chapter 26 the colonization of land by plants and fugi

Chapter 26

• The Colonization of Land by Plants and Fugi

Overview: The Greening of Earth

• For more than the first 3 billion years of Earth’s history, the terrestrial surface was lifeless

• Cyanobacteria likely existed on land 1.2 billion years ago

• Around 500 million years ago, small plants, fungi, and animals emerged on land

• Since colonizing land, plants have diversified into roughly 290,000 living species

• Land plants are defined as having terrestrial ancestors, even though some are now aquatic

• Land plants do not include photosynthetic protists (algae)

• Plants supply oxygen and are the ultimate source of most food eaten by land animals

Figure 29.1

Land plants evolved from green algae

• Green algae called charophytes are the closest relatives of land plants

Morphological and Molecular Evidence

• Many characteristics of land plants also appear in a variety of algal clades, mainly algae

• However, land plants share four key traits with only charophytes

– Rings of cellulose-synthesizing complexes– Peroxisome enzymes– Structure of flagellated sperm– Formation of a phragmoplast

Figure 29.2

• Comparisons of both nuclear and chloroplast genes point to charophytes as the closest living relatives of land plants

• Note that land plants are not descended from modern charophytes, but share a common ancestor with modern charophytes

Figure 29.3

Chara species, a pond organism

Coleochaete orbicularis, adisk-shaped charophytethat also lives in ponds (LM)

Adaptations Enabling the Move to Land

• In charophytes a layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out

• Sporopollenin is also found in plant spore walls• The movement onto land by charophyte ancestors

provided unfiltered sun, more plentiful CO2, nutrient-rich soil, and few herbivores or pathogens

• Land presented challenges: a scarcity of water and lack of structural support

• The accumulation of traits that facilitated survival on land may have opened the way to its colonization by plants

• Systematists are currently debating the boundaries of the plant kingdom

• Some biologists think the plant kingdom should be expanded to include some or all green algae

• Until this debate is resolved, we define plants as embryophytes, plants with embryos

Figure 29.4

Red algae

Chlorophytes

Charophytes

Embryophytes

ANCESTRALALGA

taeStrep

Derived Traits of Plants

• Four key traits appear in nearly all land plants but are absent in the charophytes

– Alternation of generations and multicellular, dependent embryos

– Walled spores produced in sporangia– Multicellular gametangia– Apical meristems

Alternation of Generations and Multicellular, Dependent Embryos

• Plants alternate between two multicellular stages, a reproductive cycle called alternation of generations

• The gametophyte is haploid and produces haploid gametes by mitosis

• Fusion of the gametes gives rise to the diploid sporophyte, which produces haploid spores by meiosis

• The diploid embryo is retained within the tissue of the female gametophyte

• Nutrients are transferred from parent to embryo through placental transfer cells

• Land plants are called embryophytes because of the dependency of the embryo on the parent

Figure 29.5a

Gamete fromanother plant

Haploid (n)

Diploid (2n)Gametophyte(n)

Mitosis Mitosis

Spore Gamete

MEIOSIS FERTILIZATION

Zygote

MitosisSporophyte(2n)

Alternation of generations

Figure 29.5b

Embryo

Maternal tissue

Embryo (LM) andplacental transfer cell (TEM)of Marchantia (a liverwort)

Wall ingrowths

Placental transfercell (outlined inblue)10 m

Walled Spores Produced in Sporangia• The sporophyte produces spores in organs called

sporangia• Diploid cells called sporocytes undergo meiosis

to generate haploid spores• Spore walls contain sporopollenin, which makes

them resistant to harsh environments

Figure 29.5c

SporesSporangium

Longitudinal section ofSphagnum sporangium (LM)

Sporophyte

Gametophyte

Sporophytes and sporangia of Sphagnum (a moss)

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

Figure 29.5d

Femalegametophyte

Malegametophyte

Archegonia,each with anegg (yellow)

Antheridia(brown),containing sperm

Archegonia and antheridia of Marchantia (a liverwort)

