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Chapter 30
Plant Diversity II: The Evolution of
Seed Plants
Introduction
• Seeds changed the course of plant
evolution, enabling their bearers to
become the dominant producers in most
terrestrial ecosystems
• A seed consists of an embryo and
nutrients surrounded by a protective coat
Concept 30.1 Seeds and Pollen
• In addition to seeds, the following are
common to all seed plants
– 1. Reduced gametophytes
• The gametophytes of seed plants develop within
the walls of spores that are retained within tissues
of the parent sporophyte
– 2. Heterospory
• 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
3. Ovules
• An ovule consists of
a megasporangium,
megaspore, and one
or more protective
Integuments
• Gymnosperm
megaspores have
one integument
• Angiosperm
megaspores usually
have two integuments
– 4. Pollen
• Microspores develop into pollen grains,
which contain the male gametophytes
• Pollination is the transfer of pollen to the
part of a flower containing the ovules
• Pollen eliminates the need for 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 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
Advantage of the seed
• A seed develops from the whole ovule
• A seed is a sporophyte embryo, along with its
food supply, packaged in a protective coat
• Seeds provide some evolutionary advantages
over spores:
– They may remain dormant for days to years,
until conditions are favorable for germination
– They may be transported long distances by
wind or animals
Concept 30.2 Gymnosperms
•
•
•
•
Gymnosperms means “naked seeds”
The seeds are exposed on sporophylls that form cones
Angiosperm seeds are found in fruits, which are mature ovaries
The gymnosperms 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)
Cycadophyta
• Cycads (Phylum Cycadophyta)
superficially resemble palms.
– These thrived during the Mesozoic, but
relatively few species exist today
– Palms are actually flowering plants.
Ginkophyta
• Phylum Ginkgophyta consists of only a
single extant species, Ginkgo biloba.
– This popular ornamental species has fanlike leaves that turn
gold before they fall off in the autumn.
– Landscapers usually only plant male trees because the seed
coats on female plants decay, they produce a repulsive odor (to
humans, at least). It has a high tolerance to air pollution and is a
popular ornamental tree
Gnetophyta
• Phylum Gnetophyta consists of three
very different genera.
– Weltwitschia plants, from deserts in southwestern Africa, have
straplike leaves.
– Gentum species are tropical trees or vines.
– Ephedra (Mormon tea) is a shrub of the American deserts.
Coniferophyta
• This phylum is by far the largest of the gymnosperm phyla
• Most conifers are evergreens and can carry out photosynthesis
year round
• Conifers include pines, firs, spruces, larches, yews, junipers,
cedars, cypresses, and redwoods.
Gymnosperm Life Cycle
• 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
Gymnosperm Reproduction
1. Most species produce both pollen cones and
ovulate cones.
2. A pollen cone contains hundreds of
microsporangia held on small sporophylls.
• Cells in the microsporangia undergo meiosis to form haploid
microspores that develop into pollen grains.
3. An ovulate cone consists of many scales,
each with two ovules.
4. During pollination, windblown pollen falls on
the ovulate cone and is drawn into the ovule
through the micropyle
5. The megaspore mother cell undergoes
meiosis to produce four haploid cells, one of
which will develop into a megaspore.
6. Two or three archegonia, each with an egg,
then develop within the gametophyte.
7. At the same time that the eggs are ready,
two sperm cells have developed in the pollen
tube which has reached the female
gametophyte.
8. The pine embryo, the new sporophyte, has a
rudimentary root and several embryonic
leaves.
Phylum Characteristics
• The term conifer comes from the reproductive
structure, the cone, which is a cluster of
scalelike sporophylls.
• Although there are only about 600 species of
conifers, a few species dominate vast forested
regions in the Northern Hemisphere where the
growing season is short.
• Most conifers are evergreen, retaining their
leaves and photosynthesizing throughout the
year.
• Much of our lumber and paper
comes from the wood
(actually xylem tissue) of
conifers.
• Coniferous trees are among
the largest and oldest
organisms of Earth.
– Redwoods from northern
California can grow to heights of
over 110m.
– One bristlecone pine, also from
California, is more than 4,600
years old.
