overview: feeding the world seeds changed the course of plant evolution, enabling their bearers to...

Overview: Feeding the World

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

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

• In addition to seeds, the following are common to all seed plants:

– Reduced gametophytes

The gametophytes of seed plants develop within the walls of spores retained within tissues of the parent sporophyte

– Heterospory

– Ovules

– Pollen

Sporophyte(2n)

Gametophyte(n)

Sporophyte dependent on gametophyte (mosses and other bryophytes)

Gametophyte(n)

Sporophyte(2n)

Large sporophyte and small, independent game-tophyte (ferns and other seedless vascular plants)

Microscopic femalegametophytes (n) inovulate cones(dependent)

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

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

Microscopic malegametophytes (n)in pollen cones(dependent)

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

Sporophyte (2n),(independent)

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

Heterospory: The Rule Among Seed Plants

• Seed plants evolved from plants with megasporangia, which produce megaspores that give rise to female gametophytes

• Seed plants evolved from plants with microsporangia, which 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

Integument

Megasporangium(2n)

Megaspore (n)

Unfertilized ovule Fertilized ovule

Spore wall

Male gametophyte(within germinatingpollen grain) (n)

Micropyle

Femalegametophyte (n)

Egg nucleus (n)

Dischargedsperm nucleus (n)

Pollen grain (n)

Seed coat(derived fromintegument)

Embryo (2n)(new sporophyte)

Gymnosperm seed

Food supply(femalegametophytetissue) (n)

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 can be dispersed by air or animals, eliminating the water requirement for fertilization

• If a pollen grain germinates, it gives rise to a pollen tube that discharges two 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

Gymnosperms bear “naked” seeds, typically on cones

• The gymnosperms include four phyla:

– Cycadophyta (cycads)

– Gingkophyta (one living species: Ginkgo biloba)

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

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

Cycas revoluta

Gnetum. This genus includes about 35 species of tropical trees, shrubs, and vines, mainly native to Africa and Asia. Their leaves look similar to those of flowering plants, and their seeds look somewhat like fruits.

Ephedra. This genus includes about 40 species that inhabit arid regions throughout the world. Known in North America as “Mormon tea,” these desert shrubs produce the compound ephedrine, commonly used as a decongestant.

Welwitschia. This genus consists of one species Welwitschia mirabilis, a plant that lives only in the deserts of southwestern Africa. Its strap like leaves are among the largest known.

Ovulatecones

Douglas fir. “Doug fir” (Pseudotsuga menziesii) provides more timber than any other North American tree species. Some uses include house framing, plywood, pulpwood for paper, railroad ties, and boxes and crates.

Pacific yew. The bark of Pacific yew (Taxa brevifolia) is a source of taxol, a compound used to treat women with ovarian cancer. The leaves of a European yew species produce a similar compound, which can be harvested without destroying the plants. Pharmaceutical companies are now refining techniques for synthesizing drugs with taxol-like properties.

Bristlecone pine. This species (Pinus longaeva), which is found in the White Mountains of California, includes some of the oldest living organisms, reaching ages of more than 4,600 years. One tree (not shown here) is called Methuselah because it may be the world’s oldest living tree. In order to protect the tree, scientists keep its location a secret.

Sequoia. This giant sequoia (Sequoiadendron giganteum), in California’s Sequoia National Park weighs about 2,500 metric tons, equivalent to about 24 blue whales (the largest animals), or 40,000 people. Giant sequoias are the largest living organisms and also some of the most ancient, with some estimated to be between 1,800 and 2,700 years old. Their cousins, the coast redwoods (Sequoia sempervirens), grow to heights of more than 110 meters (taller than the Statue of Liberty) and are found only in a narrow coastal strip of northern California.

Common juniper. The “berries” of the common juniper (Juniperus communis), are actually ovule-producing cones consisting of fleshy sporophylls.

Wollemia pine. Survivors of a confer group once known only from fossils, living Wollemia pines (Wollemia nobilis), were discovered in 1994 in a national park only 150 kilometers from Sydney, Australia. The species consists of just 40 known individuals two small groves. The inset photo compares the leaves of this “living fossil” with actual fossils.

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

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

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

A Closer Look at the Life Cycle of a Pine

• Key features of the gymnosperm life cycle:

– 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 is an example

Maturesporophyte(2n)

Pollencone

Microsporocytes(2n)

Longitudinalsection of pollen cone

Ovulatecone

Longitudinalsection ofovulate cone

Micropyle

Ovule

Key

Haploid (n)Diploid (2n)

Megasporocyte (2n)

Integument

Megasporangium

Germinatingpollen grainPollen

grains (n)(containing malegametophytes)

MEIOSIS

MicrosporangiumSporophyll

MEIOSIS

Germinatingpollen grain

Femalegametophyte

Archegonium

Egg (n)

Germinatingpollen grain (n)

Integument

Dischargedsperm nucleus (n)

