Chapter 27

I. structural and reproductive adaptation made the colonization of land possible: an overview of plant evolution

A. General characteristics of plants.

Plants are multicellular eukaryotes that are photosynthetic aotutrophs. 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.

As plants adapted to terrestrial life, they evolved complex bodies with cell specialization for different functions.

-Aerial plant parts are coated witha waxy cuticle that helps prevent desiccation.

-though gas exchange cannot occur across the waxy cuticle, CO2 and O2 can diffuse between the

Leaf's interior and the surrounding air through stomata, microscopic pores on the leaf's surface.

B. The Embryophyte Condition

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.

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

C. alteration of generations: a Review

Most plants reproduce sexually, and most are also capable of asexual reproduction. All plants have life cycles with an alternation of generations.

- A haploid gametophyte generation produces and alternates with a diploid sporophyte generation. The sporophyte in tun, produces gametophytes.

- The life cycles are heteromorphic; that is, sporophytes and gametophytes differ in morphology.

- The sporophyte is larger and more noticable than the gametophyte in all plants but mosses and their relatives.

A comparison of life cycles among plant divisions is instructive because:

- It points to an important trend in plant evolution: reduction of the haploid gametophyte generation and dominance of the diploid sporophyte.

- Certain life cycles features are adaptations to a terrestrial environment; for example the replacement of flagellated sperm by pollen

D. Some highlights of plant phylogeny

There are four major periods of plant evolution that opened new adaptive zones on land:

1. Origin of plants from aquatic ancestors (probably green algae) in the Silurian

(425 million years ago).

-Cuticle and jacketed gametangia evolved that protected gametes and ambryos

-vascular tissue evolved with conducting cells that transport water and nutrients throughout the plant.

2. Diversification of seedless vascular plants, such as ferns, during the early Devonian ( about 400 million years ago)

3. Origin of the seed near the end of the Devonian about 360 years ago

Seed= Plant embryo packaged with a store of food within a resistant coat.

- Early seed plants bore seeds as naked structures and evolved into gymnosperms including conifers

- conifers and ferns coexisted in the landscape for more than 200 million years

4. Emergence of flowering plants during the early Creaceous, about 130 million years ago

- Unlike gymnosperms, flowering plants bear seeds within the flower's protective ovaries.

- Most contemporary plants are flowering plants or angiosperms

E. Classification of plants

The major taxonomic category of plants is the division; it is comparable to phylum, the highest category in the animal kingdom

Common name Approximate # of extant species

nonvascular plants:

Division Bryophyta Mosses 10,000

Division Hepatophyta Liverworts 6,500

Division Anthocerophyta Hornworts 100

vascular seedless plants:

Division psilophyta Whiskferns 10 to 13

Division Lycophyta club mosses 1,000

Division pterophyta Ferns 12,00

Divsion Sphenophyta horsetails 15

Vascular seed plants:

Division Coniferophyta conifers 550

Division Cycadophyta cycads 100

Division Ginkgophyta Ginkgo 1

Division Gnetophyta Gnetae 70

Division Anthophyta flowering plants 235,000

II. Plants probably evolved from green algae called charophytes

the green algae are likely the photosynthetic protists most closely related to plants, This conclusion is based on homologies in:

- cell wall composition

- Structure and pigmentation of chloroplasts

Available evidence supports the hypothesis that plants and green algae called charophytes both evolve from a common ancestor. Researchers have found the following homologies between charophytes and plants:

1. Homologous chloroplasts.

- green algae andplants both have the accessory pigments, chlorophyllb and netacarotene

- green algae and plants both have chloroplasts with thylakoid membranes stacked asgrana.

- compared to chloroplast DNA of varios green algae, plant chloroplast DNA most closely matches that of charophytes

2.Biochemical similarity

- most gree algae and plants contain cellulose in their cell walls. Charophytes are the most plantlike in wall composition with cellulose making up 20% to 26% of the wall material.

-Charophyte peroxisomes are the only algal peroxisomes with the same enzyme composition as plant peroxisomes.

