Download Botany CHAPTER 23

CHAPTER 23
Plant Diversity
Section #1 The Evolution of Plants
I.
Plants Overcame Obstacles to Living
on Land
A. The Earth is believed to be 4.5 billion
years old. Scientists think that life on
Earth began about 3.5 billion years ago.
Fossil records indicate that nothing lived
on LAND until about 440 Million years
ago. Life was confined to the sea for the
first 3 billion years of Earth’s existence.
 B. Scientists feel that living things stayed
in the seas to avoid the intense solar
radiation that was coming from the sun.
At this time there was no protective
ozone layer in the atmosphere.
 C. With the advent of photosynthesis in
Earth’s oceans, oxygen gas began to
accumulate in the atmosphere. Some of
this oxygen was converted to ozone.
This ozone created a protective layer in
the Earth’s atmosphere that shielded the
Earth’s surface from much of the harmful
solar radiation, allowing organisms to live
on land instead of in the water.
 D. Both plants & fungi probably evolved
from multicellular protists. Multicellularity
enabled plants to develop the complex
structures that have helped them be
successful on land.
 E. Before these descendants of the
earliest plants could thrive in terrestrial
habitats, they had to overcome three
obstacles:
 1) They had to be able to absorb minerals from rocky
surfaces.
a. Mutualistic associations similar to mycorrhizae
have enabled the first plants to absorb minerals from
the rocky surfaces.
b. Mycorrhizae are symbiotic relationships
between fungi & the roots of some plants. Plants
provide carbohydrates and fungi absorb phosphorus
and other minerals that plants need.
c. 80% of all living plant species form
mycorrhizae today.
 2. They had to be able to conserve water.
 a. First plants lived near oceans, where water was
abundant. To live further away they had to develop
a way to conserve water.
 b. Plants developed a watertight outer covering
called a CUTICLE (waxy layer)
 c. Although a cuticle seals in moisture, it also seals
out the gases a plant needs for photosynthesis &
cellular respiration.
 d. The problem was solved with the
formation of specialized pore cells called
stomata. A pair of specialized epidermal
cells called guard cells border each
stoma and control its size by expanding
and contracting
 3. They had to have a way to reproduce
on land.
 a. The male gametes (sperm) of aquatic
plants were able to swim to fertilize the
female gametes. The gametes of plants that
live on land needed to be able to move in an
environment where water is not abundant.
In addition, they have to be protected from
drying out while being transferred.
 b. The eggs of the first plants were
surrounded by jackets of cells to prevent
drying, and a film of water was required
for a sperm to swim to an egg & fertilize
it.
 c. Today, mosses, ferns, and several
other groups of primitive plants still
reproduce in this way.
 d. In more advanced plants, the sperm
are enclosed in multicellular pollen grains
that keep them from drying out. This
allows them to be transmitted to female
gametes by wind or animals rather than
by water
II. A Vascular System Enabled
Plants to Thrive on Land
 A. One of the most important changes in
the structure of plants that occurred as
they adapted to land was the
development of an efficient way to move
water and other materials through the
plant body.
 B. In order to survive in environments
that have a limited water supply, most
plants need an efficient “plumbing”
system to carry water from their roots up
to their leaves and carry carbohydrates
from their leaves down to their roots.
 C. These plumbing systems consist of
specialized strands of hollow cells
connected end to end like a pipeline (see
Fig. 23-4 pg. 522)
 D. The tissues that transport water and
other materials within a plant make up
the vascular system. The word vascular
is derived from the Latin word vasculum,
meaning “vessel” or “duct”
 E. In the dominant group of plants today, the
cells of the vascular system run from near the
tips of the roots to the tips of the stems and into
the leaves. Not all plants have efficient
vascular systems, however. Of the twelve
phyla of living plants on Earth today, three
either have no vascular system or have only
very simple vascular tissue. The nine
remaining plant phyla have well-developed
vascular systems
 F. Non Vascular Plant Phyla
 Hepatophyta (Liverworts) – 6,000
species.
