-.l
CHAPTER30
PLANT DIVERSITY II:
THE EVOLUTION OF SEEDPLANTS
OUTLIITE
I.
Overview of Reproductive Adaptations of Seed Plants
A. The gametophytes of seed plants became even more reduced than the
gametophytesof seedlessvascular plants
B. In seedplants, the seedreplacedthe spore as the main meansof dispersingoffspring
C. Pollen becamethe vehicles for sperm cells in seedplants
II.
Gymnosperms
A. The Mesozoic era was the age of gymnosperms
B. The four divisions of extant gymnospenns are the cycads, the ginkgo, the
gnetophytes, and the conifers
C. The life cycle of a pine demonstratesthe key reproductive adaptations of seed
plants
Angiosperms (Flowering Plants)
A. Terrestrial adaptation continued with the refinement of vascular tissue in
angiosperms
B. The flower is the defining reproductiveadaptationof angiosperms
C. Fruits help dispersethe seedsof angiosperms
D. The life cycle of an angiospenn is a highly refined version of the alternation of
generationscommon to all plants
E. The radiation of angiosperms marks the transition from the Mesozoic to the
Cenozoic era
F. Angiosperms and animals have shapedone another'sevolution
G. Agriculture is based almost entirely on angiosperms
III.
[V.
The Global Impact of Plants
A. Plants transformed the atmosphereand climate
B. Plant diversitv is a nonrenewableresource
OBJECTIVES
After readingthis chapterand attendinglecture,the studentshouldbe ableto:
of seedplantsthat havecontributedto their successon land.
Describethe adaptations
l.
List the four divisionsof gymnosperms.
2.
Describethe structuresof ovulate and pollen conesof a pine and distinguishbetween
3.
the two.
448
Unit V The EvolutionaryHistory of BiologicalDiversity
4.
Describe the life history of a pine and indicate which structures are part of the
gametophyte generation and which are part of the sporophyte generation.
5.
Point out the major life cycle differencesin ferns and pines.
Distinguish between pollination and fertilization.
6.
7.
Describe a pine seedand indicate which structures are old sporophyte, gametophyte,
and new sporophyte.
8.
Describe how the needle-shapedleavesof pines and firs are adaptedto dry conditions.
List and give examplesof the two classesof Anthophyta.
Compare the life cycles of mosses,ferns, conifers, and flowering plants in terms of:
a. Dominant life cycle stage (gametophyte/sporophyte)
9.
10.
b. Whether they are homosporousor heterosporous
c. Mechanism of gametetransfer
I l.
12.
13.
Describe some refinementsin vasculartissuethat occurredduring angiospermevolution.
Explain how evolution of the flower enhanced the reproductive efficiency of
angiosperms.
Identiff the following floral structuresand describea function for each:
a. Sepals
f.
Anther
b. Petals
14.
I 5.
16.
17.
18.
19.
20.
e. Stigma
h. Style
c. Stamens
d. Carpels
i. Ovary
e. Filament
Describe four commonly recognized evolutionary trends in floral structure found in
various angiospermlineages.
Define fruit and explain how fruits are modified in ways that help disperse seeds.
Diagram the generalized life cycle of an angiosperm, identiff which structures are
haploid, and explain how it differs from the life cycle of a pine.
Explain the process of double fertilization and describe the fate of the polyploid
nucleus.
Explain how an angiospermseeddiffers from that of a pine.
Explain why paleobotanistshave difficulty piecing together the origin of angiosperms
and describe some current theories on how flowering plants may have evolved.
Explain how animals may have influencedthe evolution of terrestrial plants, and vice
versa.
KEYTERMS
nucellus
integuments
fiber
flower
ovule
seed
conifer
sepal
petal
tracheids
carpel
vesselelements
filament
stamen
anther
stigma
style
ovary
fruit
pollengrains
embryosac
cross-pollination
doublefertilization
cotyledons
endosperm
coevolution
Chapter30 PlantDivenity II: TheEvolutionof SeedPlants 449
LECTT]RENOTES
The emergenceof seedplants further transformedthe Earth. Seedsand other adaptationsof
gy-norp"ios and angiospermsheightenedthe ability of plants to survive and reproduceinIiu..r. terrestrialenviionments;theie plantsbecamethe principal producersin the food websof
on land.
most ecosystems
I.