3. Similarity in mitosis and cytokinesis. During cell division in charophytes and plants:

- the nuclear envelope completely disperses during late prophase

- the mitotic spindle persists until cytokinesis begins

- Cell plate formation during cytokinesis involves cooperation of microtubules, actin microfilaments, and vesicles.

4. similarity in sperm ultrastructure, charophyte sperm ultrastructure is more similar to certain plants thanto other grren algae.

5. Genetic relationship DNA and rRNA similarities in charophytes and plants provides additional evidence for the hypothesis that charophytes are the closest relatives of plants.

A. The origin of alternation of Generations in Plants

the alternation of haploid and diploid generations apparently evolved indepently among various groups of algae.

- since alternation of generations does not occur among modern charophytes, it is presumed that alternation of generations in plants has had a separate origin from alternation of generations in other algal groups

- its appearance in plants is thus analogous, not homologous, th the alternation of generations observed in various groups of algae.

How did alternation of generations evolve in plant ancestors? Coleochaete, a modern charophyte, holds some clues:

- The coleochaete thallus is haploid

- in contrast to most algae, the parental thallus of coleochaete retains the eggs, and after fertilization, the zygotes remain attach to the plant.

- Nonreproductive cells of the thallus grow around each zygote; which enlarges, undergoes meiosis, and releases haploid swimming spores

- Haploid spores develop into new individuals.

-The only diploid stage is the zygote; there is no alternation of multicellular diploid and haploid generations.

- If an ancestral charophyte delayed meiosis until after the zygote divided mitotically, ther would be a multicellular diploid generation still attached to the haploid parent. Such a life cycle would be an alternation of generations.

- If specialized gametophte cells formed protective layers around a tiny sporophyte, this hypothetical ancestor would also be a primative embryophyte.

What would be the adaptive advantage of delaying meiosis and forming a mass of diploid cells? It may maximize the production of haploid spores.

- If the zygote undergoes meiosis directly, each fertilization event results in only a few haploid spores.

- Mitotic division of the zygote to form a multicellular sporophyte amplifies the sexual product. Many diploid cells can undergo meiosis producing a large number of haploid spores, enhancing the chances of survival in unfavorable environments.

B. Adaptations to sallow water as a preadaptations for living on land

some adaptations for life in shallow water could also have been adaptive for life on land.

-Many modern charophytes live in shallow water, and some ancient charophytes may have also lived in shallow-water habitats subject to occasional drying.

- about 440 million years ago, during the transistion from Ordivician to Silurian, repeated glaciation and climatic changes caused fluctuations in the water levels of lakes and ponds.

- Natural selection may have favored shallow-water plants tolerant to periodic drying. adaptations to shallow water may also have been preadaptive for terrestrial life. For example:

- Waxy cuticles

-Protection of gametes

- Protection of developing embryos

-Eventually, accumulated adaptations made it possible for ancestral plants to live permanently above the water line, opening a new adaptive zone with:

- sunlight unfiltered by water and algae.

-soil rich minerals.

- absence of terrestrial herbivores

III. Bryophytes are embryophytes that generally lack vascular tissue and require environmental water to reproduce

The bryophytes include plants found in three divisions:

- bryophytea (mosses)

- hepatophyta (liverworts)

- anthocerophyta (hornworts)

Bryophytes ahve two adaptations that made the move onto land possible:

- a waxy cuticle that prevents desiccation

- 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 ambryo within the protective jacket of the female organ.

bryophytes are not totally free from their ancestral aquatic habitat.

-they need water to reproduce. Their flagellated sperm 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; they imbibe water and distribute it throughout the plant by the relatively slow processes of diffusion, capillary action, and cytoplasmic streaming.

Bryophytes lack woody tissues and cannot support tall plants on land; they may sprawl horizantally as mats, but always have a low profile.

1. 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

- photosynthetic occurs mostly in the small stemlike and leaflike structures found in upper parts of the plants; these structures are not homologous with stems and leaves in vascular 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.

-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.

2. Liverworts(Division Hepatophyta)

Less conspicous 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.

-Can also reproduce asexually from gemmae (small bundles of cells that bouce out

cups on the surface of the gametophyte when hit by rainwater).

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.