Figure 29.5e

Apical meristemof shoot

Developingleaves

Shoot100 m100 mRoot

Apicalmeristemof root

Apical meristems of plantroots and shoots

• Additional derived traits include Cuticle, a waxy covering of the epidermis Mycorrhizae, symbiotic associations between

fungi and land plants that may have helped plants without true roots to obtain nutrients

Secondary compounds that deter herbivores and parasites

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

Figure 29.6

(a) Fossilizedspores

Fossilizedsporophytetissue

• Those ancestral species gave rise to a vast diversity of modern plants

Figure 29.7

Origin of land plants (about 475 mya)

Origin of vascular plants (about 425 mya)

Origin of extant seed plants (about 305 mya)

1ANCESTRALGREENALGA

500 450 400 350 300 50 0

Millions of years ago (mya)

Liverworts

Mosses

Hornworts

Lycophytes (clubmosses, spikemosses, quillworts)

Pterophytes (ferns,horsetails, whisk ferns)

Gymnosperms

Angiosperms

Figure 29.7b

Liverworts

Mosses

Hornworts

Lycophytes (clubmosses, spikemosses, quillworts)

Pterophytes (ferns,horsetails, whisk ferns)

Gymnosperms

Angiosperms

lar plan

nvascu

lessvascu

• Land plants can be informally grouped based on the presence or absence of vascular tissue

• Most plants have vascular tissue; these constitute the vascular plants

• Nonvascular plants are commonly called bryophytes

• Bryophytes are not a monophyletic group; their relationships to each other and to vascular plants is unresolved

• Seedless vascular plants can be divided into clades

– Lycophytes (club mosses and their relatives)– Pterophytes (ferns and their relatives)

• Seedless vascular plants are paraphyletic, and are of the same level of biological organization, or grade

• A seed is an embryo and nutrients surrounded by a protective coat

• Seed plants form a clade and can be divided into further clades

– Gymnosperms, the “naked seed” plants, including the conifers

– Angiosperms, the flowering plants

Table 29. 1

Mosses and other nonvascular plants have life cycles dominated by gametophytes

• Bryophytes are represented today by three phyla of small herbaceous (nonwoody) plants

– Liverworts, phylum Hepatophyta– Hornworts, phylum Anthocerophyta– Mosses, phylum Bryophyta

• Bryophyte refers to all nonvascular plants, whereas Bryophyta refers only to the phylum of mosses

Figure 29.UN01

Nonvascular plants (bryophytes)

Seedless vascular plantsGymnosperms

Angiosperms

Bryophyte Gametophytes

• In all three bryophyte phyla, gametophytes are larger and longer-living than sporophytes

• Sporophytes are typically present only part of the time

Protonemata(n)

Haploid (n)Diploid (2n)

“Bud”

Malegametophyte(n)

AntheridiaSperm

ArchegoniaGametophoreSpores

Sporedispersal

Peristome

Sporangium

Femalegametophyte(n) Rhizoid

FERTILIZATION(within archegonium)Zygote

Archegonium

Embryo

Capsule(sporangium)

Youngsporophyte(2n)

MEIOSIS

Mature sporophytes

Capsule withperistome (LM) Female

gametophytes

Figure 29.8-3

• 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

Bryophyte Sporophytes

• Bryophyte sporophytes grow out of archegonia, and are the smallest and simplest sporophytes of all extant plant groups

• A sporophyte consists of a foot, a seta (stalk), and a sporangium, also called a capsule, which discharges spores through a peristome

• Hornwort and moss sporophytes have stomata for gas exchange; liverworts do not

Figure 29.9a

Sporophyte

ThallusGametophore offemale gametophyte

Marchantia polymorpha,a “thalloid” liverwort

Marchantia sporophyte (LM)

FootSeta

Capsule(sporangium)

mPlagiochila deltoidea, a “leafy” liverwort

Figure 29.9c

Polytrichum commune,hairy-cap moss

Capsule

Sporophyte(a sturdyplant thattakes monthsto grow)

Gametophyte

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

• Sphagnum, or “peat moss,” forms extensive deposits of partially decayed organic material known as peat

• Peat can be used as a source of fuel• Sphagnum is an important global reservoir of

organic carbon• Overharvesting of Sphagnum and/or a drop in

water level in peatlands could release stored CO2 to the atmosphere

Figure 29.11

Peat being harvested from apeatland

(a) “Tollund Man,” a bog mummydating from 405–100 B.C.E.

