Plants

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

Chapter 30 Green Plants

Lectures by Cheryl Ingram-Smith

Biological Science, Third Edition– Scott Freeman


Key Concepts

The green plants include both the green algae and the land plants. Green algae are an important source of oxygen and provide food for aquatic organisms; land plants hold soil and water in place, build soil, moderate extreme temperatures and winds, and provide food for other organisms.

Land plants were the first multicellular organisms that could live with most of their tissue exposed to the air. A series of key adaptations allowed them to survive on land. In terms of total mass, plants dominate today's terrestrial environments.


Key Concepts

Once plants were able to grow on land, a sequence of important evolutionary changes made it possible for them to reproduce efficiently—even in extremely dry environments.


Green Plants

• The green plants consist of the green algae and land plants.

• Green algae are important photosynthetic organisms in freshwater habitats, while land plants are the key photosynthesizers in terrestrial environments.

• Green algae have traditionally been considered protists, but we study them along with land plants for two reasons: (1) they are the closest living relatives to land plants, and (2) the transition from aquatic to terrestrial life occurred when land plants evolved from green algae.


Why Do Biologists Study the Green Plants?

• Biologists study plants not only because they are fascinating organisms but also because we could not live without them.

• Agriculture, forestry, and horticulture are among the most important endeavors supported by biological science.


Plants Provide Ecosystem Services

• An ecosystem consists of all the organisms in a particular area, along with physical components of the environment such as the atmosphere, precipitation, surface water, sunlight, soil, and nutrients.

• Plants provide ecosystem services because they add to the quality of the atmosphere, surface water, soil, and other physical components of an ecosystem.



• Plants alter the landscape in ways that benefit other organisms:

– They produce oxygen via oxygenic photosynthesis,

– They build soil by providing food for decomposers,

– They hold soil and prevent nutrients from being lost to erosion by wind and water,

– They hold water in the soil, and

– They moderate the local climate by providing shade and reducing the impact of wind on landscapes.



• Perhaps the most important ecosystem service provided by plants involves food.

• They are the dominant primary producers in terrestrial ecosystems and provide the base of the food chain in the vast majority of terrestrial habitats.

• Plants are eaten by herbivores, which are eaten by carnivores, or meat eaters. Some organisms are omnivores—those that eat both plants and animals.

• Finally, green plants are the key to the carbon cycle on the land.


The Basis of Food Chains in Terrestrial Environments

Tertiary Consumers:

Secondary Consumers:

Herbivores eat plants.

Plants form the base of the terrestrial food chain.

Secondary carnivores eat carnivores.

Carnivores eat animals.

Primary Consumers:

Producers:


Providing Food, Fuel, Building Materials, and Medicines

• Plants provide most of our food supply as well as a significant percentage of the fuel, fibers, building materials, and medicines that we use.

• Agricultural research began with the initial domestication of crop plants.

• Artificial selection for plants with certain properties has lead to dramatic changes in plant characteristics.


Crop Plants: Derived from Wild Species via Artificial Selection

ARTIFICIAL SELECTION CHANGES THE TRAITS OF DOMESTICATED SPECIES.

Oil richLess oil rich

1. Observe variation in kernel oil content.

2. Plant oil-rich seeds and grow to maturity.

3. Harvest kernels from mature plants. Repeat steps 1–3.

4. After many generations, kernel oil content increases.


Providing Food, Fuel, Building Materials, and Medicines

• Humans have historically relied on plant-based fuels such as wood and coal.

• Plants provide us with important sources of raw material for clothing and household articles.

• Woody plants provide lumber to build houses and furniture, and to make paper.

• Plants are a key source of medicines.


Humans Have Relied on Plant-Based Fuels

COAL FORMATION

Peat

1. Dead plant material accumulates in marshy or boggy habitats.

Plant-based fuels

Pressure Pressure

Sediments

Coal

2. If oxygen in water is scarce, the organic matter decays only partially, forming peat.

3. If the peat deposits are later covered by sediments and compressed, the resulting pressure and heat change them into coal.