IV. The origin of vascular tissue was an evolutionary breakthrough in the colonization of land

In addition to cuticles and jacketed sex organs, other adaptations for terrestrial life evolved in

vascular plants as thy colonized land:

-Regional specialization of the plant body. Unlike aguatic environments, terrestrial

environments spatially segregate the resources of water and light. This problem was

solved as plants evolved subterranean roots that absorb water and minerals from the soil

and an aerial shoot system of stems and leaves to make food.

-Structural support. In aquatic environments, the denser medium of water buoys plants up

toward the light, but in terrestrial environments plants must have structural support to

stand upright in air. Such support was provided as the hard material lignin was embedded

into the cellulose matrix of cell walls.

Vascular system. Regional specialization of the plant body presented the problem of

transporting substances between the root and shoot systems. This problem was solved as

A vascular system evolved with two types of conducting tissues.

Xylem= Complex, plant vascular tissue that conducts water and minerals from the roots to

the rest of the plant.

Composed of dead, tube-shaped cells that form a microscopic water-pipe system.

Cell walls are usually lignified, giving the plant structural support.

Phloem= Plant vascular tissue that conducts food throughout the plant.

Composed of living cells arranged into tubules.

Distributes sugars, amino acids and other organic nutrients.

Pollen. Pollination eliminated the need for water to transport gametes.

Seeds

Increased dominance of the diploid sporophyte.

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.

- 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.

V. 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.

C. Division Psilophyta

Psilophyta consists of only two genera: Psilotum (whiskerns) 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 of scales which lack vascular tissue.

- 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.

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.

- 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.

Spores develop into inconspicuous gametophyes. The non-photosynthetic

gametophytes are nurtured by symbiotic fungi.

- Most are homosporous (making only one type of spore which develops into a

bisexual gametophyte).

- Genus Selaginella is hetersporous, having megapores which develop into

gametophytes bearing archegonia, and microspores which develop into

gametophytes with antheridia. The gametophytes are unisexual, either male or female

Single Bisexual Eggs

Homosporous sporophyte type of gametophyte

Spore Sperm

Megaspore Female gametophye Eggs

Heteropsorous sporophyte Miscrospore Male gametophye Sperm

C. Division Spehenophyta (Horsetails)

The division Sphenophyta includes the horsetails; it survived through the Devonian and

reached its zenith during the Carboniferous period.

The only existing genus is Equistetum 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).

D. Division Pterophya (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 the modern floras; there are more than 12,000 existing species of ferns.

Fern leaves are generally much larger than those of lyocpods 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.

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.)

Ferns are homosporous and the conspicuous leafy plant is the sporophyte. (See

Campbell, Figure 27.15)

Specialized sporophylls bear sporangia on their undersides; many ferns have sporangia arranged in clusters called sori and are equipped with springlike devices that catapult into the air, where they can be blown by the wind far from their origin.

The spore is the dispersal

The free-living gametophyte is small and fragile, requiring a moist habitat.

Water 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.

E. The Coal Forests

During the Carboniferous period, much of the land was covered in shallow seas and

swamp.

Organic rubble of the plants above accumulated as pear.

When later covered by the sea and sediments, heat and pressure transformed the peat into coal.

V Reproductive adaptations catalyzed the success of the seed plants

Three life cycle modifications contributed to seed plant success.

1. Gametophytes become reduced and were retained in the moist reproductive tissue

of the sporophyte generation (not independent).

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.

VII Gymnosperms began to dominate landscapes as climates became drier at the end of the paleozoic era.

Gymnosperms appear in the fossil record much earlier than flowering plants, and they:

• Lack enclosed chambers in which seeds develop.

• Are grouped into four division : Cycadophyta, Ginkgophyta, Gnetophyta and Coniferophyta.

A. 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.

• Includes some of the tallest (redwoods and some eucalpyptus); 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.

• Thick cuticle covers the leaf.

• Stomata are in the pits, reducing water loss.

• Despite the shape, needles are megaphylls, as are leaves of all seed plants

B. The Life History of a Pine

The life cycle of a 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.

• The multicellular gametophyte generation is reduced and develops from halpoid 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 with an ovule.

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.

