Evolutionary History of Plants

Evolutionary History of Plants
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flowers, double fertilization, endosperm, fruit
common ancestor
Flowering plants
Seed
seeds
megaphylls
Vascular
Gymnosperms
vascular
tissue
Seedless
Ferns and allies
apical
growth
microphylls
embryo
protection
Bryophytes
Mosses
Hornworts
common
green
algal
ancestor
Liverworts
Nonvascular
Lycophytes
Charophytes
550
500
450
400
350
Million Years Ago (MYA)
300
250
PRESENT
5

Gametophyte
Mitosis
Mitosis
n
n
n
Spore
n
Gamete
MEIOSIS
Apical meristem
of shoot
Developing
leaves
FERTILIZATION
2n Zygote
Haploid
Diploid
Mitosis
Sporophyte
1 Alternation of generations
Archegonium
with egg
2 Apical meristems
Antheridium
with sperm
Sporangium
Spores
1 µm
3 Multicellular gametangia
4 Walled spores in sporangia

Alternation of Generations
Life cycle involves alternation of generations
Sporophyte (2n):
Multicellular individual that produces spores by
meiosis
Spores is haploid cell that will become gametophyte
Gametophyte (1n):
Multicellular individual that produces the gametes
Gametes fuse in fertilization to form zygote
Zygote is a diploid cell that becomes a s
sporophyte
7

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spores
G
a
m
e
t
o
p
h
y
t
e
seed
seed
spores
S
roots
p
o
roots
r
o
rhizoids
roots
p
h
rhizoids
(n)
y
t
Moss
Fern
Gymnosperm
Angiosperm
e
(2n)
8

Other Terrestrial Adaptations
Vascular tissue transports water and nutrients to the
body of the plant
Cuticle provides an effective barrier to water loss
Stomata bordered by guard cells that regulate
opening, and thus water loss
9

Nonvascular Plants: Bryophytes
Nonvascular plants (bryophytes)
Lack specialized means of transporting water and
organic nutrients
Do not have true roots, stems, and leaves
 Dependent sporophyte consists of a foot, stalk and
sporangium
Gametophyte is dominant generation
 Produce eggs in archegonia
 Produce sperm in antheridia
 Sperm swim to egg in
film of water to make
zygote
10

Nonvascular Plants
Hornworts (phlym Anthocerophyta) have small
sporophytes that carry on photosynthesis
Liverworts (phylum Hepatophyta) have either
flattened thallus or leafy appearance
Mosses (phylum Bryophyta) usually have a leafy
shoot, although some are secondarily flattened
 Can reproduce asexually by fragmentation
11

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4. The sporophyte:
The mature sporophyte has a foot buried in female
gametophyte tissue, a stalk, and an upper capsule (the
sporangium), where meiosis occurs and spores are
developing
sporophyte
3. The zygote:
The zygote and
developing
sporophyte are
retained within
the archegonium.
capsule
5. The spores:
When the calyptra
and lid (operculum) of
a capsule fall off, the
spores are mature.
One or two rings of
teeth project inward
from the margin of
teeth
the capsule. The
operculum
teeth close the
opening, except when
the weather is dry.
Sporangium
calyptra
Mitosis
stalk
Sporophyte
zygote
diploid (2n)
FERTILIZATION
2. Fertilization:
Flagellated sperm
produced in
antheridia swim in
external water to
archegonia, each
bearing a single
egg.
sperm
MEIOSIS
haploid (n)
Spores
foot (n)
egg
Mitosis
Archegonia
buds
Protonema
Antheridia
1. The mature
gametophytes:
In mosses, the leafy
gametophyte shoots
bear either
antheridia
or archegonia,
where
gametes are
7. The immature
gametophyte:
A spore germinates
into a male or female
protonema, the first
stage of the male
and the female
gametophytes.
Gametophytes
rhizoids
(Top): © Heather Angel/Natural Visions; (Bottom): © Bruce Iverson
12
6. Spore dispersal:
Spores are released
when they are most
likely to be dispersed
by air currents.