Ferns and other seedless vascular plants were the first plants to grow tall

• Bryophytes and bryophyte-like plants were the prevalent vegetation during the first 100 million years of plant evolution

• Vascular plants began to diversify during the Devonian and Carboniferous periods

• Vascular tissue allowed these plants to grow tall• Seedless vascular plants have flagellated sperm

and are usually restricted to moist environments

Figure 29.UN03

Seedless vascular plants

Gymnosperms

Angiosperms

Origins and Traits of Vascular Plants

• Fossils of the forerunners of vascular plants date back about 425 million years

• These early tiny plants had independent, branching sporophytes

• Living vascular plants are characterized by Life cycles with dominant sporophytes Vascular tissues called xylem and phloem Well-developed roots and leaves

Figure 29.12

Sporangia

Life Cycles with Dominant Sporophytes

• In contrast with bryophytes, sporophytes of seedless vascular plants are the larger generation, as in familiar ferns

• The gametophytes are tiny plants that grow on or below the soil surface

Figure 29.13-3

MEIOSISSporedispersal

Spore(n)

Younggametophyte

Rhizoid

Undersideof maturegametophyte(n)

Antheridium

Archegonium

FERTILIZATIONZygote(2n)

Gametophyte

Newsporophyte

Maturesporophyte(2n)

Fiddlehead (young leaf)

Sporangium

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

Evolution of Roots

• Roots are organs that anchor vascular plants• They enable vascular plants to absorb water and

nutrients from the soil• Roots 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 that is used 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 as outgrowths of stems

• Megaphylls may have evolved as webbing between flattened branches

Figure 29.14

Vascular tissue Sporangia Microphyll

(a) Microphylls (b) Megaphylls

Overtoppinggrowth

Megaphyll

Otherstemsbecomereducedandflattened.

Webbingdevelops.

Sporophylls and Spore Variations

• Sporophylls are modified leaves with sporangia• Sori are clusters of sporangia on the undersides

of sporophylls• Strobili are cone-like structures formed from

groups of sporophylls

• 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

• Heterosporous species produce megaspores, which give rise to female gametophytes, and microspores, which give rise to male gametophytes

Classification of Seedless Vascular Plants

• There are two phyla of seedless vascular plants– Phylum Lycophyta includes club mosses, spike

mosses, and quillworts– Phylum Pterophyta includes ferns, horsetails, and

whisk ferns and their relatives

Figure 29.15a

Selaginellamoellendorffii,a spike moss

Isoetesgunnii,a quillwort

Diphasiastrum tristachyum,a club moss

Strobili(clusters ofsporophylls)

2.5 cm

Figure 29.15b

Athyriumfilix-femina,lady fern

Equisetum arvense,field horsetail

Vegetative stem

Strobilus onfertile stem

Psilotumnudum,a whiskfern

Phylum Lycophyta: Club Mosses, Spike Mosses, and Quillworts

• Giant lycophytes trees thrived for millions of years in moist swamps

• Surviving species are small herbaceous plants• Club mosses and spike mosses have vascular

tissues and are not true mosses

Phylum Pterophyta: Ferns, Horsetails, and Whisk Ferns and Relatives

• Ferns are the most diverse seedless vascular plants, with more than 12,000 species

• They are most diverse in the tropics but also thrive in temperate forests

• Horsetails were diverse during the Carboniferous period, but are now restricted to the genus Equisetum

• Whisk ferns resemble ancestral vascular plants but are closely related to modern ferns

The Significance of Seedless Vascular Plants

• The ancestors of modern lycophytes, horsetails, and ferns grew to great heights during the Devonian and Carboniferous, forming the first forests