• Pollen cones have microsporangia; cells in these sporangia undergo meiosis producing haploid microspores, small spores that develop into pollen grains- the male gametophytes.

• Ovulate cones have megasporangia; cells in these sporangia undergo meiosis producing large megaspores that develop into the female gametophyte. Each ovule initially incudes a sporangium (nucellus) enclosed in protective integments with a single opening, the microplye.

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.

• A megaspore mother cell in the nucellus undergoes meiosis producing four halpoid megaspores, one of which will survive; it divides repeatly by mitosis producing the immature female gametophyte.

• Two or three archegoniam, each with an egg, then develop within the multicellular gametophyte.

• 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.

• 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.

• Scales of the ovulated 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 seeding.

C. 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.

• This large change marks the end of the Paleozoic era and the beginning of the Mesozoic era.

VIII The evolution of flowers and fruits contributed to the radiation of angiosperms

Flowering plants are the most widespread and diverse.

• There is only division, Anthophyta, with two classes, Monocotyledones (monocots) and Dicotyledones (dicots).

• Most use insects and animals for transferring pollen, and, therefore, are less dependent on wind and have less random pollination.

Vascular tissue became more refined during angiosperm evolution.

• Conifers have tracheids, water-conducting cells that are:

An early type of eylem cell.

Elongate, tapered cells that function in both mechanical support and water

movement up the plant.

• Most angiosperms also have vessel elements that are:

Shorter, wider cells that the more primitive tracheids.

Arranged end to end forming continuous tubes.

Compared to tracheids, vessel elements are more specialized for conducting

water, but less specialized for support.

• Angiosperm xylem is reinforced by other cell types called fibers, which are:

Specialized for support with a thick lignified wall.

Evolved in conifers. (Note that conifer xylem contains both fibers and tracheids,

but not vessel elements.)

A. The Flower

Flower = The reproductive structure of an angiosperm which is a compressed shoot with

four whorls of modified leaves.

Parts of the Flower:

Sepals- Sterile, enclose the bud.

Petals- Sterile, aid in attracting pollinators.

Stamen- Produces the pollen.

Carpel- Evolved from a seed-bearing leaf that became

rolled into a tube.

Stigma- Part of the carpel which is a sticky

structure that receives the pollen.

Ovary- Part of the carpel that protects the

ovules, which develop into seeds after fertilization.

Four Evolutionary Trends in Various Angiosperm Lineages:

1. The number of floral parts have become reduced.

2. Floral parts have become fused.

3. Symmetry has changed from radial to bilateral.

4. The ovary has dropped below the petals and sepals, where the ovules are

better protected.

B. The Fruit

Fruit= A ripened ovary that protects dormant seeds and aids in their dispersal; some

fruits (like apples) incorporate other floral parts along with the ovary.

Aggregate Fruit = Several ovaries that are part of the same flower(e.g. raspberry)

Multiple Fruit = One that develops from several separate flowers (e.g. pineapple)

Modifications of fruits that help disperse seeds include:

• Seeds are within fruits that act as kites or propellers to aid in wind dispersal.

• Fruits are modified as burrs that cling to animal fur

• Fruits are edible and seeds pass through the digestive tract of herbivores unharmed

• dispersing seeds miles away.

C. Life Cycle of an Angiosperm

Life cycles of angiosperms are herterosporous andthe two types of sporangia are found in the flower:

• microsporangia in anthers produce microspores that form male gametophytes.

• magasporiangia in ovules produce magespores that develop in female gametophytes

Immature male gametophytes:

• are pollen grains, which develop within the anthers of stamens.

• each pollen grain has two haploid nuclei that will participate in double fertilization characteristic of angiosperms.

Female gametophytes:

• do not produce an archegonium

• are located within an ovule

• consit of only a few cells- and embryo sac with eight haploid nuclei in seven cells (a large central cell has two haploid nuclei)

• one of the cells is the egg

An outline of the angiosperm life cycle follows:

• pollen from the anther lands on the sticky stigma at the carpel's tip; most flowers do not self-pollinate, but have mechanisms to ensure cross-pollination.

• the pollen grain germinates on the stigma by growing a pollen tube down the style of the carpel

• when it reaches the ovary, the pollen tube grows through its micropyle and discharges two sperm cells into the embryo sac.