What energy sources do you think will be important in the future?

Wood

CoalPetroleum and natural gas



Plant-based fuels

What energy sources do you think will be important in the future?

Wood

CoalPetroleum and natural gas



COAL FORMATION

Peat

1. Dead plant material accumulates in marshy or boggy habitats.

Pressure Pressure

Sediments

Coal

2. If oxygen in water is scarce, the organic matter decays only partially, forming peat.

3. If the peat deposits are later covered by sediments and compressed, the resulting pressure and heat change them into coal.


How Do Biologists Study Green Plants?

• To understand how green plants originated and diversified, biologists use three tools:

(1) They compare the fundamental morphological features of various green algae and green plants,

(2) They analyze the fossil record of the lineage, and

(3) They assess similarities and differences in DNA sequences from homologous genes to estimate phylogenetic trees.

• Let’s consider each of these complementary strategies.


Analyzing Morphological Traits

• Biologists have long hypothesized that green algae are closely related to plants on the basis of several key morphological traits, including their chloroplast and cell wall structures.

• The green algae include species that are unicellular, colonial, or multicellular and that live in marine or freshwater habitats.

• Based on morphology, the major phyla of plants are grouped into three categories: nonvascular plants, seedless vascular plants, and seed plants.



• Nonvascular plants lack vascular tissue—specialized groups of cells that conduct water or dissolved nutrients from one part of the plant body to another.

• Seedless vascular plants have well-developed vascular tissue but do not make seeds.


Morphological Diversity: Nonvascular and Seedless Vascular Plants

Nonvascular plants do not have vascular tissue to conduct water and provide support.

Seedless vascular plants have vascular tissue but do not make seeds.

Hepaticophyta (liverworts) Anthocerophyta (hornworts) Bryophyta (mosses)

Pteridophyta (ferns)Sphenophyta (horsetails)Psilotophyta (whisk ferns)

Lycophyta (lycophytes or club mosses)



Nonvascular plants do not have vascular tissue to conduct water and provide support.

Hepaticophyta (liverworts) Anthocerophyta (hornworts) Bryophyta (mosses)



Seedless vascular plants have vascular tissue but do not make seeds.

Pteridophyta (ferns)Sphenophyta (horsetails)Psilotophyta (whisk ferns)

Lycophyta (lycophytes or club mosses)



• A seed consists of an embryo and a store of nutritive tissue, surrounded by a tough protective layer.

• Seed plants have vascular tissue and make seeds.


Morphological Diversity: Seed Plants

Seed plants have vascular tissue and make seeds.

Cycadophyta (cycads) Ginkgophyta (ginkgo) Other conifers (redwoods, junipers, yews)

Gnetophyta (gnetophytes) Pinophyta (pines, spruces, firs) Anthophyta (angiosperms or flowering plants)



• Within the seed plants, gymnosperms produce seeds that do not develop in an enclosed structure.

• In the flowering plants, or angiosperms, seeds develop inside a protective structure called a carpel.


Using the Fossil Record

• The fossil record for green algae begins 700–725 million years ago (mya). The fossil record for land plants begins about 475 mya.

• The fossil record for plants is massive and is broken up into five intervals, each of which encompasses a major event in the diversification of land plants.

• According to the fossil record, the green algae appear first, followed by the nonvascular plants, seedless vascular plants, and seed plants. Angiosperms appear in the fossil record about 150 mya.


The Fossil Record of Land Plants

Cooksonia pertoni Seed fern leavesCones from Araucaria mirabilis, an early gymnosperm Archaefructus, an early angiosperm

Angiosperms abundant

Present

Diversification of flowering plantsBoth wet and dry environments blanketed with green plants for the first time

Gymnosperms abundant

Extensive coal-forming swamps

Carboniferous: Lycophytes and

horsetails abundant

Most major morphological innovations: stomata, vascular tissue, roots, leaves

Silurian-Devonian explosion

Origin of land plants

First evidence of land plants: cuticle, spores, sporangia

475 mya 444 359 299 145


Evaluating Molecular Phylogenies

• The phylogenetic tree of green plants shown in Figure 30.9 shows at least eight important points:

(1) Land plants probably evolved from green algae.