• Increased growth and photosynthesis removed CO2 from the atmosphere and may have contributed to global cooling at the end of the Carboniferous period

• The decaying plants of these Carboniferous forests eventually became coal

Fern Lycophyte trees Horsetail

Tree trunkcovered withsmall leaves

Lycophyte treereproductivestructures

Figure 29.16

Figure 29.UN02

Figure 29.UN04

Homosporous spore production

Heterosporous spore production

Sporangiumon sporophyll

Singletype of spore

Typically abisexualgametophyte

Megasporangiumon megasporophyll

Megaspore Femalegametophyte

Malegametophyte

SpermMicrosporeMicrosporangiumon microsporophyll

Gametophyte

Mitosis Mitosis

Spore Gamete

MEIOSIS FERTILIZATION

Zygote

Mitosis

Sporophyte

Haploid

Diploid

Alternation of generations

Apical meristemof shoot

Developingleaves

Apical meristems

Archegoniumwith egg

Antheridiumwith sperm

Sporangium Spores

Multicellular gametangia Walled spores in sporangia

Figure 29.UN05

Figure 29.UN06

Overview: Transforming the World

• Seeds changed the course of plant evolution, enabling their bearers to become the dominant producers in most terrestrial ecosystems

• Seed plants originated about 360 million years ago

• A seed consists of an embryo and nutrients surrounded by a protective coat

• Domestication of seed plants had begun by 8,000 years ago and allowed for permanent settlements

Figure 30.1

Concept 30.1: Seeds and pollen grains are key adaptations for life on land

• 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 that are retained within tissues of the parent sporophyte

Figure 30.2

PLANT GROUP

Mosses and othernonvascular plants

Gametophyte

Sporophyte

Sporophyte(2n)

Gametophyte(n)

Dominant

Reduced, dependent ongametophyte for nutrition

Dominant

Reduced, independent(photosynthetic andfree-living)

Ferns and other seedlessvascular plants

Dominant

Reduced (usually microscopic), dependent on surroundingsporophyte tissue for nutrition

Seed plants (gymnosperms and angiosperms)

AngiospermGymnosperm

Microscopicfemalegametophytes(n) insidethese partsof flowers

Microscopic femalegametophytes (n) insideovulate cone

Microscopicmalegametophytes(n) insidethese partsof flowers

Microscopic malegametophytes (n)inside pollencone

Sporophyte (2n)Sporophyte (2n)

Sporophyte(2n)

Gametophyte(n)

Example

Heterospory: The Rule Among Seed Plants

• The ancestors of seed plants were likely homosporous, while seed plants are heterosporous

• Megasporangia produce megaspores that give rise to female gametophytes

• Microsporangia produce microspores that give rise to male gametophytes

Ovules and Production of Eggs

• An ovule consists of a megasporangium, megaspore, and one or more protective integuments

• Gymnosperm megaspores have one integument• Angiosperm megaspores usually have two

integuments

Figure 30.3-1

Immatureovulate cone

Integument (2n)

Spore wall

Megaspore (n)

(a) Unfertilized ovule

Megasporangium(2n)

Pollen grain (n)Micropyle

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

• 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 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

Figure 30.3-3

Immatureovulate cone

Integument (2n)

Spore wall

Megaspore (n)

Femalegametophyte (n)

Egg nucleus(n)

Dischargedsperm nucleus(n)

Pollen tubeMale gametophyte (n)

(a) Unfertilized ovule

Megasporangium(2n)

Pollen grain (n)Micropyle

(b) Fertilized ovule (c) Gymnosperm seed

Seedcoat

Sporewall

Foodsupply (n)

Embryo (2n)

• Seeds provide some evolutionary advantages over spores

– 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

Gymnosperms bear “naked” seeds, typically on cones

• Gymnosperms means “naked seeds”• The seeds are exposed on sporophylls that form

cones• Angiosperm seeds are found in fruits, which are

mature ovaries

Figure 30.UN01

Angiosperms

Gymnosperm Evolution

• Fossil evidence reveals that by the late Devonian period some plants, called progymnosperms, had begun to acquire some adaptations that characterize seed plants