Seedless Vascular Plants: Lycophyta
Club Mosses (phylum Lycophyta)
Typically, branching rhizome sends up short aerial
stems
Leaves are microphylls (have only one strands of
vascular tissue)
Most likely evolved as a simple side extensions of the
stem
Sporangia occur on surfaces of sporophylls

13
Grouped into club-shaped strobili

Seedless Vascular Plants
Roots evolved as lower extensions of the stem
 today’s lychophytes, also called club mosses, include
three groups of 1,150 species:
o Ground pines (Lycopodium)
o Spike mosses (Selaginella)
o Quillwort (Iseotes)
14

Seedless Vascular Plants: Pteridophytes
Includes Ferns and their Allies (horsetails and
whisk ferns)
Have megaphylls (broad leaves)


allow plants to effectively collect solar energy
Produce more food and the possibility of producing more offspring
than plants without megaphylls
Horsetails

rhizome produces tall aerial stems
 Contains whorls of slender, green branches
 Small, scale like leaves also form whorl at the joint
15

Microphylls and Megaphylls
single strand of
vascular tissue
a. Microphyll
branched
vascular tissue
Megaphyll
One branch began to
dominate the stem system.
branched
stem system
b. Megaphyll evolution process
16
The side branches flattened
into a single plane.
Tissue filled in the spaces
between the side branches.
megaphyll
leaf

Ferns
Whisk Ferns (phylum Psilotophyta)
Branched rhizome has rhizoids (extensions)
Mutualistic mycorrhizal fungus helps gather nutrients
Ferns (phylum Pterophyta)
Large conspicuous fronds
 divide into leaflets
Dominant sporophyte produces windblown spores
17

Figure 29.13-3
Key
Haploid (n)
Diploid (2n)
MEIOSIS
Spore
dispersal
Spore
(n)
Rhizoid
Underside
of mature
gametophyte
(n)
Sporangium
Sporangium
Antheridium
Young
gametophyte
Mature
sporophyte
(2n)
Sorus
New
sporophyte
Sperm
Archegonium
Egg
Zygote
(2n)
FERTILIZATION
Gametophyte
Fiddlehead (young leaf)
1 µm
In contrast with bryophytes, sporophytes
of seedless vascular plants are the
larger generation, as in familiar ferns

Seed Plants
Seed plants are the most plentiful plants in the
biosphere
Seed coat and stored food allow an embryo to survive
harsh conditions during long period of dormancy
Heterosporous
Drought-resistant pollen grains
Ovule develops into seed
19

Gymnosperms
 Gymnosperms have ovules and seeds exposed on the surface of sporophylls
 Conifers

Conifers, as well as other gymnosperm phyla, bear cones

Tough, needlelike leaves of pines conserve water with a thick cuticle and recessed stomata

Considered a “soft” wood because it consist primarily of xylem tissue (water flow)
 Cycads

Large, finely divided leaves that grow in clusters at the top of the stem

Pollen and seed coats on separate plants
 Ginkgoes

Dioecious: Some trees producing seeds and others producing pollen
 Gnetophytes

20
None have archegonia

Figure 30.3-3
If a pollen grain germinates,
Immature
it gives rise to a pollen tube
ovulate cone that discharges sperm into
the female gametophyte
within the ovule
Female
gametophyte (n)
Integument (2n)
Spore wall
Micropyle
Pollen grain (n)
(a) Unfertilized ovule
Spore
wall
Egg nucleus
(n)
Megaspore (n)
Megasporangium
(2n)
Seed
coat
Discharged
sperm nucleus
(n)
Pollen tube
Male gametophyte (n)
(b) Fertilized ovule
Food
supply (n)
Embryo (2n)
(c) Gymnosperm seed
A seed is a sporophyte embryo, along with
its food supply, packaged in a protective
coat

Angiosperms
Ovules are always enclosed within diploid tissues
Two classes of flowering plants