(2) The green algal group called Charaphyceae is the sister group to land plants—meaning that Charaphyceae are

their closest living relative.

(3) The green plants are monophyletic, meaning that a single common ancestor gave rise to all of the green algae and

land plants.


The Phylogeny of Green Plants

Green plants

Land plants

Vascular plants

Seed plants

Gymnosperms Angiosperms

Flowers

Seeds

Vascular tissue

Ability to live on land

Chloroplasts containing chlorophyll a + b and -carotene

Green algae

Nonvascular plants

Seedless vascular plants

EukaryaArc

haea

Bacte

ria

(red a

lgae

)

(ulv

ophytes

)

(cole

ochae

tes)

(sto

neworts

)

Rhodophyta

Ulvophyc

eae

Coleoch

aeto

phycea

e

Charap

hycea

e

Hepat

icophyt

a

Anthoce

rophyt

a

Bryophyt

a

(live

rworts

)

(horn

worts)

(moss

es)

(lyco

phytes

)

(whis

k fe

rns)

(hors

etai

ls)

(fern

s)

Lycophyt

a

Psilo

tophyt

a

Sphenophyt

a

Pterid

ophyta

Cycad

ophyta

Ginkg

ophyta

Other

conife

rs

Gnetophyt

a

Pinophyt

a

Anthophyt

a

(cyc

ads)

(gin

kgo)

(redwoods

et a

l.)

(gnet

ophytes

)

(pin

es e

t al.)

(angio

sper

ms)


Evaluating Molecular Phylogenies

(4) The green algae group is paraphyletic; the land plants are monophyletic.

(5) The nonvascular plants are the earliest-branching, or most basal, groups among land plants.

(6) The tree supports the hypothesis that water-conducting cells and tissues evolved gradually, with simpler structures preceding more complex ones.

(6) The seed plants are a monophyletic group, as are the gymnosperms.

(8) Seeds and flowers evolved only once.


Themes in the Diversification of Green Plants

• The story of land plants is the story of adaptations that allowed photosynthetic organisms to move from aquatic to terrestrial environments.


How Did Plants Adapt to Dry Conditions?

• Plants had to adapt to conditions in which only a portion, if any, of their tissues are wet. Tissues that are exposed to air tend to dry out and die.

• The adaptations that solved the water problem arose in two steps: (1) prevention of water loss from cells, and

(2) transportation of water from tissues with access to water to tissues without access.


Preventing Water Loss: Cuticle and Stomata

• A critical adaptation was cuticle. Cuticle is a waxy, watertight sealant that covers the aboveground parts of the plant and gives them the ability to survive in dry environments.

• However, the cuticle also keeps necessary CO2 out of the plant. Another critical adaptation was the stoma (plural stomata). Gas exchange is accomplished through a stoma, which consists of an opening called a pore surrounded by specialized guard cells. The pore opens and closes as the guard cells change shape.


The Most Fundamental Plant Adaptations to Life on Land

Leaf cross section

Cuticle is a waxy layer that prevents water loss from stems and leaves.

Stomata have pores that allow gas exchange in photosynthetic tissues.

Stoma

Guard cells

Pore

Moist photosynthetic cells

Cuticle


Transporting Water: Vascular Tissue and Upright Growth

• The first land plants probably lacked rigidity and grew low to the ground.

• The evolution of vascular tissue allowed early plants to both support erect stems and transport water from roots to aboveground tissues.

• Vascular tissue most likely evolved in a series of gradual steps that provided an increasing level of structural support, allowing plants to grow more upright.