Figure 30.4

• Living seed plants can be divided into two clades: gymnosperms and angiosperms

• Gymnosperms appear early in the fossil record about 305 million years ago and dominated Mesozoic (251–65 million years ago) terrestrial ecosystems

• Gymnosperms were better suited than nonvascular plants to drier conditions

• Angiosperms began to replace gymnosperms near the end of the Mesozoic

• Angiosperms now dominate more terrestrial ecosystems

• Today, cone-bearing gymnosperms called conifers dominate in the northern latitudes

• The gymnosperm consist of four phyla– Cycadophyta (cycads)

– Gingkophyta (one living species: Ginkgo biloba)

– Gnetophyta (three genera: Gnetum, Ephedra, Welwitschia)

– Coniferophyta (conifers, such as pine, fir, and redwood)

Phylum Cycadophyta• Individuals have large cones and palmlike leaves• These thrived during the Mesozoic, but relatively

few species exist today

Figure 30.5a

Cycas revoluta

Phylum Ginkgophyta• This phylum consists of a single living species,

Ginkgo biloba• It has a high tolerance to air pollution and is a

popular ornamental tree

Ginkgo bilobaleaves andfleshy seeds

Ginkgo biloba pollen-producing tree

Figure 30.5b

Phylum Gnetophyta• This phylum comprises three genera• Species vary in appearance, and some are

tropical whereas others live in deserts

Figure 30.5d

Ovulate conesGnetum

Ephedra

Welwitschia

Phylum Coniferophyta• This phylum is by far the largest of the

gymnosperm phyla• Most conifers are evergreens and can carry out

photosynthesis year round

Figure 30.5e

Douglas fir

Common juniper

European larch

Sequoia

Wollemi pine Bristlecone pine

The Life Cycle of a Pine: A Closer Look

• Three key features of the gymnosperm life cycle are

– Dominance of the sporophyte generation

– Development of seeds from fertilized ovules

– The transfer of sperm to ovules by pollen

• The life cycle of a pine provides an example

• The pine tree is the sporophyte and produces sporangia in male and female cones

• Small cones produce microspores called pollen grains, each of which contains a male gametophyte

• The familiar larger cones contain ovules, which produce megaspores that develop into female gametophytes

• It takes nearly three years from cone production to mature seed

Pollencone

Microsporocytes(2n)

Microsporangia

Microsporangium (2n)

Pollengrains (n)

MEIOSIS

Figure 30.6-1

Ovulatecone

Pollencone

Microsporocytes(2n)

Microsporangia

Microsporangium (2n)Survivingmegaspore (n)

MEIOSIS

Megasporangium (2n)Pollengrain

Pollengrains (n)

MEIOSIS

Megasporocyte (2n)

Integument

Figure 30.6-2

Ovulatecone

Pollencone

Microsporocytes(2n)

Microsporangia

Archegonium

Survivingmegaspore (n)

MEIOSIS

Pollengrains (n)

MEIOSIS

Femalegametophyte

Megasporocyte (2n)

Integument

Spermnucleus (n) Egg nucleus (n)

Pollentube

FERTILIZATION

Figure 30.6-3

Ovulatecone

Pollencone

Microsporocytes(2n)

Microsporangia

Seedling

Archegonium

Survivingmegaspore (n)

MEIOSIS

Pollengrains (n)

MEIOSIS

Femalegametophyte

Megasporocyte (2n)

Integument

Spermnucleus (n) Egg nucleus (n)

Pollentube

Seed coat (2n)

FERTILIZATION

Foodreserves (n)

Embryo(new sporophyte)(2n)

Figure 30.6-4

The reproductive adaptations of angiosperms include flowers and fruits

• Angiosperms are seed plants with reproductive structures called flowers and fruits

• They are the most widespread and diverse of all plants

Figure 30.UN02

Angiosperms

Characteristics of Angiosperms

• All angiosperms are classified in a single phylum, Anthophyta, from the Greek anthos for flower