22
Monocotyledones (Monocots) – 1 cotyledon
Eudicotyledones (Dicots) – 2 cotyledons

Monocots vs Eudicots
 Number of cotyledons -- The cotyledons are the "seed leaves" produced by the embryo. They serve to





absorb nutrients packaged in the seed, until the seedling is able to produce its first true leaves and begin
photosynthesis.
 The number of cotyledons found in the embryo is the actual basis for distinguishing the two classes
of angiosperms, and is the source of the names Monocots ("one cotyledon") and Eudicots ("two
cotyledons").
Number of flower parts -- Monocot flowers = divisible by three, usually three or six. Dicot flowers =
multiples of four or five
 This character is not always reliable, however, and is not easy to use in some flowers with reduced
or numerous parts.
Leaf veins – Monocots = veins which run parallel the length of the leaf. Eudicots, = numerous veins
which reticulate between the major ones.
Stem vascular arrangement -- Vascular tissue occurs in long strands called vascular bundles. These
bundles are arranged within the stem of eudicots to form a cylinder, appearing as a ring of spots when
you cut across the stem. In monocots, these bundles appear scattered through the stem, with more of
the bundles located toward the stem periphery than in the center.
Root development -- In most eudicots (and gymnosperms) the root develops from the lower end of the
embryo, from a region known as the radicle. The radicle gives rise to an apical meristem which
continues to produce root tissue for much of the plant's life. By contrast, the radicle dies in monocots,
and new roots arise from nodes in the stem.
 These roots may be called prop roots when they are clustered near the bottom of the stem.
Secondary growth – Gymnosperm and eudicots increase their diameter through secondary growth,
producing wood and bark. Monocots (and some dicots) do not produce wood.

Figure 30.13ea
Monocot
Characteristics
Eudicot
Characteristics
Embryos
Two cotyledons
One cotyledon
Leaf
venation
Veins usually
netlike
Veins usually
parallel
Stems
Vascular tissue
scattered
Vascular tissue
usually arranged
in ring

Figure 30.13eb
Monocot
Characteristics
Eudicot
Characteristics
Roots
Taproot (main root)
usually present
Root system
usually fibrous
(no main root)
Pollen
Pollen grain with
one opening
Pollen grain with
three openings
Flowers
Floral organs
usually in
multiples of three
Floral organs
usually in multiples
of four or five

The Flower
Peduncle (flower stalk) expands at tip into a receptacle
 Bears sepals, petals, stamens, and carpels, all attached to receptacle in
whorls

Each stamen consists of an anther and a filament (stalk)

Carpel has three major regions

Ovary – swollen base

Style – elevates stigma

Stigma – sticky receptor of pollen grains

fruit
 Calyx (collection of sepals) protects flower bud before it opens
 Corolla (collection of petals)
26

Figure 38.2b
Anther
Germinated pollen grain (n)
(male gametophyte)
Ovary
Ovule
Embryo sac (n)
(female gametophyte)
Pollen tube
Egg (n)
FERTILIZATION
Sperm (n)
Key
Zygote
(2n)
Mature sporophyte
plant (2n)
Haploid (n)
Diploid (2n)
(b) Simplified angiosperm
life cycle
Germinating
seed
Seed
Seed
Simple
fruit
Embryo (2n)
(sporophyte)