 Simplest plants; small, having a
dominant gametophyte with a
flattened or “leafy” body that lacks
vascular tissue, a cuticle, stomata,
roots, stems, and leaves.
 Anthocerophyta (Hornworts) 100
species
 Small, with a flattened, dominant
gametophyte that has stomata but
lacks vascular tissue, roots, stems,
and leaves
Liverwort & Hornwort
 Liverwort
 Hornwort
 Bryophyta (Mosses) 10,000 species
 Small: most have simple vascular
tissue, a sporophyte consisting of a
bare stalk and a spore capsule, and a
dominant, “leafy” green gametophyte
that lacks roots, stems, & leaves.
Moss
G.Vascular Plant Phyla
 1) Psilotophyta (Whisk ferns) 21 species
 Seedless, with a small, independent
gametophyte and a dominant sporophyte
that is highly branched and has tiny
leaves but is not differentiated into roots
and stems
 Sphenophyta (Horsetails) 15 species
 Seedless, with a small, independent
gametophyte and a dominant sporophyte
consisiting of roots and ribbed and
jointed stems with soft needlelike leaves
at the joints.
Whisk Ferns & Horsetails
 Lycophyta (Club Mosses)1,000 species
 Seedless, with a small independent
gametophyte and a dominant, mosslike
sporophyte with roots, stems, and leaves.
 Pterophyta (ferns) 12,000 species
 Seedless, with a small, independent
gametophyte and a dominant sporophyte
consisting of roots, horizontal stems, and
leaves called fronds; spores are
produced in clusters of sporangia on
lower surfaces of leaves.
Club Moss & Ferns
 Coniferophyta (Conifers) 100 species
 Gymnosperms with palm-like leaves;
produce male and female cones on
separate plants
 Cycadophyta (Cycads) 100 species
 Gymnosperms with palmlike leaves
 Produce male and female cones on
separate plants
 Ginkgophyta (Ginkgo) 1 species
 Gynmosperm deciduous tree with fanlike
leaves; produces conelike male
reproductive structures and uncovered
seeds on separate individuals
 Gnetophyta (Gnetophytes) 70 species
 Gymnosperms; diverse group of shrubs
and vines
Conifer, Ginkgo,
Gnetophyte, Cycad
 Anthophyta (Flowering Plants) 250, 000
species
 Angiosperms (seed plants with tiny
gametophytes, a large sporophyte, and
ovules enclosed by an ovary); a very
diverse group including trees, shrubs,
vines, and herbs that produce flowers
and fruits.
Angiosperms – flowering plants
III. Plants Evolved With
Alternation of Generations
 A Plants evolved with a distinctive pattern of
development in their life cycles.
 B. Among many algae, the zygote is the only
diploid (2n) cell, and it undergoes meiosis
immediately after fertilization (zygotic meiosis)
to form haploid (n) cells.
 C. In early plants, however, meiosis was
delayed. The zygote divided by mitosis to
produce many diploid cells that persist for a
long portion of the life cycle.
 D. As a result, plants developed alife
cycle in which a multicelled haploid
individual that produces gametes, the
gametophyte, alternates with a
multicelled diploid individual that
produces spores, the sporophyte.
 E. The pattern among life cycles in
which a haploid individual alternates with
a diploid individual is called Alternation of
Generations.
 F. As plants evolved, however, a
fundamental difference arose between
the life cycles of the simpler, nonvascular
plants and those of the more complex,
vascular plants:
 1) In the nonvascular plants (mosses &
liverworts), the gametophyte generation is the
dominant (most noticeable ) generation.
 2) In the vascular plants (ferns, gymnosperms,
& angiosperms), the sporophyte generation is
dominant. In fact, the very tiny gametophytes
of most vascular plants grow within tissues of
the sporophytes
 1. In the nonvascular plants (mosses &
liverworts), the gametophyte generation is the
dominant (most noticeable) generation.
