Chapter 30

Plant Diversity II

Seed Plants

Evolution of Seed Plants (angiosperms and gymnosperms)

Seeds changed the course of plant evolution

– Enabling their bearers to become the dominant producers in most terrestrial ecosystems

– Seed plants arose about 360 million years ago

Key Terrestrial Adaptations for Success of Seed Plants

1. Reduced gametophytes

2. Heterospory

3. Ovules

4. Pollen

5. Seeds

Advantages of Reduced Gametophytes

• The gametophytes of seed plants

– Microscopically small (miniaturization of gametophytes)

– Develop within the walls of spores retained within tissues of the parent sporophyte.

– Receive nutrients and protection from environmental stress, such as drought, from sporophyte tissue.

– The reduced gametophytes of seed plants are protected in ovules and pollen grains

• Gametophyte/sporophyte relationships

Sporophyte dependent on gametophyte (mosses and other bryophytes).

(a) Large sporophyte and small, independent gametophyte (ferns and other seedless vascular plants).

(b)

Microscopic femalegametophytes (n) inovulate cones(dependent)

Sporophyte (2n),the flowering plant(independent)

Microscopic malegametophytes (n)inside these partsof flowers(dependent)

Microscopic malegametophytes (n)in pollen cones(dependent) Sporophyte (2n)

(independent)

Microscopic femalegametophytes (n)inside these partsof flowers(dependent)

Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms).

(c)

Gametophyte(n)

Gametophyte(n)

Sporophyte(2n)

Sporophyte(2n)

BryophyteSeedless Vascular Plant

Gymnosperm Angiosperm

Seed Plants are Heterosporous

• megaspores give rise to female gametophytes (which develop within the ovule)

• microspores give rise to male gametophytes (which develop in the pollen grain)

Ovules and Production of Eggs

• An ovule consists of

– A megasporangium, megaspore, and protective integuments

(a) Unfertilized ovule. In this sectional view through the ovule of a pine (a gymnosperm), a fleshy megasporangium is surrounded by a protective layer of tissue called an integument. (Angiosperms have two integuments.)

Integument

Spore wall

Megasporangium(2n)

Megaspore (n)

Pollen and Production of Sperm

• Microspores develop into pollen grains

– Which contain the male gametophytes of plants

• Pollination

– Is the transfer of pollen to the part of a seed plant containing the ovules

• Evolution of pollen which can be dispersed by air or animals contributed greatly to the success and diversity of seed plants

– Eliminated the water requirement for fertilization that is required by bryophytes and seedless vascular plants (remember flagellated sperm of these plants must swim through a water film to reach the egg cells)

• If a pollen grain germinates

– It gives rise to a pollen tube that discharges two sperm into the female gametophyte within the ovule

(b) Fertilized ovule. A megaspore develops into a multicellular female gametophyte. The micropyle,the only opening through the integument, allowsentry of a pollen grain. The pollen grain contains amale gametophyte, which develops a pollen tubethat discharges sperm.

Spore wall

Male gametophyte(within germinatingpollen grain) (n)

Femalegametophyte (n)

Egg nucleus (n)

Dischargedsperm nucleus (n)

Pollen grain (n)Micropyle

Sperm of seed plants does not require water or motility because the pollen tube conveys the sperm to the egg

When a pollen grain germinates it grows a pollen tube.

Seeds

– When sperm fertilizes egg, the zygote forms and develops into the sporophyte embryo

– Seeds develop from the whole ovule

– A seed is: a sporophyte embryo, along with its food supply, packaged in a protective coat

Gymnosperm seed. Fertilization initiatesthe transformation of the ovule into a seed,which consists of a sporophyte embryo, a food supply, and a protective seed coat derived from the integument.

(c)

Seed coat(derived fromIntegument)

Food supply(femalegametophytetissue) (n)

Embryo (2n)(new sporophyte)

Gymnosperms

• Gymnosperms bear “naked” seeds, typically on cones.(“NAKED” when compared to angiosperms where seeds develop in enclosed chambers called ovaries…fruits are mature ovaries)

• Gymnosperms include many well-known conifers

– cone-bearing trees, including pine, fir, and redwood

Gymnosperm Phyla

• The gymnosperms include four plant phyla

– Cycadophyta

– Gingkophyta

– Gnetophyta

– Coniferophyta

Gymnosperm Diversity

Gnetum

Ephedra

Ovulate cones

Welwitschia

PHYLUM GNETOPHYTA

PHYLUM CYCADOPHYTA PHYLUM GINKGOPHYTA

Cycas revoluta

Cycads: Large cones and palm-like leaves. Thrived during Mesozoic era.

Ginkgo biloba is the only extant species of this phylum

This phylum contains three genera of tropical or desert plants

Gymnosperm Diversity

Douglas fir

Pacificyew

Common juniper

Wollemia pine

Bristlecone pine Sequoia

PHYLUM

Coniferophyta

Approx. 600 species of conifers.