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Stamen
Carpel
stigma
anther
style
filament
ovary
ovule
7. The sporophyte:
The embryo within a seed
is the immature sporophyte.
When a seed germinates,
growth and differentiation
produce the mature
sporophyte of a flowering plant.
The ovules:
In an ovule (megasporangium)
within an ovary, meiosis
produces four megaspores.
Mitosis
stigma
fruit
(mature ovary)
Sporophyte
1. The stamen:
An anther at the top of a stamen
has four pollen sacs. Pollen
grains are produced in pollen
seed
(mature ovule)
6. The seed:
The ovule now develops
into the seed, which
contains an embryo and
food enclosed by a
protective seed coat. The
wall of the ovary and
sometimes adjacent parts
develop into a fruit that
2. The pollen sacs:
In pollen sacs
(microsporangia) of the
anthe ,r meiosis produces
microspores.
Pollen sac
(microsporangium)
Ovule
(megasporangium)
The carpel:
The ovary at the base of a carpel
contains one or more ovules. The
contents of an ovule change
during the flowering plant life
cycle.
seed coat
embryo
endosperm (3n)
style
ovary
Anther
Seed
diploid (2n)
FERTILIZATION
MEIOSIS
haploid (n)
(Mature male gametophyte)
tube cell
ovule wall
polar nuclei
sperm
pollen
tube
sperm
egg
tube cell
nucleus
Microspores
Megaspores
ovule wall
antipodals
polar nuclei
egg
to
mi
pollen
tube
generative cell
sis
Pollination
to
mi
degenerating
megaspores
sis
5. Double fertilization:
On reaching the ovule, the
pollen tube discharges the
sperm. One of the two
sperm migrates to and
fertilizes the egg, forming
a zygote; the other unites
with the two polar nuclei,
producing a 3n (triploid)
endosperm nucleus. The
endosperm nucleus
divides to form
MEIOSIS
Pollen grain
synergids
Embryo sac
(mature female gametophyte)
4. The mature male gametophyte:
A pollen grain that lands on the carpel of the same
type
of plant germinates and produces a pollen tube,
which grows within the style until it reaches an ovule
in the ovary. Inside the pollen tube, the generative cell
nucleus divides and produces two nonflagellated
sperm. A fully germinated pollen grain is the mature
28
The mature female gametophyte:
The ovule now contains the mature female
gametophyte (embryo sac), which typically
consists of eight haploid nuclei embedded in a
mass of cytoplasm. The cytoplasm differentiates
into cells, one of which is an egg and another of
which contains two polar nuclei.
3. The microspores:
Each microspore in a pollen sac undergoes mitosis to become
an immature pollen grain with two cells: the tube cell and the
generative cell. The pollen sacs open, and the pollen grains
are windblown or carried by an animal carrier, usually to other
flowers. This is pollination.
The megaspores:
Inside the ovule of an ovar y , three megaspores disintegrate,
and only the remaining one undergoes mitosis to become a
female gametophyte.

Figure 30.2
PLANT GROUP
Mosses and other
nonvascular plants
Gametophyte Dominant
Sporophyte
Ferns and other seedless
vascular plants
Seed plants (gymnosperms and angiosperms)
Reduced, independent
(photosynthetic and
free-living)
Reduced (usually microscopic), dependent on surrounding
sporophyte tissue for nutrition
Reduced, dependent on
Dominant
gametophyte for nutrition
Dominant
Gymnosperm
Sporophyte
(2n)
Sporophyte
(2n)
Microscopic female
gametophytes (n) inside
ovulate cone
Gametophyte
(n)
Angiosperm
Microscopic
female
gametophytes
(n) inside
these parts
of flowers
Example
Gametophyte
(n)
Microscopic male
gametophytes (n)
inside pollen
cone
Sporophyte (2n)
Microscopic
male
gametophytes
(n) inside
these parts
of flowers
Sporophyte (2n)

Bryophytes
Gametophyte dominant;
sporophyte dependent;
gametophyte
independent
Seedless vascular
plants
Sporophyte dominant;
sporophyte initially
dependent;
gametophyte
independent
Seed plants
Sporophyte dominant;
sporophyte independent;
gametophyte
dependent & microscopic
Gymnosperms
Gametophytes develop
inside cones
Angiosperms
Gametophytes develop
inside flowers

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Ground Tissue
Parenchyma cells
–
–
–
–
–
Have thin and flexible primary walls
Lack secondary walls
Are the least specialized
Perform the most metabolic functions
Retain the ability to divided and diffrentiate
Collenchyma cells help support young parts of the plant shoot
 They have thicker and uneven cell walls
 The cells provide flexible support without restraining growth
Sclerenchyma cells are rigid because of thick secondary walls
strengthened with lignin
 They are dead at functional maturity
 There are two types:
1. Sclereids are short and irregular in shape and have thick
lignified secondary walls
2. Fibers are long and slender and arranged in threads