 2. In the vascular plants (ferns, gymnosperms,
& angiosperms), the sporophyte generation is
dominant. In fact, the very tiny gametophytes
of most vascular plants grow within tissues of
the sprophytes.
IV. The First Plants
Lacked a Vascular system
 A. The first plants to successfully make
the transition to living on land probably
had no vascular system for transporting
materials throughout their bodies. All
materials had to be transported by
osmosis and diffusion, which greatly
limited the maximum size of the plant.
 B. Only two phyla of living plants, the
liverworts (Hepatophyta) and hornworts
(Anthocerophyta) completely lack a
vascular system. The members of these
two groups have historically been
grouped with the mosses and are still
frequently referred to as “bryophytes”.
 However, botanists no longer think that
these three groups of relatively simple
plants are directly related to one another.
 C. The genus Marchantia is a common
liverwort seen in Fig. 23-6 on pg. 525.
 D. The word “wort” meant “herb” in Old
English.
 E. The name liverwort dates back to
the Middle Ages, when it was
thought that plants resembling
certain body parts might contain
substances that could cure diseases
of those body parts.
 While the shape of some liverworts
resembles a liver, the dominant
gametophyte of most liverworts
consists of simple leaflike and
stemlike structures.
 F. Projections called rhizoids help
anchor liverworts to the surfaces on
which they grow.
 G. Gametes are formed by mitosis in
separate multicellular structures.
 1) Archegonia produce eggs
 2) Antheridia produce sperm
 H. When water is available, the
sperm swim to a nearby
archegonium and fertilize the egg
within it. The resulting zygote grows
into a very tiny diploid sporophyte.
V. Mosses Have Simple
Vascular Tissue
 A. The mosses (phylum Bryophyta)
include many species in which a central
strand of specialized conducting cells
distributes water and carbohydrates
throughout the plant. These conducting
strands make up what is called vascular
tissue
 This tissue is very simple in
bryophytes because the conducting
cells lack thickened walls
 B. Because their vascular tissue is
so simple, mosses are still grouped
with the liverworts and hornworts,
and all three groups are considered
to be nonvascular plants.
 C. The three phyla of nonvascular
plants share an important similarity.
Their life cycles, represented in fig.23-7
on pg. 526, are dominated by the
gametophyte generation. The
archegonia and antheridia of mosses are
produced on separate gametophytes.
 D. The moss sporophyte consists of
a bare stalk that supports a spore
capsule, or sporangium, in which
haploid spores are produced by
meiosis
VI. Vascular Plants Are
Characterized by Several
Features
 A. The first vascular plants appeared
approximately 430 million years ago, but
only incomplete fossils of these plants
have been found.
 B. The earliest known vascular plants for
which there are relatively complete
fossils are Rhynia and Cooksonia. (See
Fig. 23-8 on pg. 527
 C. Today, vascular plants occupy almost
all terrestrial habitats except those
perpetually covered by ice and snow.
Unlike the nonvascular plants and the
earliest vascular plants, many modern
vascular plants grow very tall.
 D. All vascular plants are distinguished
by the following features:
 1) A Dominant Sporophyte --- In
contrast to nonvascular plants, the
life cycles of vascular plants are
dominated by a diploid sporophyte
that is much larger than the
gametophyte.
 2) Specialized Conducting Tissue –
 a) Phloem --- Relatively soft-walled cells
that conduct carbohydrates away from the
areas where they are made
 b) Xylem – Hard-walled cells that transport
water and dissolved minerals up from the
roots.
 3) Distinctive Body Form- a) SHOOT – Above ground structures,
including the stem and leaves.
 b) ROOT – Below ground structures
 c) MERISTEM – Zones of actively dividing
cells that produce plant growth.
Section #2
 The Evolution of Seeds
I. First Vascular Plants
Lacked Seeds
 A. The first forests were composed
vascular plants that did not produce
seeds.
 B. Like the nonvascular plants, ferns,
and other seedless vascular plants have
swimming sperm and require a film of
water for fertilization. Forests of these
plants flourished in the warm, humid
climate of the late Paleozoic era
 C. In these forests, plenty of water was
available for successful reproduction.