Overview of Reproductive Adaptations of SeedPlants
Threelife cycle modificationscontributedto terrestrialseedplant success:
l. Reductionof the gametophyte.They wereretainedin the moist reproductive
tissueof the sporophytegeneration(not independent).
2'
:"?il;:l'jiil,"o.o
packaged
with a roodsupplywithina
into embryos
protective seed coat.
Seedsreplacedsporesas main meansof dispersal.
3. Evolution of pollen. Plants were no longer tied to water for fertilization.
A. The gametophytes of seed plants became even more reduced than the
gametophytes of seedlessvascular plants
While the gametophytesof seedlessvascular plants develop in the soil as an
independerit generation, those of seed plants are reduced in size and retained within the
moiit reproductive tissue of the sporophytegeneration.
.
This evolutionary trend reverses the gametophyte-sporophyte relationship
observedin bryophytes(see Campbell, Figure 30.1).
. Dominance of the diploid generation may afford protection from solar
radiation-induced mutations of the genome (damaging radiation is more
extensive on land than in aquatic habitats).
'
B. In seed plants, the seed replaced the spore as the main means of dispersing
offspring
The relatively harsh terrestrial environment led to the development of resistant
structures for the dispersal of offspring.
. Bryophytes and seedlessvascular plants produce and release hardy single-celled
spores.
. Seedsare morehardybecauseof their multicellularnature'
A seedconsistsof a sporophyteembryotogetherwith a food supply surroundedby a
protectivecoat.
. The sporophytes
do not releasetheir spores,but retainthem in their sporangia,
also containsa gametophyte.
the
sporophyte
result,
as a
in that they possesstwo different kinds of sporangia,
All seedplantsare heterosporous
eachproducinga differenttype of spore.
. Megasporangiaproduce megasporesthat give rise to female egg-containing
gametoPhYtes.
. Microsporangia
producemicrosporesthat give rise to male sperm-containing
gametoPhYtes.
with the megasporangia.
of the seedis associated
The development
. The megasporangium
of seedplants is not a chamber,but a fleshy structure
calleda nucellus.
(contribute
. Additional tissuescalledintegumentssurroundthe megasporangium
to the Protectivecoat).
-l
450
Unit V The EvolutionaryHistory of BiologicalDiversity
.
.
.
.
The resulting structure-megaspore, megasporangium,and integuments-is
calledan ovule (seeCampbell,Figure 30.2a).
The female gametophytedevelopswithin the wall of the megasporeand is
nourishedby the nucellus.
If the egg cell of a female gametophyte is fertilized by a spenn cell (see
Campbell,Figure 30.2b),the zygotedevelopsinto a sporophyteembryo.
The resultingsporophyte-containing
ovule developsinto a seed(seeCampbell,
Figure30.2c).
C. Pollen became the vehicles for sperm cells in seed plants
The microsporesdevelopinto pollen grains,which in turn, matureto form the male
gametophores
of seedplants.
. Pollen grainsare coatedwith a resilientpolymer, sporopollenin(seeChapter
2e).
.
Pollen grains can be carried away by wind or animals (e.g., bees) following
releasefrom microsporangia.
A pollen grain nearan ovule will extenda tube throughspenncells into the female
gametophytewithin the ovule.
. In somegymnosperms,
the spermcells are flagellated(ancestralcondition).
. Other gymnospenns (including conifers) and angiosperms do not have
flagellatedspermcells.
tr.
Gymnosperms
A. The Mesozoic era was the age of gymnosperms
Gymnospermsappearin the fossil recordmuch earlierthan flowering plants.
Gymnospermsmost likely descended
from Devonianprogymnosperm
and were
seedless.
. Seedsevolvedby the endof the Devonian.
. Adaptiveradiationduring the Carboniferousand Permianperiodsled to today's
divisions.
. During the Permian, Earth becamewanner and drier; therefore, lycopods,
horsetails,and ferns (previously dominant) were largely replacedby conifers
and their relatives,the cycads(two divisionsof gymnosperms).
. This largechangemarksthe end of the Paleozoicera and the beginningof the
Mesozoicera.
Gymnospermslack enclosedchambers(ovaries)in which seedsdevelop.
B. The four divisions of extant gymnospermsare the cycads,the ginkgo, the
gnetophytes,and the conifers
Campbell,Figure30.3,showsthe four divisionsof living gymnospenns.