Pollentube

Egg nucleus (n)FERTILIZATION

Embryo(new sporophyte)(2n)

Food reserves(gametophytetissue) (n)

Seed coat(derived fromparentsporophyte) (2n)

Seedling

Seeds on surfaceof ovulate scale

Survivingmegaspore (n)

The reproductive adaptations of angiosperms include flowers and fruits

• Angiosperms are flowering plants

• These seed plants have reproductive structures called flowers and fruits

• They are the most widespread and diverse of all plants

Characteristics of Angiosperms

• All angiosperms are classified in a single phylum, Anthophyta

• The name comes from the Greek anthos, flower

Flowers

• The flower is an angiosperm structure specialized for sexual reproduction

• 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

Stamen

Filament

Anther

StigmaCarpel

Style

Ovary

Petal

Receptacle

Ovule

Sepal

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

• Various fruit adaptations help disperse seeds

• Seeds can be carried by wind, water, or animals to new locations

Tomato, a fleshy fruit with softouter and inner layers of pericarp

Ruby grapefruit, a fleshy fruitwith a hard outer layer and softinner layer of pericarp

Milkweed, a dry fruit that splitsopen at maturity

Walnut, a dry fruit that remainsclosed at maturity

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

Wings enable maplefruits to be easilycarried by the wind.

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

The barbs of cockleburs facilitate seed dispersal by allowing these fruits to hitchhike on animals.

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 central cell of the female gametophyte and initiates development of food-storing endosperm

• The endosperm nourishes the developing embryo

Anther

Mature flower onsporophyte plant(2n)

Key

Haploid (n)

Diploid (2n)

MicrosporangiumMicrosporocytes (2n)

MEIOSIS

Microspore (n)

MEIOSIS

Ovule withmegasporangium (2n) Male

gametophyte(in pollengrain)

Ovary

Generative cell

Tube cell

Megasporangium(n)

Survivingmegaspore(n)

Female gametophyte(embryo sac)

Antipodal cellsPolar nucleiSynergidsEggs (n)

Pollentube

Sperm(n)

Pollengrains

Pollentube

Style

Stigma

Pollentube

Sperm

Eggs nucleus (n)

Discharged sperm nuclei (n)

Germinatingseed

Zygote (2n)

FERTILIZATION

Nucleus ofdevelopingendosperm (3n)

Embryo (2n)

Endosperm(foodsupply) (3n)

Seed coat (2n)

Seed

Angiosperm Evolution

• Clarifying the origin and diversification of angiosperms poses fascinating challenges to evolutionary biologists

• Angiosperms originated at least 140 million years ago

• During the late Mesozoic, the major branches of the clade diverged from their common ancestor

Fossil Angiosperms

• Primitive fossils of 125-million-year-old angiosperms display derived and primitive traits

• Archaefructus sinensis, for example, has anthers and seeds but lacks petals and sepals

Carpel

Stamen

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

5 cm

Artist’s reconstruction ofArchaefructus sinensis

An “Evo-Devo” Hypothesis of Flower Origins

• Scientist Michael Frohlich hypothesized how pollen-producing and ovule-producing structures were combined into a single flower

• He proposed that the ancestor of angiosperms had separate pollen-producing and ovule-producing structures

Angiosperm Diversity

• The two main groups of angiosperms are monocots and eudicots

• 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

Amborella trichopoda Water lily (Nymphaea “Rene Gerald”)

Star anise (Illiciumfloridanum)

BASAL ANGIOSPERMS

Southern magnolia(Magnoliagrandiflora)

HYPOTHETICAL TREE OF FLOWERING PLANTS

MAGNOLIIDS

Am

bo

rella

Sta

r an

ise

and

rel

ativ

es

Wat

er li

lies

Mag

no

liid

s

Mo

no

cots

Eu

dic

ots

Orchid(Lemboglossumrossii)

MonocotCharacteristics

One cotyledon

Embryos

Two cotyledons

EudicotCharacteristics

Californiapoppy(Eschscholziacalifornia)

EUDICOTSMONOCOTS

Pygmy date palm (Phoenix roebelenii)

Veins usuallyparallel

Vascular tissuescattered

Stems

Pyrenean oak(Quercuspyrenaica)

EUDICOTSMONOCOTS

Leafvenation

Vascular tissueusually arranged

in ring

Veins usuallynetlike

Dog rose (Rosa canina), a wild roseRoot system

usually fibrous(no main root)

Roots

Lily (Lilium“Enchantment”)

EUDICOTSMONOCOTS

Taproot (main root)usually present

Barley (Hordeum vulgare), a grass

Floral organsusually in

multiples of three

Pollen

Zucchini (CucurbitaPepo), female(left), andmale flowers

EUDICOTSMONOCOTS

Pollen grain withthree openings

Stigma

Ovary

Anther

Filament

Floral organs usuallyin multiples of

four or five

Pollen grain withone opening

Flowers

Pea (Lathyrusnervosus, LordAnson’sblue pea), a legume

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 flower pollinated byhoneybees.

