Plant Diversity 1
How Plants Colonized Land
From Water to Land
 In order to survive the transition from
water to land, it was necessary for plants
to make adaptations for obtaining water
and to prevent them from drying out.
Reproduction in Water
Plants
 Algae reproduce with flagellated sperm
cells which require water for fertilization.
 Land plants must be able to reproduce
without water.
Surviving UV radiation
 Once plants emerge from the protective
cover of water, the genetic material
(DNA) is more susceptible to damage by
ultraviolet radiation.
Major Plant Adaptations
for Survival on Land:
 1. Except for primitive bryophytes, the
dominant generation of all plants is the diploid
sporophyte generation.
 A diploid structure is more apt to survive
genetic damage because two copies of each
chromosome allows recessive mutations to be
masked.
Major Plant Adaptations
for Survival on Land:
 2. All plants possess a cuticle, a waxy
covering on aerial parts that reduces
desiccation (drying out).
Major Plant Adaptations
for Survival on Land:
 3. The development of a vascular system
in plants further reduced their
dependency on water.
 Without a vascular system, all cells must
be reasonably close to water.
 A vascular system allows
for water to be distributed
throughout the plant.
Major Plant Adaptations
for Survival on Land:
 4. In the more primitive plants, flagellated
sperm require water to swim to the eggs.
 In the more advanced plants, the sperm,
packaged as pollen, are adapted for
delivery by wind or animals.
Major Plant Adaptations
for Survival on Land:
 5. In the most advanced division of
plants, the Anthophyta*, the
gametophytes are enclosed (and thus
protected) inside an ovary.
 *These are the flowering
plants.
Major Plant Adaptations
for Survival on Land:
 6. The most advanced plants (Conifers
and Flowering plants) have developed
adaptations to seasonal variations in the
availability of water and light. Ex:
Deciduous Trees
shed their leaves to
minimize water loss
during the dormant
season.
Desert wildflowers germinate, grow,
bloom and produce seeds during the
brief time there is water after a rain.
Key Idea!
 Understand how the various plant types
evolved:
 Chlorophytes → Bryophytes →
Seedless Vascular Plants →
Gymnosperms → Angiosperms
Chlorophyta—Green Algae
 Scientists have found enough evidence to conclude
that they are the common ancestors of land plants.
 They have both chlorophyll a and b.
 They have cellulose cell walls and store their
carbohydrates as starch.
 Some species show trends towards multicellular
colonies.
Microasterius
Cosmarium
Desmidium
Volvox colony
Bryophytes
 Bryophytes are
considered the most
primitive land plants.
 They were the first to
evolve from the
chlorophytes.
 They include
mosses, liverworts,
and hornworts.
Bryophytes’ Evolution
from Water:
 In order to survive on
land where water is not
unlimited, two
evolutionary adaptations
helped:
 1) A waxy cuticle to
protect against water
loss
 2) the packaging of
gametes in structures
known as gametangia.
The gametangia provide a safe
haven because the fertilization
and development of the zygote
occur within the protected
structure.
Alternation of Generations
in Plants:
 In plants, a multicellular sporophyte (diploid, 2N)
alternates with a multicellular gametophyte (haploid, 1N)
In plants, meiosis
produces spores. Spores
are haploid cells that
divide by mitosis to
become a multicellular
haploid organism, the
gametophyte.
Gametes are produced by
the gametophyte by
mitosis since it is already
haploid.
Alternation of Generations
in Plants:
 When both the gametophyte and sporophyte
stages are multicellular, the life cycle
demonstrates an alternation of generations.
The gametes fuse
and produce a
diploid cell that
grows by mitosis to
become the
sporophyte.
Specialized cells in
the sporophyte
divide by meiosis to
produce spores.
Bryophyte Reproduction
 In bryophytes, the gametes are packaged in
structures called gametangia.
 Bryophyte sperm is produced by the male
gametangia, called antheridia.
 Bryophyte eggs are produced by the female
gametangia, called archegonium.