The conifers are the largestdivision of gymnosperms.
. Most are evergreens:pines, firs, spruces, larches, yews, junipers, cedars,
cypresses,
andredwoodsall belongto this division.
. Includessome of the tallest (redwoodsand some eucalyptus);largest (giant
andoldest(bristleconepine)living organisms.
sequoias);
. Most lumberand paperpulp is from conifer wood.
.""''"
:'Tfi;*$f.f;,jiJd;;::::eaves
pan,s
.faseed
Chryter30 PlantDiversitytr: TheEvolutionof SeedPlants 451
C. The life cycle of a pine demonstratesthe key reproductive adaptations of seed
plants
by the following:
conifer, is characterized
The life cycle of pine, a representative
. The multicellularsporophyteis the most conspicuous
stage;the pine tree is a
sporophyte,with its sporangialocatedon cones.
. The multicellulargametophytegenerationis reducedand developsfrom haploid
sporesthat are retainedwithin sporangia.
= The male gametophyteis the pollen grain; there is no antheridium.
+ The female gametophyteconsistsof multicellular nutritive tissue and an
archegoniumthat developswithin an ovule.
developfrom
male and femalegametophytes
Conifer life cyclesare heterosporous;
produced
cones.
by
separate
different types of spores
. Treesof most pine speciesbearboth pollen conesand ovulate cones,which
developon differentbranches.
. Pollen coneshave microsporangia;
cells in these sporangiaundergomeiosis
producinghaploidmicrospores,smallsporesthat developinto pollen grainsthe male gametophytes.
. Ovulate cones have megasporangia;
cells in these sporangiaundergomeiosis
producinglarge megasporesthat develop into the female gametophyte (see
Campbell, Figure 30.4). Each ovule initially includesa megasporangium
(nucellus)enclosedin protective integumentswith a single opening, the
micropyle.
It takesnearly threeyearsto completethe pine life cycle, which progressesthrougha
complicatedseriesof eventsto producematureseeds.
. Windblown pollen falls onto the ovulate cone and is drawn into the ovule
throughthe micropyle.
. The pollen grain germinatesin the ovule, forming a pollen tube that beginsto
digestits way throughthe nucellus.
o I megasporemother cell in the nucellusundergoesmeiosis producing four
one of which will survive; it growsand dividesrepeatedly
haploid megaspores,
by mitosis producingthe immaturefemale gametophyte.
. Two or three archegonia, each with an egg, then develop within the
multicellulargametophyte.
. More than a year after pollination, the eggsare ready to be fertilized; two
spermcells havedevelopedand the pollen tube has grown through the nucellus
to the femalegametoPhyte.
. Fertilization occurswhen one of the spennnuclei unites with the egg nucleus.
All eggsin an ovule may be fertilized,but usuallyonly one zygotedevelopsinto
an embryo.
. The pine embryo, or new sporophyte, has a rudimentary root and several
in the female gametophyte,which nourishes
embryonic leaves.It is embedded
the embryountil it is capableof photosynthesis.The ovule has developedinto
a pine seed,which consists of an embryo (2n), its food source (n), and a
surroundingseedcoat (2n) derivedfrom the integumentsof the parenttree.
. Scalesof the ovulateconeseparate,andthe winged seedsare carriedby the wind
to new locations. Note, that with the seedplants, the seedhas replacedthe
sporeas the modeof dispersal.
r I seedthat landsin a habitableplacegerminates,its embryo emergingas a pine
seedling.
-l
452
Unit V The EvolutionaryHistory of BiologicalDiversity
m.
Angiosperms (Flowering Plants)
Flowering plants are the most widespreadand diverse;250,000 speciesare now known.
. There is only one division, Anthophyta, with two classesoMonocotyledones
(monocots) and Dicotyledones (dicots).
. Most use insects and animals for transferring pollen, and therefore, are less
dependenton wind and have less random pollination.
A. Terrestrial adaptation continued with refinement of vascular tissue in
angiosperms
Vascular tissue becamemore refined during angiospermevolution.
.
Conifers have tracheids (seeCampbell,Figure 30.5), water-conductingcells that
are:
= An early type of xylem cell
= Elongated, tapered cells that function both in mechanical support and water
movement up the plant
. Most angiosperms also have vessel elementsthat are:
+ Shorter,wider cells than the more primitive hacheids
Arranged end to end forming continuous tubes
= Comparedto tracheids,vesselelementsare more specializedfor conducting
water, but less specialized for support
Angiosperm xylem is reinforcedby other cell types calledfibers, which are:
+ Specialized for support with a thick lignified wall
= Evolved in conifers. (Conifer xylem contains both fibers and tracheids, but
not vessel elements.)