 D. Ferns are the most abundant and
most familiar group of seedless vascular
plants today. Though they are found
throughout the world, ferns are most
abundant in the tropics.
 E. Many ferns are small, measuring
only a few centimeters in diameter.
However, some of the largest living
plants are tree ferns that can have
trunks more than 24 meters tall and
leaves up to 5 meters long.
 F. The fern life cycle, illustrated in Fig.
23-11 , represents an intermediate stage
in the revolutionary change that took
place in plant life cycles. Remember,
the life cycles of nonvascular plants are
dominated by a gametophyte that
supports a smaller, dependent
sporophyte.
 G. In ferns and other seedless
vascular plants, however, the
sporophyte is dominant, and the
gametophyte is smaller,
independent, and self-sufficient.
 H. The fern gametophyte is a thin,
heart-shaped photosynthetic plant
that lives in moist places and is
usually no more than 1 cm in
diameter.
 I. Fern gametophytes produce eggs in
archegonia and sperm in antheridia, both
of which are located on the lower surface
of the plant. In ferns, the archegonia and
antheridia are produced by the same
individual. In other seedless vascular
plants, the male and female structures
are produced by separate gametophytes.
 J. When a film of water is available,
sperm are able to swim to eggs and
fertilize them.
 K. Fern sporophytes consist of roots,
horizontal underground stems called
rhizomes, and long, often highly divided
leaves called fronds. Clusters of sporeproducing sporangia form on the lower
surfaces of fronds.
II. First Seed Plants Were
Gymnosperms
 A. Seeds apparently arose only
once among the vascular plants, as
the plant life cycle continued to shift
toward a more dominant sporophyte
generation and a more reduced
gametophyte generation.
 B. Of the 5 phyla of living seed plants,
four are collectively called gymnosperms.
The word gymnosperm comes from the
Greek words gymnos, meaning “naked”
and sperma, meaning “seed,” and refers
to the fact that gymnosperm seeds do not
develop within a fruit.
 C. First appearing about 380 million years
ago, gymnosperms were the first seed plants.
 D. The flowering plants, or angiosperms,
evolved from gymnosperms and make up the
fifth phylum of seed plants. The word
angiosperm comes from the Greek words
angeion, meaning “case” and sperma,
meaning “seeds” and refers to the fact that
angiosperm seeds develop within a fruit.
 E. First appearing between 150 and 200
million years ago, angiosperms are the
most recently evolved of all plant phyla.
 F.. As Fig. 23-12 illustrates, the
gametophytes of seed plants have
become highly reduced during the course
of evolution.
 1) Developing from spores that are
produced within the tissue of the
sporophyte individuals, the gametophytes
of seed plants are entirely dependent
upon those sporophyte individuals for
nutrients and water.
 2) Seed plants produce two kinds of
gametophytes:
 a) A very tiny male gametophyte, or
microgametophyte, that produces
sperm.
 b) Arelatively large female
gametophyte, or megagametophyte,
that produces eggs.
 3) Thus, the spores that produce
the microgametophytes are called
microspores and those that produce
the megagametophytes are called
megaspores.
 a) A pollen grain, which consists of
only a few haploid cells surrounded
by a thick protective wall, is a mature
microspore that contains a
microgametophyte.
 b) Each megagametophyte develops
from a megaspore within an ovule, a
multicellular structure that is part of
the sporophyte. If the inside of an
ovule is fertilized, the ovule and its
contents becomes a seed.
 G. Wind, insects, or other animals
transport pollen grains to the female
reproductive structures that contain
ovules. The transportation of pollen
grains from a plant’s male reproductive
structures to a female reproductive
structure of a plant of the same species
is called pollination.
 H. When a pollen grain reaches a female
reproductive structure, the pollen grain
cracks open. A pollen tube then grows
from the pollen grain to an ovule and
enables a sperm to pass directly to an
egg.