Most are evergreen. (larch, tamarack, dawn redwood are deciduous)

Gymnosperm Evolution

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

• Gymnosperms appear early in the fossil record and dominated the Mesozoic terrestrial ecosystems

Life Cycle of a Pine

• Key features of the gymnosperm life cycle include

– Dominance of the sporophyte generation, the pine tree

– The development of seeds from fertilized ovules

– The role of pollen in transferring sperm to ovules

Pine Life Cycle…The movie

Ovule

Megasporocyte (2n)

Integument

Longitudinalsection ofovulate cone

Ovulatecone

Pollencone

Maturesporophyte(2n)

Longitudinalsection ofpollen cone

Microsporocytes(2n)

Pollengrains (n)(containing malegametophytes)

MEIOSIS

Micropyle

Germinatingpollen grain

Megasporangium

MEIOSIS

SporophyllMicrosporangium

Survivingmegaspore (n)

Germinatingpollen grain

ArchegoniumIntegumentEgg (n)

Femalegametophyte

Germinatingpollen grain (n)

Dischargedsperm nucleus (n)

Pollentube

Egg nucleus (n)FERTILIZATION

Seed coat(derived fromparentsporophyte) (2n)

Food reserves(gametophytetissue) (n)

Embryo(new sporophyte)(2n)

Seeds on surfaceof ovulate scale

Seedling

Key

Diploid (2n)Haploid (n)

Life Cycle of a Pine

A pollen cone contains many microsporangia held in sporophylls. Each microsporangium contains microsporocytes (microspore mothercells). These undergo meiosis, giving rise tohaploid microspores that develop into pollen grains.

3

In mostconifer species,

each tree hasboth ovulate

and pollencones.

1

A pollen grainenters throughthe micropyleand germinates,forming a pollentube that slowlydigeststhrough themegasporangium.

4

While thepollen tubedevelops, themegasporocyte(megasporemother cell)undergoes meiosis,producing fourhaploid cells. Onesurvives as amegaspore.

5

The female gametophytedevelops within the megasporeand contains two or threearchegonia, each with an egg.

6

By the time the eggs are mature,two sperm cells have developed in thepollen tube, which extends to thefemale gametophyte. Fertilization occurswhen sperm and egg nuclei unite.

7

Fertilization usually occurs more than a year after pollination. All eggs

may be fertilized, but usually only one zygote develops into an embryo. The

ovule becomes a seed, consisting of an embryo, food supply, and seed coat.

8

An ovulate cone scale has twoovules, each containing a mega-sporangium. Only one ovule is shown.

2

Angiosperms

– Are commonly known as flowering plants

– Are seed plants that produce the reproductive structures called flowers and fruits

– Are the most widespread and diverse of all plants (over 250,000 species…about 90% of all plant species)

Flowers• The flower is an angiosperm structure specialized for

sexual reproduction

• A flower is a specialized shoot with modified leaves

– Sepals, which enclose the flower

– Petals, which are often brightly colored and attract pollinators

– Stamens, which produce pollen

– Carpels, which produce ovules

Anther

Filament

Stigma

Style

Ovary

Carpel

Petal

ReceptacleOvule

Sepal

Stamen

Fruits

Typically consist of a mature ovary

(b) Ruby grapefruit, a fleshy fruitwith a hard outer layer andsoft inner layer of pericarp

(a) Tomato, a fleshy fruit withsoft outer and inner layersof pericarp

(c) Nectarine, a fleshyfruit with a soft outerlayer and hard innerlayer (pit) of pericarp

(e) Walnut, a dry fruit that remains closed at maturity

(d) Milkweed, a dry fruit thatsplits open at maturity

Pericarp is the thickened wall of the fruit.

Fruit adaptations can enhance seed dispersal

• Fruits can be carried by wind, water, or animals to new locations, enhancing seed dispersal

Wings enable maple fruits to be easily carried by the wind.

(a)

Seeds within berries and other edible fruits are often dispersed in animal feces.

(b)

The barbs of cockleburs facilitate seed dispersal by allowing the fruits to “hitchhike” on animals.

(c)

The Angiosperm Life Cycle

• In the angiosperm life cycle

– Double fertilization occurs when a pollen tube discharges two sperm into the female gametophyte within an ovule

– One sperm fertilizes the egg, while the other combines with two nuclei in the center cell of the female gametophyte and initiates development of food-storing endosperm

– Some flowers self-pollinate, but most species have mechanisms that promote cross-pollination which enhances genetic variability.

• The endosperm

– Nourishes the developing embryo

Angiosperm fertilization…movie

The Angiosperm Life Cycle

Key

Mature flower onsporophyte plant(2n)

Ovule withmegasporangium (2n)

Female gametophyte(embryo sac)

Nucleus ofdevelopingendosperm

(3n)

Dischargedsperm nuclei (n)

Pollentube

Male gametophyte(in pollen grain)

Pollentube

Sperm

Survivingmegaspore(n)

Microspore (n) Generative cell

Tube cell

Stigma

OvaryMEIOSIS

MEIOSIS

Megasporangium(n)

Pollengrains

EggNucleus (n)

Zygote (2n)

Antipodal cellsPolar nucleiSynergidsEgg (n)

Embryo (2n)

Endosperm(foodSupply) (3n)

Seed coat (2n)

Seed

FERTILIZATION

Haploid (n)

Diploid (2n)

Anther

Sperm(n)

Pollentube

Style

Microsporangium

Microsporocytes (2n)

GerminatingSeed

Anthers contain microsporangia.Each microsporangium contains micro-sporocytes (microspore mother cells) thatdivide by meiosis, producing microspores.