=
.
B. The flower is the defining reproductive adaptation of angiosperms
Flower: The reproductivestructureof an angiospennwhich is a compressedshoot
with four whorls of modified leaves(seealso Campbell,Figure 30.6)
Parts of the flower:
Sepals - Sterile, enclose the bud
Petals - Sterile, aid in athacting
pollinators
Stamen - Producesthe pollen
Carpel - Evolved from a seed-bearing
leaf that became rolled into a tube
Stigma - Part of the carpel that is a
sticky structure that receives the
pollen
Ovary - Part of the carpel that
protects the ovules, which develop
into seeds after fertilization
Shm
'
Anfs
'
Stiqmal
l
style ".*,
|
ovary)
There are four evolutionary trends in various angiosperm lineages:
I . The number of floral parts have become reduced.
2. Floral parts have becomefused.
3. Symmetry has changedfrom radial to bilateral.
4. The ovary has dropped below the petals and sepals,where the ovules are better
protected.
Chapter30 PlantDivenity tr: TheEvolutionof SeedPlants 453
C.
F'ruits help disperse the seeds of angiosperms
Fruit : A ripened ovary that protects dormant seedsand aids in their dispersal; some
fruits (like apples) incorporate other floral parts along with the ovary (see Campbell,
Figure 30.7)
Aggregate fruits : Several ovaries that are part of the same flower (e.g., raspberry)
Multiple fruit = One that develops from several separateflowers (e.g., pineapple)
Modifications of fruits that help disperseseedsinclude:
.
Seedswithin fruits that are shaped like kites or propellers to aid in wind
dispersal
. Burr-like fruit that cling to animal fur
.
Edible fruit with tough seeds which pass through the digestive tract of
herbivores unharmed,dispersing seedsmiles away
D. The life cycle of an angiosperm is a highly refined version of the alternation
of generations common to all plants
Life cycles of angiospennsare heterosporous(in common with all seedplants) and the
two types of sporangiaare found in the flower (seeCampbell, Figure 30.8):
. Microsporangia in anthersproduce microsporesthat form male gametophytes.
. Megasporangia in ovules produce megaspores that develop into female
gametophytes.
Immature male gametophytes:
. Are pollen grains, which develop within the anthers of stamens
. Each pollen grain has two haploid nuclei that will participate in double
fertilization characteristic of angiosperrns.
Female gametophytes:
.
Do not produce an archegonium
. Are located within an ovule
.
Consist of only a few cells: an embryo sac with eight haploid nuclei in seven
cells (a large central cell has two haploid nuclei)
. One of the cells is the egg
An outline of the angiospermlife cycle follows:
. Pollen from the anther lands on the sticky stigma at the carpel's tip; most
flowers do not self-pollinate,but have mechanismsto ensurecross-pollination.
. The pollen grain germinateson the stigma by growing a pollen tube down the
style of the carpel.
.
When it reachesthe ovary, the pollen tube grows through its micropyle and
dischargestwo sperm cells into the embryo sac.
.
Double fertilization occurs as one spenn nucleus unites with the egg to form a
diploid zygote; the other speffn nucleus fuses with two nuclei in the embryo
sac'scentral cell to form triploid (3n) endosperm.
. After double fertilization, the ovule matures into a seed.
The seedis a mature ovule, consistingof:
l. Embryo. The zygote developsinto an embryo with a rudimentary root and one
(in monocots)or two (in dicots) cotyledons or seed leaves.
2. Endosperm.The triploid nucleusin the embryo sac divides repeatedly forming
triploid endosperm,rich in starch and other food reserves.
3. Seedcoat. This is derived from the integuments(outer layers of the ovules).
Monocots and dicots use endospermdifferently.
454
Unit V The EvolutionaryHistory of BiologicalDiversity
.
.
Monocot seedsstore most food in the endosperm.
Dicots generally restock most of the nutrients in the developing cotyledons.
In a suitable environment the seed coat ruptures and the embryo emergesas a seedling,
using the food stored in the endospermand cotyledons.