 I. Thus, in seed plants there is no need
for a film of water during the fertilization
process.
III. Most Living
Gymnosperms are Conifers
 A. Members of the most familiar
phylum of gymnosperms are trees
that produce seeds in cones and
thus are called conifers.
 1) Phylum Coniferophyta includes cedar,
cypress, fir, hemlock, pine, redwood,
spruce, and yew trees.
 a. The tallest living vascular plants, the
giant redwoods of coastal California
and Oregon, are conifers.
 1) Phylum Coniferophyta includes cedar,
cypress, fir, hemlock, pine, redwood,
spruce, and yew trees.
 a. The tallest living vascular plants, the
giant redwoods of coastal California
and Oregon, are conifers.
 b. One of the biggest redwoods, a giant
sequoia (Sequoia gigantea) named after
General Sherman of the Civil War, stands more
than 80 m tall and measures 20 m around its
base.
 c. Some individuals of another, much smaller
species of conifer, the bristlecone pine (Pinus
longaeva) that lives in the Rocky Mountains,
are more than 5,000 years old– the oldest
trees in the world!
 2) Most conifers have needle-like leaves
that are an adaptation for limiting water
loss. Conifers are often found growing in
seasonally dry regions of the world,
including the vast taiga forests of the
northern latitudes
 B. The Life cycle of a Conifer (Fig. 23-14 pg.
533)
 1. Conifers form two kinds of cones which
can be seen in Fig. 23-13.
 a. Seed cones produce ovules on the
surface of their scales. At the time of
pollination, the scales of a seed cone are
open, exposing the ovules
 b) Pollen cones produce pollen
grains within sacs that develop on
the surface of their scales. The
pollen grains of conifers are small
and light, and they are carried by
wind to seed cones.
 2. In pines and some other conifers, each
pollen grain has a pair of air sacs that help to
carry it in the wind. Because it is very unlikely
that any particular pollen grain will be carried to
a seed cone of the same species, a great
many pollen grains are needed to ensure that
at least a few succeed in pollinating the
species’ seed cones. For this reason, pollen
cones produce huge quantities of pollen grains.
 3) When a pollen grain lands near the
ovule on a scale of a female cone, a
slender pollen tube grows out of the
pollen grain and into the ovule. Thus, the
pollen tube delivers a sperm to the egg
inside the ovule.
 4. Fertilization occurs when the
sperm fuses with the egg, forming a
zygote that is the beginning of a new
sporophyte generation.
 5) Instead of growing directly into an
adult sporophyte (a tree)– just as you
grow directly into a n adult from a
zygote– the zygote first develops into a
small embryo and then becomes
dormant.
 6) While its further growth is
postponed, the zygote and the
sporophyte tissues that surround
and protect it form a seed.
 C. Other Gymnosperms (Fig. 23-15 pg.
533)
 1. Cycads (Phylum Cycadophyta), the
dominant land plant during the Jurassic
period, have short stems and palmlike
leaves and are still widespread
throughout the tropics.
 2) The only living species of ginkgo
(Phylum Ginkgophyta), the maidenhair
tree (Ginkgo biloba), has fan-shaped
leaves that are shed in the autumn.
 3) The gnetophytes (Phylum
Gnetophyta) are all very unusual.
Welwitschia mirabilis, perhaps the most
bizarre of all plants, is a gnetophyte that
grows in the harsh Namib Desert of
southwestern Africa.
IV. What is a seed?
 A. By providing the offspring of
plants with several survival
advantages, seeds have had an
enormous influence on the evolution
of plants on land.
 B. As you can see in Fig. 23-16, a
seed is a sporophyte plant embryo
surrounded by a protective coat.
The hard cover of a seed is called
the seed coat.
 C. Formed from the sporophyte tissue of
the parent plant, the seed coat protects
the embryo and other tissues in the seed
from drying out.