Evolutionary Sequence Observed in Water-Conducting Cells

Simple water-conducting cells

First vascular tissue Tracheids Vessel elements

Ends have gaps in secondary cell wall (inside) Ends have gaps

through primary and secondary cell walls

Primary wall (with cellulose)

Secondary wall (with lignin)





Lignin

Little structural support. Found in fossils and present-day mosses

Some structural support. Found in fossils

Increased structural support. Found in all vascular plants

Found in gnetophytes and angiosperms



Simple water-conducting cells


Little structural support. Found in fossils and present-day mosses



First vascular tissue


Lignin

Some structural support. Found in fossils



Tracheids

Ends have gaps in secondary cell wall (inside)



Increased structural support. Found in all vascular plants



Vessel elements

Ends have gaps through primary and secondary cell walls



Found in gnetophytes and angiosperms


Transporting Water: Vascular Tissue and Upright Growth

• A primary adaptation for upright growth was found in the cell walls of early water-conducting cells. They were strengthened with a molecule called lignin, a structural polymer built from six-carbon rings. Today, the presence of lignin in the cell walls of water-conducting cells is the defining feature of vascular tissues.

• Later the advanced water-conducting cells called tracheids evolved.

• Finally the most specialized type of water-conducting cell appeared, which are vessel elements.


Mapping Evolutionary Changes on the Phylogenetic Tree

• The major innovations that allowed plants to adapt to life on land are shown in Figure 30.12.

Animation: Plant Evolution and the Phylogenetic Tree


Innovations That Allow Plants to Adapt to Life on Land

Land plants

Vessel elements Vessel

elements

WoodTrue leaves

TracheidsRoots

Vascular tissue

Stomata

PoresCuticle

Sporopollenin-encased spores or zygotes

Vessel elements evolved more than once

Most key innovations for living on land evolved only once

Red a

lgae

Ulvophyt

esCole

ochae

tes

Stonew

orts

Liver

worts

Hornw

ortsM

osses

Early

vas

cula

r

plants

(foss

ils

only)

Lycophyt

esW

hisk

fern

sHors

etai

lsFer

nsCyc

ads

Gin

kgo

Redw

oods et

al.

Gnet

ophytes

Pines

et a

l.Angio

sper

ms


How Do Plants Reproduce in Dry Conditions?

• Innovations that were instrumental for efficient plant reproduction in a dry environment include the development of:

– spores that resist drying,

– gametes that were produced in complex, multicellular structures, and

– embryos that were retained on the parent plant and were nourished by it.


Land Plants as Embryophytes

• Early land plants contained specialized reproductive organs called gametangia that protected gametes from drying and damage. All living land plant groups but angiosperms still have these.

• Individuals produce distinctive male and female gametangia:

– The sperm-producing structure is the antheridium;

– The egg-producing structure is the archegonium.

• In land plants, called embryophytes, the zygote is retained on the gametophyte after fertilization and develops into a multicellular embryo that remains attached to the parent and is nourished by it.


Alternation of Generations

• All land plants undergo alternation of generations.

• In this process, individuals have a multicellular haploid phase called the gametophyte and a multicellular diploid phase known as the sporophyte.

• The two phases of the life cycle are connected by distinct types of reproductive cells—gametes and spores.


Alternation of Generations

• Alternation of generations always involves the same five key events:

(1) Haploid gametophytes produce haploid gametes by mitosis.

(2) Two gametes unite during fertilization to form a diploid zygote.

(3) The zygote divides by mitosis and develops into a multicellular, diploid sporophyte.

(4) The sporophyte produces haploid spores by meiosis.

(5) Spores divide by mitosis and develop into a haploid gametophyte.


All Land Plants Undergo Alternation of Generations

FERTILIZATION

Gametophyte (n; multicellular,

haploid)MITOSIS MEIOSIS

Sporophyte (2n; multicellular,

diploid)

Spores (n)

Haploid (n)

Diploid (2n)MITOSIS

MITOSIS

Gametes (n) Zygote (2n)


Changing Trends in Life Cycles

• In land plants, the relationship between gametophyte and sporophyte is highly variable.

• Gametophyte-dominated life cycles evolved early, as shown by the moss in Figure 30.17a; sporophyte-dominated life cycles evolved later, as shown by the fern in Figure 30.17b.