E
The radiation of angiosperms marks the transition from the Mesozoic to the
Cenozoic era
Angiosperms showed a relatively sudden appearancein the fossil record with no clear
transitional links to ancestors.
.
Earliest fossils are early Cretaceous(approximately 130 million years ago)
. dominant, as they are today.
There are two theories about their sudden appearance:
l. Angiospermsoriginatedwhere fossilizationwas unlikely (they are an artifact of
an imperfect fossil record).
2, Angiosperms evolved and radiated relatively abruptly (punctuatedequilibrium).
Perhapsangiospermsevolved from seedferns, an extinct group of unspecialized
gymnosperms.
F.
Angiosperms and animals shaped one another's evolution
Terrestrial plants and animals have coevolved, a consequenceof their interdependence.
Coevolution: Reciprocal evolutionary responsesamong two or more interacting
species;adaptive change in one speciesis in responseto evolutionary changein the
other species.
Coevolution between angiospermsand their pollinators led to diversity of flowers.
.
Some pollinators are specific for a particular flower. The pollinator has a
monopoly on a food sourceand guaranteesthe flower's pollen will pollinate a
flower of the samespecies(see Campbell,Figure 30.9).
.
Often, the relationshipbetween angiospermsand their pollinators is not species
specific; a pollinator may not dependexclusively on one flower species,or a
flower species may not depend exclusively on one species of pollinator.
However, flower color, fragrance, and structure are usually adaptations for types
of pollinators, such as various speciesof beesor hummingbirds.
Edible fruits of angiospermshave coevolvedwith animals that can disperseseeds.
Animals become attracted to ripening fruits as they:
.
Become softer, more fragrant, and higher in sugar
.
Change to a color that attracts birds and mammals, animals which are large
enoughto dispersethe seeds
G.
is based almost entirely on angiosperms
Angiosperms provide nearly all our food: fruit, vegetable crops, and grains, such as
corn, rice, wheat.
Flowering plants are also used for other purposes,such as:
.
Fiber
Agriculture
.
.
.
Medication
Perfume
Decoration
Through agriculture, humans have influenced plant evolution by artificially selecting
for plants that improved the quantity and quality of foods and other crops.
. Many of our agricultural plants are so genetically removed from their origins
that they probably could not survive in the wild.
Chapter30 PlantDiversityII: TheEvolutionof SeedPlants 455
.
v.
cultivatedcrops that requirehuman intervention to water,
As a consequence,
and even to plant their
fertilize, provideprotectionfrom insectsand disease,
disasters.
human-caused
and
seeds,arevulnerableto natural
The Global ImPact of Plants
A. Plants transformed the atmosphereand the climate
In addition to being the primary producersof the terrestrialenvironment,plants also
changedthe physical environmentof Earth.
. They decreased
atmosphericcarbondioxide,resultingin global cooling.
. The cooler environment may have made terrestrial life more habitable for
other organisms.
B. Plant diversity is a nonrenewableresource
plant diversity is a nonrenewable
resource,and the irrevocableextinction of plant
rate.
unprecedented
an
at
speciesis occurring
. The explodinghumanpopulationdemandsspaceand naturalresources.
. The toll of habitatdestructionis greatestin the tropics becausethis is where:
live
+ Most sPecies
the humanpopulationlives and humanpopulationgrowth is
half
than
+ More
fastest
clearingfor agriculture
+ Most deforestationis causedby slash-and-burn
so do thousandsof plant and animalspecies.
As the forestdisappears,
. Habitat destructionalso endangersanimal speciesthat dependon plants in the
tropical rainforest.
. Habitat destructionby humanshas not beenlimited to the tropics' Europeans
eliminated most of- their forests centuries ogo, and in North America,
destructionof habitatis endangeringmany species'
There are many reasonsto value plant diversity and to find ways to protect it'
. Ecosystemsare living treasuresthat can regenerateonly slowly.
. Humansdependon plants for productssuch as medicines,food and building
materials.
. We still know so little of the 250,000known plant species.(Food agricultureis
basedon only abouttwo dozenspecies')
REFERENCES
1999'
Campbell,N., et al.Biotogtt.5thed. Menlo Park,California:Benjamin/Cummings,
Worth
Raven, P.H., R.F. Evert and S.E. Eichhorn. Biology of Plants. 6th ed' New York:
Inc., 1988.
Publishers,