 D. In addition to their role in protecting a
plant embryo, seeds have enabled plants
to become better adapted to living on
land in at least three other respects:
 1) Dispersal—Seeds enable the offspring
of plants, which are anchored in one
place by their roots, to be dispersed to
new locations. Many seeds have
appendages, such as wings, that help
wind, water, or animals carry them away
from their parent plant.
 The dispersal of a plant’s offspring
prevents the parent and offspring from
competing with each other for water,
nutrients, light, and living space. Seed
dispersal also facilitates the migration of
a plant species to new habitats.
 2) Nourishment– Most kinds of seeds
have abundant food stored in them.
Playing a role similar to that of the yolk in
an egg, this food supply is a ready
source of energy for a plant embryo as it
starts its growth. Thus, seeds offer a
young plant nourishment during the
critical period just after germination when
the seedling must establish itself.
 3) Dormancy-- Once a seed has fallen
to the ground, it may lie dormant for
many years. When conditions are
favorable, particularly when moisture is
present, the seed will begin to grow into a
young plant.
 By remaining dormant until conditions
improve, seed enable plants to postpone
development during unfavorable
conditions such as a drought or a cold
period. Thus, seeds aid in synchronizing
the growth of a new plant with the season
of the year.
Section #3
 The Evolution of Flowers
I.
Angiosperms Achieved
Evolutionary Success On
Land
 A. 90% of all living plants—more
than 250,000 species of trees,
shrubs, herbs, fruits, vegetables,
and grains—are angiosperms.
 B. Virtually all of your food comes
directly or indirectly from angiosperms.
In fact, more than half of the calories that
humans consume come from just three
species of angiosperms:
 1. rice
 2. corn
 3. wheat
II. What is a Flower
 A. Flowers are the reproductive organs
of angiosperms.
 B. Many flowers are sophisticated
structures that are adapted to enable
insect pollination:
 1) Bright colors attract the attention of
insects
 2) Nectar, which is a sugary secretion of
many flowers, induces insects to enter a
flower.
 3) Pollen-bearing structures coat the
insects with pollen while they are visiting
a flower.
 4) Then, when the insects visit another
flower, they carry the pollen into that
flower.
 C. The basic structure of a flower, seen
in Fig. 23-18 pg. 536, consists of the four
concentric whorls (circles) of appendages
described below:
 1) Calyx – The outermost whorl of a flower,
derived from the Greek word kalyx, meaning
“cup”. The calyx is made of the sepals,
which are modified leaves that protect the
flower from damage while it is a bud.
 2) Corolla– Aname derived from the
Latin word corona, meaning “crown”.
The corolla consists of the petals, which
are also modified leaves. The petals are
often colored or scented to attract
pollinators.
 3) Androecium– The word comes from
the Greek words andros, meaning
“male”, and oikos, meaning “house”. The
androecium produces the
microgametophytes (pollen grains). It is
made up of one or more stamens, which
consist of slender, threadlike filaments
that are each topped by a pollencontaining sac called an anther.
 4) Gynoecium– The term comes from
the Greek words gyne, meaning “female”
and oikos, meaning “house”. The
gynoecium houses the ovules, in which
the megagametophytes develop. It
consists of one or more pistils that are
found in the center of a flower.
 The pistil is made of three parts:
 a. Ovary– The swollen lower portion that
houses the ovules.
 b. Style– A slender stalk that rises from the
ovary.
 c. Stigma – The swollen, sticky tip of the
style where pollen lands.
D. Flowers may or may
not have all four whorls
 1. A flower that has all four whorls of
appendages is called a Complete flower.
 2. If a flower has both a gynoecium and an
androecium, it is a perfect flower.
 3. Many flowers lack either a gynoecium or an
androecium and are called imperfect flowers.
 4. Incomplete flowers are those that lack any
one of the four whorls
III. Flowering Plants
Coevolved With Animals
 A. Insects do not visit flowers at random.
Instead, certain insects are attracted by
particular flowers. Insects and plants
have coevolved so that certain insects
specialize in visiting particular kinds of
flowers.