Gametophyte-Dominated Life Cycles Evolved Early

Haploid (n)

Diploid (2n)

Eggs form in archegonia

Sperm form in antheridia

Sperm swim to egg

Spores (n) are produced in sporangia by meiosis, dispersed by wind

Mature sporophyte (2n)

Egg (n)Zygote

(2n)Developing sporophyte

(2n)

Developing sporophyte

(2n)

Archegonium

Mature gametophyte (n)

Mature gametophyte (n)

Developing gametophyte Spore (n)

Mosses: Gametophyte is large and long lived; sporophyte depends on gametophyte for nutrition.

FERTILIZATIONMITOSIS

MITOSIS MEIOSIS


Sporophyte-Dominated Life Cycles Evolved Later

Ferns: Sporophyte is large and long lived but, when young, depends on gametophyte for nutrition.

Spore (n, dispersed

by wind)

Mature gametophyte (n, underside)

Sperm develop in antheridia

1 mm

Eggs develop in archegonia

Sperm swim to egg

Zygote (2n)

Archegonium

Sporophyte (2n; develops on gametophyte)

Developing gametophyte

(n)

Spores are produced in sporangia

Gametophyte (n; side view)


FERTILIZATION

MITOSIS

MITOSIS

MEIOSIS


Heterospory

• Another important innovation found in seed plants is heterospory, the production of two distinct types of spore-producing structures and thus two distinct types of spores, male and female.

• All of the nonvascular plants and most of the seedless vascular plants are homosporous. Homosporous plants produce a single type of spore that develops into a bisexual gametophyte that produces both eggs and sperm.


Heterosporous Plants


Heterospory

• The two types of spore-producing structures in heterosporous species are microsporangia and macrosporangia.

• Microsporangia produce microspores that develop into male gametophytes that produce sperm.

• Macrosporangia produce megaspores that develop into female gametophytes that produce eggs.

• Thus, the gametophytes of seed plants are either male or female, but never both.


Pollen

• When pollen evolved, heterosporous plants lost their dependence on water for fertilization.


Seeds

• Seeds allow embryos to be dispersed to a new habitat, away from the parent plant.

• Seeds are often dispersed by wind, water, or animals.

• The evolution of heterospory, pollen, and seeds triggered a dramatic radiation of seed plants starting about 290 mya.

• The life cycle of a gymnosperm, showing heterospory, is illustrated in Figure 30.21.


Heterospory in Gymnosperms


Developing sporophyte Seed (disperses

via wind or animals)

Embryo (2n)

Ovules (contain megasporangia)

Ovulate cone

Female gametophyte (n)

Archegonia

Eggs (n)

Mother cell (2n)

Pollen produces sperm

Cones with microsporangia

Microspore (n) forms pollen grain

Pollen grain (male gametophyte

Megasporangium

Pollen grain

Megaspore divides to form female gametophyte (n), which produces archegonia and eggs by mitosis. (Only one egg is fertilized and develops.) Note that the red dots here and elsewhere represent nuclei

Pollen grains disperse via wind

Four meiotic products; one is large and forms the megaspore (n)

Three meiotic products die

MEIOSISMITOSIS

POLLINATION

MEIOSIS

MITOSISFERTILIZATION


Flowers

• Flowering plants, or angiosperms, are the most diverse land plants living today. About 250,000 species have been described, and more are discovered each year.

• The success of angiosperms in terms of geographical distribution, number of individuals, and number of species revolves around a reproductive organ: the flower.


Flowers

• Flowers contain two key reproductive structures: stamens and carpels.

• The stamen contains the anther, where microsporangia develop.

• The carpel contains the ovary in which the ovules are found. Ovules contain the megasporangia.

• The evolution of the flower is an elaboration of heterospory, with the key innovation being the evolution of the ovary.

• Heterospory in angiosperms is illustrated in Figure 30.22; this figure shows the key features of microspore and megaspore production.