 The gametangia provide a safe haven because
the fertilization and development of the zygote
occur within the protected structure.
Liverworts
 Liverworts are very
small plants (usually
less than 1” in size)
 Like all Bryophytes,
they live in very moist
places since they
reproduce with
flagellated sperm that
must swim through
water to fertilize the
eggs.
Liverworts
 Like other bryophytes, liverworts
do not have vascular tissues to
carry water.
 This lack of vascular tissue (plus
the fact they have flagellated
sperm cells) results in a
dependence on water.
 For this reason, bryophytes must
live in damp areas so they don’t
dry out.
Hornworts
Reproductive
structures at
the tips release
spores
Hornworts
 The group's common
name "hornwort"
refers to the tall
narrow sporophytes
which are embedded
in the top of the
plant.
Mosses
 Mosses are special
because, unlike in all
other plants, the
dominant generation
in their life cycle is
the haploid
gameophyte. (What
you see when you
look at a moss plant
is a collection of
haploid cells!)
Mosses
 The moss sporophyte (2N) is tiny, shortlived, and relies on the gametophyte (1N)
for nutritional support.
Tracheophytes—The First
Vascular Plants
 The transition from
water to land was
tricky—how to get
nutrients and water
from the soil up to all
parts of the plant?
 The answer: Phloem
and Xylem—
Vascular tissues that
transport materials.
Vascular Plants
 Xylem—water
superhighway,
transporting water
from the soil
throughout all parts
of the plant
 Phloem—Sugar food
superhighway,
transporting sugar
and nutrients.
Tracheophytes—First
Vascular Plants
 The first vascular
plants to evolve did
not have seeds.
 They live their lives as
sporophytes (2N).
 They produce spores
(haploid) which are
bisexual (neither male
nor female)
Ferns-Spores on Leaves
 The spores of ferns live on the underside
of the leaves. The spores will germinate
to form tiny gametophytes under the leaf
surface.
Fern Life Cycle
 http://academic.kellogg.edu/herbrandson
c/bio111/animations/0124.swf
Seed Plants
 Seeds are packages containing an
embryo and the food to feed the
developing embryo that is surrounded by
a nice protective shell.
Evolution of Seed Plants
 3 major changes led to the evolution of seed
plants:
 1. Further decline in the prominence of the
gametophyte generation of the life cycle.
 2. The birth of pollination
 3. The evolution of the seed.
Gymnosperms
 The first major seed plants to surface
were the gymnosperms (“naked” “seeds”)
The most important
gymnosperms are
the Conifers.
Their reproductive
structures are
cones.
They include pine
trees, firs, cedars
and redwoods.
Gymnosperms
Gymnosperms produce both
male and female gametes.
Gymnosperms transport their
sperm through the use of pollen.
These plants survive well in dry
conditions and keep their leaves
all year.
They usually have needles for
leaves.
Pollen
 Pollen is the sperm-bearing male gametophyte
(1N).
 Pollen grains have a hard coat that protects the
sperm cells during the process of their
movement from the stamens to the pistil of
flowering plants or from the male cone to the
female cone of conifers.
Gymnosperms Life Cycle
Flowering Plants
 The final major plant evolutionary
category to branch off are the flowering
plants.
 Today, there are more of these than any
other plants.
Angiosperms
 Flowers are the
reproductive
structures of the
angiosperms
(“covered” “seeds”)
Angiosperms
 The flower is a major evolutionary
advancement because:
 It attracts pollinators such as insects and birds
 The ovules are protected inside the ovary
 The ovary develops into
a fruit which fosters the
dispersal of seeds by wind,
insects, birds, mammals and
other animals.
Angiosperm Life Cycle
Major Evolutionary Trends
in the Plant Kingdom:
 Dominant gametophyte generation →
dominant sporophyte generation
 Nonvascular → vascular
 Seedless → seeds
 Motile sperm → pollen
 Naked seeds → seeds in flowers