• Angiosperms have two key adaptations– Flowers

– Fruits

Flowers

• The flower is an angiosperm structure specialized for sexual reproduction

• Many species are pollinated by insects or animals, while some species are wind-pollinated

• A flower is a specialized shoot with up to four types of modified leaves

– 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

Stamen Anther

Filament

StigmaCarpel

Figure 30.7

Fruits

• A fruit typically consists of a mature ovary but can also include other flower parts

• Fruits protect seeds and aid in their dispersal• Mature fruits can be either fleshy or dry

Animation: Fruit Development

Angiosperm Diversity

• Angiosperms comprise more than 250,000 living species

• Previously, angiosperms were divided into two main groups

– Monocots (one cotyledon)– Dicots (two dicots)

• DNA studies suggest that monocots form a clade, but dicots are polyphyletic

• The clade eudicot (“true” dicots) includes most dicots

• The rest of the former dicots form several small lineages

• Basal angiosperms are less derived and include the flowering plants belonging to the oldest lineages

• Magnoliids share some traits with basal angiosperms but evolved later

Basal Angiosperms• Three small lineages constitute the basal

angiosperms• These include Amborella trichopoda, water lilies,

and star anise

Water lily

Star anise

Amborella trichopoda

Basal AngiospermsFigure 30.13a

Monocots• More than one-quarter of angiosperm species

are monocots

Orchid

Monocots

Pygmy date palm

Anther

Filament

Stigma

Barley, a grass

Figure 30.13c

Eudicots• More than two-thirds of angiosperm species are

eudicots

California poppy Dog rose

Pyrenean oak

Snow pea Zucchini

EudicotsFigure 30.13d

Figure 30.13eMonocot

CharacteristicsEudicot

Characteristics

Embryos

One cotyledon Two cotyledons

Leafvenation

Veins usuallyparallel

Veins usuallynetlike

Vascular tissuescattered

Vascular tissueusually arranged

in ring

Root systemusually fibrous(no main root)

Taproot (main root)usually present

Pollen

Pollen grain withone opening

Pollen grain withthree openings

Flowers

Floral organsusually in

multiples of three

Floral organsusually in multiples

of four or five

Evolutionary Links Between Angiosperms and Animals

• Animals influence the evolution of plants and vice versa

– For example, animal herbivory selects for plant defenses

– For example, interactions between pollinators and flowering plants select for mutually beneficial adaptations

Figure 30.14

• Clades with bilaterally symmetrical flowers have more species than those with radially symmetrical flowers

• This is likely because bilateral symmetry affects the movement of pollinators and reduces gene flow in diverging populations

Figure 30.15

Human welfare depends greatly on seed plants

• No group of plants is more important to human survival than seed plants

• Plants are key sources of food, fuel, wood products, and medicine

• Our reliance on seed plants makes preservation of plant diversity critical

Products from Seed Plants

• Most of our food comes from angiosperms• Six crops (wheat, rice, maize, potatoes, cassava,

and sweet potatoes) yield 80% of the calories consumed by humans

• Modern crops are products of relatively recent genetic change resulting from artificial selection

• Many seed plants provide wood• Secondary compounds of seed plants are used

in medicines

Table 30.1

Threats to Plant Diversity

• Destruction of habitat is causing extinction of many plant species

• In the tropics 55,000 km2 are cleared each year• At this rate, the remaining tropical forests will be

eliminated in 200 years • Loss of plant habitat is often accompanied by

loss of the animal species that plants support

• At the current rate of habitat loss, 50% of Earth’s species will become extinct within the next 100–200 years

• The tropical rain forests may contain undiscovered medicinal compounds

Figure 30.16

A satellite imagefrom 2000 showsclear-cut areas inBrazil surroundedby dense tropicalforest.

By 2009, muchmore of this sametropical forest hadbeen cut down.

Figure 30.16a

A satellite imagefrom 2000 showsclear-cut areas inBrazil surrounded bydense tropical forest.

chapter 26 the colonization of land by plants and fugi

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aquatic land plants

living species land

land animals

colonization of land

plants systematists

small plants

pathogens land

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