Concept 30.3 Angiosperms
• Better known as flowering plants and these
vascular seed plants have two key adaptations
- flowers and fruits.
• Most diverse and geographically widespread of
all plants.
• About 250,000 known species of angiosperms.
• All angiosperms are placed in a single phylum,
the phylum Anthophyta.
The Flower
• The flower is an angiosperm structure
specialized for reproduction.
– In many species, insects and other animals
transfer pollen from one flower to female sex
organs of another.
– Some species that occur in dense
populations, like grasses, rely on the more
random mechanism of wind pollination.
• A flower is a specialized shoot with four
circles of modified leaves: sepals, petals,
stamens, and carpals.
Parts of the Flower
• The sepals at the base of the flower are
modified leaves that enclose the flower before it
opens.
• The petals lie inside the ring of sepals.
– These are often brightly colored in plant species that
are pollinated by animals.
– They typically lack bright coloration in wind-pollinated
plant species.
• Neither the sepals or petals are directly involved
in reproduction.
• Stamens, the male reproductive organs,
produce microspores that will give rise to
gametophytes.
– A stamen consists of a filament and anther
where pollen is produced.
• Carpals are female megaspores and their
products, female gametophytes.
– At the tip of the carpal is a sticky stigma that
receives pollen.
– A style leads to the ovary at the base of the
carpal.
Fruit
• A fruit is a mature ovary.
• Fruits protect dormant seeds and aid in their
dispersal.
• If a flower is not pollinated, the fruit does not
develop, and the entire flower withers and falls
away.
• In some plants, such as dandelions and maples,
the fruit functions like a kite or propeller,
enhancing wind dispersal.
• Many angiosperms use animals to carry seeds.
• Various fruit adaptations help disperse seeds
• Seeds can be carried by wind, water, or animals to new
locations
Angiosperm Reproduction
• All angiosperms are heterosporous, producing
microspores that form male gametophytes and
megaspores that form female gametophytes.
– The immature male gametophytes are contained within
pollen grains and develop within the anthers of
stamens.
• Each pollen grain has two haploid cells.
– Ovules, which develop in the ovary, contain the female
gametophyte, the embryo sac.
• It consists of only a few cells, one of which is the egg.
- Most flowers have mechanisms to ensure crosspollination between flowers from different plants of
the same species
• A pollen grain that has landed on a stigma
germinates and the pollen tube of the male
gametophyte grows down to the ovary
• The ovule is entered by a pore called the micropyle
• Double fertilization occurs when the 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 triploid endosperm nourishes the developing
embryo
• Within a seed, the embryo consists of a root and
two seed leaves called cotyledons
Angiosperm Diversity
• The two main groups of angiosperms are
monocots (one cotyledon) and eudicots
(“true” dicots)
• The clade eudicot includes some groups
formerly assigned to the paraphyletic
dicot (two cotyledons) group
• Monocots
– More than one-quarter of angiosperm species
are monocots
• Eudicots
– More than two-thirds of angiosperm species
are eudicots
Concepts 30.4 Our Need for 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
• 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
Problems Facing Plants
• Demand for space and natural resources resulting
from the exploding human population is extinguishing
plant species at an unprecedented rate.
• 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 plants = loss of potential that plant could
have at impacting human life
– Almost all of our food is based on cultivation of only
about two dozen species.
Figure 30.16
A satellite image
from 2000 shows
clear-cut areas in
Brazil surrounded
by dense tropical
forest.
4 km
By 2009, much
more of this same
tropical forest had
been cut down.
Figure 30.UN04
Five Derived Traits of Seed Plants
Reduced
gametophytes
Heterospory
Microscopic male and
female gametophytes
(n) are nourished and
protected by the
sporophyte (2n)
Male
gametophyte
Female
gametophyte
Microspore (gives rise to
a male gametophyte)
Megaspore (gives rise to
a female gametophyte)
Ovules
Integument (2n)
Megaspore (n)
Ovule
(gymnosperm) Megasporangium
(2n)
Pollen
Pollen grains make water
unnecessary for fertilization
Seeds
Seeds: survive
better than
unprotected
spores, can be
transported
long distances
Seed coat
Food supply
Embryo