• double fertilization occurs as one sperm nucleus unites with the egg to form a diploid zygote; the other sperm nucleus fuses with two nuclei in the embryo sac's central cell to form triploid (3n) endosperm.

• after double fertilization, the ovule matures into a seed.

The seed is a mature ovule consisting of:

1. Embryo. the zygote develops into an embryo with a rudimentary root and one (in monocots) or two (in dicots) cotyledons or seed leaves

2. Endosperm the triploid nucleus in the embryo sac divides repeatedly forming triploid endosperm, rich in starch and other food reserves.

3. Seed coat. this is derived from the integuments (outer layers of the ovules)

Monocots and dicots use endosperm differntly.

• monocot seeds store most food in the endosperm;

• discots generally restock most of the nutrients in the developing cotyledons

In a suitable environment the seed coat ruptures and the embryo emerges as a seedling, using the food stored in the endosperm and cotyledons.

D. The rise of angoisperms

Angiosperms showed a relatively sudden appearance in the fossil record with no clear transitional links to ancestors.

• earliest fossils are early cretaceous (approximately 120 million years ago).

• by the end of the cretaceous (65 millionyears ago), angiosperms became dominant, as they are today.

There are two theories about their sudden disappearance;

1. Artifact of an imperfect fossil record: angiosperms originated where fossilization was unlikely.

2. Punctuated equilibrium: angiosperms did evolve and radiate relatively abruptly.

Angiosperms perhaps evolve from seed ferns, an extinct group of unspecialized gymnosperms.

E. Relationships between Angiosperms and animals

terrestrial plants and animals have coevolved- a consequence of their interdependence

coevolution= reciprocal evolutionary responses among two or more interacting species; adaptive change in one species is in response to evolutionary change in the other species. for example:

• coevolution between angiosperms and their pollinators led to diversity of flowers.

- some pollinators are specific for a particular flower. The pollinator has a monopoly on a food source and gaurentees the flower's pollen will pollinate a flower of the same species.

- often, the relationship between angiosperms and their pollinators is not species specific; a pollinator may not depend exclusively on one flower species, or a flower species may not depend exclusively on one species of pollinator. However, flower color, fragrance and structure are usually adaptations for types of pollinators, such as various species of bees or hummingbirds.

• edible fruits of angiosperms have coevolved with animals that can disperse seeds. animals become attracted to ripening fruits as they:

- become softer, more fragnent and higher in sugar

-change to a color that attracts birds and mammals, animals arge enough to disperse the seeds.

F. Angiosperms and agriculture

Angiosperms provide nearly all our food:

• fruit

• vegetable crops

• grains, such as corn, rice,wheat

flowering plants are also used for other purpose, such as:

• fiber

• medication

• perfume

• decoration

Through agriculture, humans have influenced plant evolution by artificially selecting for plants that improved the wuantity and quality of foods and other crops.

• many of our agriculture plants are so genetically removed fromt heir origins that they probably could not survive in the wild

• as a consequence, cultivated crops that require human intervention to wate, fertilize, provide protection from insects and disease, and even to plant their seeds, are vulnerable to natural and human caused disasters

•

IX. Plant diversity is a nonrenewable resource

plant diversity is a nonrenewable resource, and the irrevocable extinction of plant species is occurring at an unprecedented rate.

• the exploding human population demands space and antural resources.

• the toll of habitat destruction is greatest in the tropics becasue this is where:

-most species live

-more than half the human population lives and human population growth is fastest.

-most deforestation is caused by slash-and-burn clearing for agriculture

As the forest diassapears, so do thousands of plant and animal species.

• habitat destruction also endangers animal species that depend on plants in the tropical rainforest.

• habitat desruction by humans has not been limited to the tropics. Europeans eliminated most of their forests centuries ago, and in North america, destruction of habitat is endangering many species.

There are many reasons to value plant diversity and to find ways to protect it.

• ecosystems are living treasures that can regenerate only slowly.

• Humans depend on plants for products such as medications, food and building materials.

• We still know so little of the 250,000 known plant species. (food agriculture is based on only about two dozen species).