A flower pollinated by hummingbirds.

A flower pollinated by nocturnal animals.

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

Threats to Plant Diversity

• Destruction of habitat is causing extinction of many plant species

• Loss of plant habitat is often accompanied by loss of the animal species that plants support

Animations and Videos

• Life Cycle of a Conifer

• Life Cycle of a Pine

• Parts of the Vascular Plant

• Parts of a Flower

• Double Fertilization

• Plant Fertilization

• Seed Development

• Fruit Development

Animations and Videos

• Life Cycle of an Angiosperm

• Life Cycle of a Flowering Plant

• Chapter Quiz Questions – 1

• Chapter Quiz Questions - 2

The reason that two sperm nuclei travel down the pollen tube is that

• they both stimulate growth of the pollen tube.

• one fertilizes the egg, and the other combines with the two polar nuclei.

• one fertilizes the egg, and the other fertilizes the synergid.

• one is for fertilizing the egg, and one directs the pollen tube toward the micropyle.

• if one fails in fertilization, there is a backup nucleus.

Adaptations that have contributed to the success of plants in terrestrial environments include seeds, vascular tissue, cuticle, and flowers. There are other significant adaptations as well. Which of the following correctly lists the adaptations in the order in which they appeared in the fossil record?

a) seeds, vascular tissue, flowers, cuticle

b) cuticle, seeds, flowers, vascular tissue

c) cuticle, vascular tissue, seeds, flowers

d) vascular tissue, cuticle, seeds, flowers

These four adaptations (cuticle, vascular tissue, seeds, and flowers) are key characters that define four major modern plant groups. Complete the following table using the key provided.

a) ferns (and relatives) b) bryophytes (mosses) c) flowering plants d) gymnosperms

The reason that two sperm nuclei travel down the pollen tube is that

• they both stimulate growth of the pollen tube.

• one fertilizes the egg, and the other combines with the two polar nuclei.

• one fertilizes the egg, and the other fertilizes the synergid.

• one is for fertilizing the egg, and one directs the pollen tube toward the micropyle.

• if one fails in fertilization, there is a backup nucleus.

In dicotyledon seed germination, usually the first organ to emerge from the seed is the

• shoot tip, as the most important function to get under way is breaking through the soil.

• hypocotyl, as the most important function to get under way is breaking through the soil.

• cotyledon(s), as the most important function to get under way is photosynthesis.

• radicle, as the most important function to get under way is the absorption of water and nutrients.

• root, as the most important function to get under way is the anchoring of the plant to the soil.

All of the following statements regarding double fertilization in the angiosperms is true except which one?

• The angiosperm embryo is diploid.

• The pollen tube reaching the micropyle results in the death of one of the synergids.

• The fertilized angiosperm ovule consists of diploid and triploid cells.

• The megasporangium gives rise to four haploid eggs through meiosis.

• One of the pollen nuclei contributes to the genome of the endosperm.

The flowers of wind-pollinated plants

• usually have flat blades to direct the wind.

• produce sticky pollen.

• are usually green and inconspicuous.

• are usually white and small.

• usually occur on separate male and female plants.

An accessory fruit is one that

• grows from a lateral meristem.

• develops from tissue other than the ovary.

• develops from the pericarp.

• consists of multiple receptacles.

• is not necessary for reproduction.

Which of the following lists an aggregate fruit, a multiple fruit, and an accessory fruit, in that order?

• guava, blackberry, cherry

• watermelon, orange, fig

• apple, pear, cherry

• pineapple, tangerine, cranberry

• strawberry, fig, pear

Scientific Skills ExercisesOne unusual opportunity to test how long seeds can remain viable occurred when seeds from date palm trees (Phoenix dactylifera) were discovered under the rubble of a 2,000-year-old fortress near the Dead Sea. Scientists measured the fraction of carbon-14 that remained in three of the ancient date palm seeds: two that were not planted and one that was planted and germinated. (For the germinated seed, the scientists used a seed-coat fragment found clinging to a root of the seedling.) The table shows their results.

Data from S. Sallon, et al., Germination, genetics, and growth of an ancient date seed. Science 320:1464 (2008).

An ancient date palm seed germinated and grew into a healthy plant. What was the source of nutrients for the young sporophyte embryo during germination of the date palm seed?

a) sunlight

b) food stored within the seed

c) water taken up by the germinating seed

d) the surrounding soil

Why is seed dormancy adaptive for many plant species?

a) Seed dormancy increases the longevity of an individual sporophyte plant.

b) Seed dormancy allows seeds to germinate when conditions are favorable for seedling growth.

c) Seed dormancy ensures that a seedling does not compete with its parents.

d) Seed dormancy ensures that a seedling does not germinate when a source of food is not available for the young seedling.

overview: feeding the world seeds changed the course of plant evolution, enabling their bearers to...

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