Heterospory in Angiosperms

Megasporangium

Sperm travel down growing pollen tube to reach egg

Pollen lands near female gametophyte; produces pollen tube and sperm

Pollen grains disperse via wind or animals (the red dots here and elsewhere are nuclei)

Nutritive tissue

Embryo (2n)

Developing sporophyte

Seed (disperses via wind or animals)

Zygote (2n)

Endosperm (3n) forms nutritive tissue in seed

Mature sporophyte flower (2n)

Flower

Ovule

Ovary

Carpel

Stamen

Anther

Microspore (n) forms pollen grain

Pollen grain (male gametophyte)

EggMegaspore (n: retained in ovary)

Female gametophyte (n: retained in ovary)

MEIOSISMITOSIS

POLLINATION

MEIOSISMITOSIS

MITOSIS

FER

TILI

ZATI

ON


Flowers

• Flowers vary in size, structure, scent, and color in order to attract different pollinators.

• The evolution of the flower made efficient pollination possible.


Fruits

• A fruit is a structure that is derived from the ovary and encloses one or more seeds.

• The evolution of fruit made efficient seed dispersal possible.


The Transition to Land: Summary

• The adaptations that allow land plants to reproduce in dry environments are summarized in Figure 30.26.


Innovations That Allow Plants to Reproduce Efficiently on Land

Land plants

Vascular plants

Seed plants

Gymnosperms

Green algae

Nonvascular plants


HeterosporyFruit

Flowers

SeedsPollen

Heterospory

Sporophyte-dominated life cycle

Alternation of generationsRetention of embryo on parent

Complex gametangiaThick-walled spores

Retention of egg on parentSimple gametangia

Red a

lgae

Ulvophyt

esCole

ochae

tes

Stonew

orts

Liver

wortsHorn

wortsM

osses

Lycophyt

esW

hisk

fern

sHors

etai

lsFer

ns

Cycad

sGin

kgo

Redwoods

et a

l.

Gnetophyt

es

Pines

et a

l.Angio

sper

ms


The Angiosperm Radiation

• Angiosperms represent one of the great adaptive radiations in the history of life.

• The diversification of angiosperms is associated with three key adaptations: (1) vessel elements, (2) flowers, and (3) fruits.

• These adaptations allow angiosperms to transport water, pollen, and seeds efficiently.


The Angiosperm Radiation

• On the basis of morphological traits, angiosperms have traditionally been divided into the two major groups named monocots and dicots.

• Monocots have one cotyledon, the first leaf, while dicots have two.

• Monocots as a group are monophyletic; dicots are paraphyletic. Because dicots are not a natural grouping, most biologists call them eudicots.


Monocots Are Monophyletic, but Dicots Are Paraphyletic

Oldest living angiosperm

lineages

Several lineages related to magnolias EudicotsNon-a

ngiosp

erm

s

Monoco

ts

Angiosperms

Green plants

Lineages in orange were traditionally called dicots, but this tree shows that dicots are not a natural grouping


Key Lineages of Green Plants

• The evolution of cuticle, stomata, and water-conducting tissues allowed green plants to grow on land, where resources for photosynthesis are abundant.

• Once the green plants were on land, the evolution of gametangia, retained embryos, pollen, seeds, and flowers enabled them to reproduce efficiently even in very dry environments.

• Let’s explore the diversity of green plants today.


Green Algae

• The green algae are a paraphyletic group that totals about 7000 species.

• Like land plants, their chloroplasts have a double membrane and chlorophylls a and b, but relatively few accessory pigments.

• Green algae are important primary producers in nearshore ocean environments and in all types of freshwater habitats.

• Green algae live in close association with an array of other organisms.


Green Algae Are Paraphyletic

Green algae

Land plantsRed

alg

ae

Ulvophyt

es

Coleoch

aete

s

Stonew

orts


Coleochaetophyceae (Coleochaetes)

• Most coleochaetes grow as flat sheets of cells (Figure 30.31), and the multicellular individuals are haploid.