1 Microspores formpollen grains (containingmale gametophytes). Thegenerative cell will divideto form two sperm. Thetube cell will produce thepollen tube.

2

In the megasporangiumof each ovule, themegasporocyte divides bymeiosis and produces fourmegaspores. The survivingmegaspore in each ovuleforms a female gametophyte(embryo sac).

3

After pollina-tion, eventuallytwo sperm nucleiare discharged ineach ovule.

4

Double fertilization occurs. One spermfertilizes the egg, forming a zygote. Theother sperm combines with the two polarnuclei to form the nucleus of the endosperm,which is triploid in this example.

5

The zygotedevelops into an

embryo that ispackaged alongwith food into aseed. (The fruit

tissues surround-ing the seed are

not shown).

6

When a seedgerminates, the

embryo developsinto a mature

sporophyte.

7

Early embryo has a rudimentary root and Cotyledons (seed leaves)

Angiosperm Evolution• Angiosperms originated at least 140 million years ago

• Primitive fossils of 125-million-year-old angiosperms

– Display both derived and primitive traitsCarpel

Stamen

Archaefructus sinensis, a 125-million-year-old fossil.

(a)

Artist’s reconstruction of Archaefructus sinensis

(b)

5 cm

Angiosperm Diversity

• The two main groups of angiosperms

– Are monocots (one cotyledon) and eudicots (true dicots)

• Basal angiosperms

– Are less derived and include the flowering plants belonging to the oldest lineages

• Magnoliids

– Share some traits with basal angiosperms but are more closely related to monocots and eudicots

Angiosperm Diversity

Amborella trichopoda Water lily (Nymphaea “Rene Gerard”)

Star anise (Illicium floridanum)

BASAL ANGIOSPERMS

HYPOTHETICAL TREE OF FLOWERING PLANTS

MAGNOLIIDS

Am

bo

rell

a

Wat

er l

ilie

s

Sta

r an

ise

and

rel

ativ

es

Mag

no

liid

s

Mo

no

cots

Eu

dic

ots

Southern magnolia (Magnoliagrandiflora)

Angiosperm Diversity

Orchid(Lemboglossumfossii)

MonocotCharacteristics

Embryos

Leafvenation

Stems

Roots

Pollen

Flowers

Pollen grain withone opening

Root systemUsually fibrous(no main root)

Vascular tissuescattered

Veins usuallyparallel

One cotyledon Two cotyledons

Veins usuallynetlike

Vascular tissueusually arranged

in ring

Taproot (main root)usually present

Pollen grain withthree openings

Zucchini(CucurbitaPepo), female(left) andmale flowers

Pea (Lathyrus nervosus,Lord Anson’sblue pea), a legume

Dog rose (Rosa canina), a wild rose

Pygmy date palm (Phoenix roebelenii)

Lily (Lilium“Enchant-ment”)

Barley (Hordeum vulgare), a grass

Anther

Stigma

Californiapoppy(Eschscholziacalifornica)

Pyrenean oak(Quercuspyrenaica)

Floral organsusually in

multiples of three

Floral organs usuallyin multiples of

four or fiveFilament Ovary

EudicotCharacteristics

MONOCOTS EUDICOTS

Monocots Eudicots

Evolutionary Links Between Angiosperms and Animals

• Pollination of flowers by animals and transport of seeds by animals

– Are two important relationships in terrestrial ecosystems

(a) A flower pollinated by honeybees. This honeybee is harvesting pollen and nectar (a sugary solution secreted by flower glands) from a Scottish broom flower. The flower has a tripping mechanism that arches the stamens over the beeand dusts it with pollen, some ofwhich will rub off onto the stigmaof the next flower the bee visits.

(c) A flower pollinated by nocturnal animals. Some angiosperms, such as this cactus, depend mainly on nocturnal pollinators, including bats. Common adaptations of such plants include large, light-colored, highly fragrant flowers that nighttime pollinators can locate.

(b) A flower pollinated by hummingbirds.The long, thin beak and tongue of this rufous hummingbird enable the animal to probe flowers that secrete nectar deep within floral tubes. Before the hummer leaves, anthers will dust its beak and head feathers with pollen. Many flowers that are pollinated by birds are red or pink, colors to which bird eyes are especially sensitive.

Pollinators movie

Importance of Seed Plants

• Human welfare depends greatly on seed plants– No group is more important to

human survival than seed plants. Agriculture is based almost entirely on angiosperms.

• Humans depend on seed plants for

– Food

– Wood

– Many medicines

Threats to Plant Diversity

• Plants are a renewable resource, but plant diversity is not.

• Destruction of habitat

– Is causing extinction of many plant species and the animal species they support

– 50 million acres of tropical rainforest are cleared each year