 B. An insect recognizes a particular color
pattern and searches for flowers with that
pattern. As a result, a particular insect
carries pollen from the flowers on one
individual to the flowers of another
individual of the same species. This
specificity is the key to successful insect
pollination, making it much more effective
than wind pollination.
 1. bees are the most numerous insect
pollinators.
 a. Today, there are over 20,000 species of
bees.
 b. Bees locate sources by odor first, then by
color. They are usually attracted to yellow or
blue flowers. These flowers often have lines
of dots that guide the bee to the location of
the nectar.
 c. While inside a flower, bees become
coated with pollen. This coating is far
from accidental. Most of the bees visiting
flowers actively seek the pollen, which is
rich in protein for their larvae.
 2. Butterflies tend to visit flowers that
have “landing platforms” on which they
can perch. These flowers are typically
tube-shaped and filled with nectar that
they can reach by uncoiling a long, hoselike mouthpart.
 3. Flies pollinate flowers that smell like
rotting meat.
 4. Moths, which visit flowers at
night, pollinate white, heavily
scented flowers that are easy to
locate in dim light.
 C. Many angiosperms are pollinated by
animals other than insects.
 1. Red flowers are typically visited by
hummingbirds. Birds, which have keen
vision, have a poor sense of smell. Knowing
this, it is not surprising that red flowers
usually lack a strong odor.
 2. Certain angiosperms have large,
heavily scented, and pale-colored
flowers that open at night. These
flowers are pollinated by another
nighttime visitor, bats.
 D.
Some angiosperms have
reverted to wind pollination, a
characteristic of their ancestors.
Examples include oaks, birches,
and grasses
IV. Double Fertilization Provided
Large Food Reserves in Seeds
 A. Unlike the seeds of gymnosperms,
the seeds of angiosperms develop a
highly nutritious tissue, called
endosperm, that originates at the same
time that an egg is fertilized.
 1. In some angiosperms (corn & wheat) the
endosperm is still present in mature seeds.
 2. In other angiosperms ( beans & peas)
the endosperm is completely transferred
into the embryo by the time the seed is
mature. The food reserves are then
stored in the embryo’s fleshy, leaflike
cotyledons (seed leaves).
 B. The angiosperms are divided into two
classes based on the number of
cotyledons in their seeds.
 1. Dicots (class Dicotyledones) have two
cotyledons. They typically have flower parts
in multiples of four or five, netlike veins, and
vascular bundles arranged in a ring.
 2. Monocots (class
Monocotyledones) have only one
cotyledon. They typically have
flower parts in multiples of three,
parallel veins, and vascular bundles
scattered throughout the cortex.
 C. Figure 23-22 on page 539 shows how
the angiosperm embryo and endosperm
originate at fertilization.
 1. The microgametophytes of seed
plants contain two sperm cells.
 2. In most gymnosperms, one of these
sperm dies.
 3. In angiosperms, however, both sperm
fuse with certain cells of the
megagametophyte.
 a. One sperm fuses with the egg, forming
the zygote.
 b. The other sperm fuses with the haploid
nuclei of the two other cells produced by
meiosis, forming a triploid (3n) cell that gives
rise to the endosperm.
 4. The term double fertilization is
used to describe the process by
which two sperm fuse with cells of
the megagametophyte to produce
both a zygote and the endosperm.
V. Fruits Enabled Efficient
Seed Dispersal
 A. A fruit consists of a mature ovary that
contains one or more seeds and often
includes other flower parts.
 B. Animals, which aid in pollination
because they are attracted to and obtain
nourishment from flowers, also aid in
seed dispersal.
 1. many mammals and birds are attracted to
and eat fruits that are fleshy and tasty.
 2. As fruits ripen, they often change from
green and odorless to brightly colored and
sweet smelling.
 3. The mature seeds within such ripe fruits are
often resistant to chewing and digestion.
 C. Most fruits, however, are not eaten by
animals.
 1. Some are specialized for sticking to an
animal’s fur
 2. Others are adapted for floating on wind
currents or water.