• The coleochaetes are strictly freshwater algae that grow attached to aquatic plants or over submerged rocks.


Coleochaetes Are Thin Sheets of Cells

Coleochaete scutata


Ulvophyceae (Ulvophytes)

• Ulvophytes range from unicellular to multicellular.

• Many of the large green algae in habitats along ocean coastlines are Ulvophytes.

• They are important primary producers in aquatic areas.


Green Algae: Primary Producers in Aquatic Environments

Ulva lactuca


Charaphyceae (Stoneworts)

• Stoneworts get their common name because they usually accumulate crusts of calcium carbonate over their surfaces.

• They are multicellular; some species can be a meter or more in length.

• These freshwater algae are a good indicator that water is not polluted.


Stoneworts Can Form Beds on Lake Bottoms

Chara globularis (tall) and C. fibrosa (short)


Nonvascular Plants ("Bryophytes")

• The nonvascular plants, or bryophytes, are the most basal lineages of land plants.

• These plants grow low to the ground, as most do not have any water-conducting cells.

• The evolutionary relationships among the three lineages with living representatives—mosses, liverworts, and hornworts—are still unclear.


Bryophytes: Monophyletic or Paraphyletic?

Land plants

Non-vascular plants

Green algaeLiv

erworts

Hornworts

Moss

es


Bryophyta (Mosses)

• Mosses are common in moist forests, but they can also be abundant in extreme environments. Under severe conditions, they can dry out and then rehydrate later.

• Most mosses cannot grow taller than a few centimeters because they lack a true vascular tissue.


Moss Species Can Become Dormant When Conditions Are Dry

Moss in dry weather Moss in wet weather


Hepaticophyta (Liverworts)

• Liverworts are commonly found growing on damp forest floors or riverbanks, often in dense mats.

• They can grow on bare rock or tree bark and after they die they contribute to the initial stages of soil formation.


Liverworts Thrive in Moist Habitats

Marchantia bryophyta


Anthocerophyta (Hornworts)

• In hornworts, the sporophytes look like horns and have stomata.

• Some species harbor symbiotic cyanobacteria that fix nitrogen.


Hornworts Have Horn-Shaped Sporophytes

Phaeocerus leavis


Seedless Vascular Plants

• The seedless vascular plants are a paraphyletic group that forms a grade (a sequence of lineages that are not monophyletic) between the nonvascular plants and the seed plants.

• All species of seedless vascular plants have conducting tissues with cells that are reinforced with lignin, forming vascular tissue.

• Seedless vascular plants depend on the presence of water for reproduction.

• Tree-sized lycophytes and horsetails are abundant in the fossil record, but most living today (except some ferns) are much smaller.


The Seedless Vascular Plants Are Paraphyletic

Land plants


Lycophyt

es

Whis

k fe

rns

Horset

ails

Ferns


Lycophyta (Lycophytes, or Club Mosses)

• Fossil lycophytes were up to 40 meters tall, but the 1000 species of lycophytes living today are all small in stature.

• Lycophytes are the most ancient plant lineage with roots, the belowground system of tissues and organs that anchors the plant and is responsible for absorbing water and mineral nutrients.

• Most lycophytes live on the forest floor or on the branches or trunks of tropical trees.


Lycophytes Living Today Are Small in Stature

Lycopodium species


Psilotophyta (Whisk Ferns)

• Only six species of whisk ferns are living today; this group has no fossil record.

• Whisk ferns live only in tropical regions.

• They very simple morphologically, and lack both leaves and roots.


Psilotophytes Are Extremely Simple Morphologically

Tmesipteris species


Sphenophyta (Horsetails)

• Horsetails are prominent in the fossil record of land plants, but only 15 species are known today.

• Their name comes from the brushy appearance of the stems and branches in some species.

• Horsetails live in wet habitats such as stream banks or marsh edges. They can flourish in waterlogged soils by allowing oxygen to diffuse down their hollow stems.