Roots
A root is an organ with important functions:
1. anchoring the plant
2. Absorbing minerals and water
3. Storing carbohydrates
In most plants, absorption of water and minerals
occurs near the root hairs, where vast numbers of
tiny root hairs increase the surface area

Figure 35.14
Epidermis
Cortex
Endodermis
Vascular
cylinder
100 µm
(a) Root with xylem and
phloem in the center
(typical of eudicots)
50 µm
Pericycle
Core of
parenchyma
cells
Xylem
Phloem
Endodermis
Pericycle
Xylem
Phloem
100 µm
(b) Root with parenchyma in the
center (typical of monocots)
Key
to labels
Dermal
Ground
Vascular

Figure 35.13
Cortex
Vascular cylinder
Epidermis
Root hair
Zone of
differentiation
Key
to labels
Dermal
Ground
Vascular
Zone of
elongation
Zone of cell
division
(including
apical
meristem)
Root cap
Mitotic
cells
100 µm

Most eudicots and gymnosperms have a taproot
system, which consists of:
A taproot, the main vertical root
Lateral roots, or branch roots, that arise from the taproot
Most monocots have a fibrous root system, which
consists of:
Adventitious roots that arise from the stems or leaves
Lateral roots that arise from adventitious roots

Stems
 A stem is an organ consisting of
 An alternating system of nodes, the point at which leaves are attached
 Internodes, the stem segments between nodes
 An axillary bud is a structure that has the potential to form a lateral
shoot, or branch
 An apical bud, or terminal bud, is located near the shoot tip and
causes elongation of a young shoot
 Apical dominance helps to maintain dormancy in most axillary buds
 Many plants have modified stems (eg. Rhizoids, bulbs, stolons, tuber)

The vascular tissue of a stem or root is collectively
called the stele
In angiosperms the stele of the root is a solid
central vascular cylinder
The stele of stems and leaves is divided into
vascular bundles, strands of xylem and phloem

Figure 35.17
Phloem
Xylem
Sclerenchyma
(fiber cells)
Pith
Epidermis
Cortex
Vascular
bundle
Ground
tissue
Ground tissue
connecting
pith to cortex
1 mm
(a) Cross section of stem with
vascular bundles forming a
ring (typical of eudicots)
Epidermis
Key
to labels
Dermal
Ground
Vascular
Vascular
bundles
1 mm
(b) Cross section of stem with
scattered vascular bundles
(typical of monocots)

Seeds
Seed coat, or testa can be impenertrable, especially in long
dormancy need
Cotyledons are leaf-like structures in the seed that provide
nourishment while the seed germinates
Radicle = embryonic root
Plumule = seed shoot
Epicotyl = portion of stem above the point where the stem is
attached to the cotyledon
Hypocotyl = portion of stem below the cotyledon
Endosperm = source of nutrients
Hillum = point of attachment of seed to the ovary wall
Micropyle = small opening near hillum

Secondary growth increases the diameter of stems and
roots in woody plants
 Secondary growth occurs in stems and roots of
woody plants but rarely in leaves
 The secondary plant body consists of the tissues
produced by the vascular cambium and cork
cambium
 Secondary growth is characteristic of gymnosperms
and many eudicots, but not monocots

Lateral meristems add thickness to woody plants,
a process called secondary growth
There are two lateral meristems: the vascular
cambium and the cork cambium
The vascular cambium adds layers of vascular
tissue called secondary xylem (wood) and secondary
phloem
The cork cambium replaces the epidermis with
periderm, which is thicker and tougher

Figure 35.11
Primary growth in stems
Epidermis
Cortex
Primary phloem
Shoot tip (shoot
apical meristem
and young leaves)
Axillary bud
meristem
Primary xylem
Pith
Vascular cambium
Secondary growth in stems
Lateral
Cork
meristems
cambium
Cork cambium
Periderm
Cortex
Primary
phloem
Root apical
meristems
Secondary
phloem
Pith
Primary
xylem
Secondary
xylem
Vascular
cambium