as seedlings

Some seeds land close to their parent sporophyte plant, but others are transferred far distances by plants and animals

GYMNOSPERM EVOLUTION

Gymnosperms bear “naked” (not enclosed in ovaries) seeds, typically conesEarly gymnosperms lived in carboniferous

ecosystems and took over the seedless plants before (horsetails, lycophytes, ferns)

Carboniferous Period380 million years ago Plants begin to acquire characteristics of seed

plants Progymnosperms- transitional species of

seedless vascular plants 1st seed bearing plants appear in fossil record

1200 million years after the first angiosperms

GYMNOSPERM EVOLUTION

Permian period Drier climate that favored the spread of

gymnosperms Mostly changed the plant life in seas Changed terrestrial life

Amphibians decreased in diversity and replaced with reptiles

CarboniferousPermianPaleozoicMesozoic

CYCADOPHYTA

Next largest group after conifers (cone bearing conifers)

Large cones and palm-like leaves 130 species survive todays

Thrived during Mesozoic era

GINKOPHYTA

Ginkgo biloba is only surviving species Fan-like leaves that turn gold in fall Tolerates air pollution well

GNETOPHYTA PHYLUM

• Gnetum 35 species of tropical trees, shrubs, or vines Located in Africa and Asia Seeds look like fruits Leaves look like those of flowering plants

• Ephedra 40 species that inhabit arid regions worldwide Arid- extremely dry

GNETOPHYTA PHYLUM CONT.

Coniferophyta Largest phyla (600 species) Large trees Evergreens

Douglas fir, European larch, Common juniper, Wollemi pine, Bristlecone pine, Sequoia

Welwitschia 1 species = Welwitschia mirabilis Lives in deserts of southwest Africa Strap-like leaves (largest leaves ever known)

30.3: THE REPRODUCTIVE ADAPTATIONS OF ANGIOSPERMS INCLUDE FLOWERS AND FRUITS

flowering plants; seed plants that produce the reproductive structures called flowers or fruits.

Most diverse and widespread of all plants More than 250,000 species About 90% of plant species

ANGIOSPERMS

Flower: structure specialized for sexual reproduction Pollination occurs by wind, insects, and other animals Sepal: Usually green and enclose the flower before it

opens (ex: rosebud) Petals: Interior to the sepals, brightly colored and aid

in attracting pollinators Stamens: Produce microspores that develop into pollen

grains containing male gametophytes Filament: Stalk of the stamen Anther: Terminal sac of the stamen where pollen is produced

Carpels: Make megaspores and their products, female gametophytes

Stigma: Sticky tip of the carpel that receives pollen. Style: Leads from the stigma to the ovary at the base

of the carpal

Fruits: A mature ovary of a flower Fruit protects dormant seeds and often aids in their

dispersal. Can either be fleshy (oranges, grapes) or dry (beans,

nuts, and grains) Seeds are spread through animals, water (coconuts),

and wind (Maple tree helicopters)

Terms: Generative Cell: divides and forms two sperm Tube Cell: Produces a pollen tube Gametophyte: The multicellular haploid form that

produces haploid gametes by mitosis. The haploid gametes unite and develop into sporophytes

Embryo Sac: Female gametophyte Crosspollination: The transfer of pollen from an anther

of a flower on one plant to the stigma of a flower on another plant of the same species Enhances genetic variability

Endosperm: Developed from the rapid division of the fertilized nucleus of the central cell of the female gametophyte Tissue rich in starch and other food reserves that nourish the

developing embryo Double Fertilization: One fertilization event produces a

zygote and the other produces a triploid cell Unique to angiosperms

ANGIOSPERM EVOLUTION

Originated at least 140 million years ago During the late Mesozoic period, the major

branches of the clade diverged from their common ancestor

Fossils, phylogenic analyses, and developmental studies offer insights into the origin of flowers

Increased dominance of plants in Cretaceous era

TYPES OF ANGIOSPERMS Magnolids: most closely related to eudicots; more

than 8,000 species Ex: Magnolias

Basal Angiosperms: Member of a clade of 3 early-diverging lineages Ex: Water Lily, Star Anise, Amborella Trichopoda

TYPES OF ANGIOSPERMS

Monocots: 70,000 species (1/4 of angiosperms); one cotyledon (sporophyte embryo with a rudimentary root and one or two seed leaves); veins usually parallel; vascular tissue scattered; root system usually fibrous (no main root); pollen grain with one opening; floral organs usually in groups of threeEx: Orchids, Pygmy date palm, Lily, Barley

TYPES OF ANGIOSPERMS

Eudicots: 170,000 species (2/3 of angiosperms); two cotyledons, veins usually netlike; vascular tissue usually arranged in ring; taproot (mainroot) usually present; pollen grain with three openings; floral organs usually in multiples of four of five Ex: California Poppy, Pyrenean Oak, Dog Rose, Snow Pea,

Zucchini

30.4 HUMAN WELFARE DEPENDS GREATLY ON SEED PLANTS

We rely on seed plants for food, fuel, wood, and medicine. preservation of plant diversity is

critical because of our reliance on themPRODUCTS FROM SEED PLANTS

MOST OF OUR FOOD COMES FROM ANGIOSPERMS

Flowering plants provide nearly all our food.

Just six crops—wheat, rice, maize, potatoes, cassava, and sweet potatoes

yield 80% of all calories consumed by humans.

Also depend on angiosperms to feed livestock

Today’s crops are the products of a relatively recent burst of genetic change, resulting from artificial selection after the domestication of plants 13,000 years ago.

How did wild plants change so dramatically in such a relatively short time? The answer is likely a combination of

deliberate and unconscious selection for plants with desirable traits, such as large fruits and lack of toxins.

Angiosperms also provide important edible foods such as coffee, chocolate, and spices. Spices come from many plant parts such as vanilla and

mustard come from fruits and seeds Gymnosperms and angiosperms are sources of

wood, which is absent in all living seedless plants and consists of an accumulation of tough-walled xylem cells. Wood is the primary source of fuel for much of the

world. Wood pulp is used to make paper the world’s most widely used construction material.

Humans depend on seed plants for medicines. Most cultures have a tradition of

herbal medicine. Scientific research has identified

the relevant secondary compounds in many of these plants, leading to the synthesis of many modern medicines.

THREASTS TO PLANT DIVERSITY Although plants are a renewal

resource, plant diversity is not. Plant diversity is a nonrenewable

resource. **

The demand for space and natural resources are extinguishing plant species at a very fast rate because of human population Due primarily to the slash-and-burn clearing of forests

for agriculture, tropical forests may be completely eliminated within 25 years.

When the forests go so does thousands of plant species and the animals that depend on these plants.

The destruction of these areas is an irrevocable loss of these nonrenewable resources.

The rate of loss is faster than the Permian and Cambrian extinctions….forever changing the evolutionary history of land plants and many other organisms

While the loss of species is greatest in the tropics In addition to the ethical concerns that many

people have concerning the extinction of living forms, there are also practical reasons to be concerned about the loss of plant diversity.

People depend on plants for food, building materials, and medicines. We have explored the potential uses for only a tiny

fraction of the 290,000 known plant species. For example, Almost all of our food is based on

cultivation of only about two dozen species. fewer than 5,000 plant species as potential

sources of medicines The tropical rain forests and other plant communities

may be a medicine chest of healing plants that could be extinct before we even know they exist.