Horsetails: Separate Reproductive and Vegetative Stalks

Equisetum arvense


Pteridophyta (Ferns)

• Ferns are by far the most species-rich seedless vascular plants, with 12,000 known species.

• They are particularly abundant in the tropics.

• The growth habits of ferns are highly variable, and ferns range in size from just a few centimeters tall to 20-meter-tall trees.

• Ferns are the only seedless vascular plants to have large, well-developed leaves. These leaves give the plant a large surface area with which to capture sunlight for photosynthesis.


Ferns Range from Small to Tree Sized

Gonocormus minutus Dicksonia antarcticaFerns range in size.


Seed Plants

• The seed plants are a monophyletic group that consists of the gymnosperms and the angiosperms.

• Seed plants are defined by the production of seeds and pollen grains.


The Seed Plants Are a Monophyletic Group

Land plants

Seed plants

Gymnosperms

Cycad

s

Gingko

Redwoods

et a

l.

Gnetophyt

es

Pines

et a

l.

Angiosp

erm

s


Gymnosperms: Cycadophyta (Cycads)

• Cycads resemble palms but are not closely related to them.

• Cycads were extremely abundant when dinosaurs were living (150–65 mya), but now only about 140 species exist.

• Most species of cycads live in the tropics.

• Cycads harbor large numbers of symbiotic, nitrogen-fixing cyanobacteria; the nitrogen they fix provides nutrients for the cycads and for nearby plants.


Ferns Resemble Palms but Are Not Closely Related to Them

Cycas revoluta


Gymnosperms: Ginkgophyta (Ginkgos)

• Although ginkgos have an extensive fossil record, only one species is alive today.

• Unlike most gymnosperms, the ginkgo is deciduous, and individual trees are either male or female.


The Gingko Tree Is a “Living Fossil”

Ginkgo bilobaGinkgo huttoni

Living ginkgoFossil ginkgo


Gymnosperms: Gnetophyta (Gnetophytes)

• The gnetophytes comprise about 70 species in three genera. One genus consists of vines and trees from the tropics. The second is made up of desert-dwelling shrubs in southwestern North America. The third is an unusual plant called Welwitschia with only two large leaves aboveground that grows in the deserts of southwestern Africa.

• Gnetophytes have vessel elements in addition to tracheids.


Welwitschia Is an Unusual Plant

Welwitschia mirabilis


Gymnosperms: Pinophyta (Pines, Spruces, Firs)

• The Pinophyta and other conifers have a reproductive structure, the cone, in which microsporangia and megasporangia are produced.

• All living species make wood as a support structure.

• Pines have a unique arrangement of needle-like leaves.

• Pines are common on sandy soils, and spruces and firs are common in very cold environments.

• These are among the most abundant trees on the planet, as well as some of the most long-lived. One bristlecone pine is known to be at least 4900 years old.


Pollen-Bearing Cones and Ovulate Cones

Picea abies

Cones that produce macrosporangia and eggs

Cones that produce macrosporangia and pollen

Picea abies

Pollen


Gymnosperms: Other Conifers

• All of the species in this lineage are large shrubs or trees. Redwood trees are the world’s largest plants.

• Most have narrow leaves; some have scale-like, overlapping leaves. These leaves allow them to thrive in dry or very cold habitats.

• Like other conifers, these species produce cones.

• The species in this group are wind pollinated. Depending on the species, the seeds are dispersed by wind or by birds or mammals.


Some Species in This Group Have Scale-Like Leaves

Thuja plicata


Anthophyta (Angiosperms)

• The angiosperms are the most species-rich of the land plants, with over 250,000 known species.

• They range in size from less than half a millimeter to massive oak trees. They thrive in environments from desert to freshwater to rain forests.

• The defining adaptation of angiosperms is the flower.

• The vascular tissue of most angiosperms contains both tracheids and vessel elements.

• In most terrestrial habitats today, angiosperms supply the food that supports virtually every other species, including humans.


Flower That Produces Both Pollen and Eggs

Ornithogalum dubium

Animal-pollinated flower (this species produces both pollen and eggs in the same flower)

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