Plant Diversity
From Waterworld to Dry Land
The Greening of Earth

Looking at a lush landscape

It is difficult to imagine the land without any
plants or other organisms
Figure 29.1
Plants

Why Study Plants?





Plants Provide Ecosystem Services
Domestication and Selective Breeding
Plant-Based Fuels and Fibers
Bioprospecting
How to Study Plants



Analyzing Morphological Traits
Using the Fossil Record
Evaluating Molecular Phylogenies
Themes in the Diversification of Plants

The Transition to Land, I: How Did Plants Adapt to
Dry Conditions?


Preventing Water Loss: Cuticle and Stomata
Transporting Water: Vascular Tissue and Upright Growth
The Transition to Land, II: How Do Plants Reproduce in
Dry Conditions?
 Retaining and Nourishing Offspring: Land Plants as
Embryophytes
 The Evolution of Pollen
 The Evolution of the Seed
 The Evolution of the Flower
The Angiosperm Radiation
Why Study Plants?
Among the most important endeavors
supported by biological science

Agriculture
 Forestry
Horticulture

Tens
of thousands of biologists are employed in
research designed to increase the productivity
of plants and create new ways of using them
that benefit people.
Plants Provide
Ecosystem Services
Produce oxygen via oxygenic photosynthesis
 Build soil by providing food for
decomposers
 Hold and prevent nutrients from being lost
by erosion by wind and water
 Hold water
 Moderate the local climate.

Plants are eaten by
herbivores
which are eaten by
carnivores
which are eaten by
omnivores—organisms
that eat both plants
and animals.
Omnivores feed at
several different
levels in the
terrestrial food
chain.
Domestication and Selective Breeding
Humans have actively selected seeds to plant the next generation of
crops, a process called artificial selection
Plant-Based Fuels and Fibers
In addition to food, humans have
depended on plants for cooking and heating
fuels and as a source of fibers for clothing
and other things

Wood
has been replaced by other fuels.
Today, the primary interest in woody plants
is for building materials and fibers used in
papermaking.
Bioprospecting
The effort to find naturally occurring
compounds

Drugs
Fragrances
Insecticides
Herbicides
fungicides.
Hydroponics
The liquid culture of plants
 Can be used to harvest large
quantities of plant chemicals.

How Do Biologists Study Green
Plants?

To understand how green plants
originated and diversified, biologists use
three tools:
1.
2.
3.
They compare the fundamental morphological
features of various green algae and green
plants;
They analyze the fossil record of the lineage;
and
They assess similarities and differences in
molecular traits such as the DNA sequences
from selected genes
Morphological Traits

Important phyla of plants are
grouped into three categories:
1.
2.
3.
nonvascular plants
seedless vascular plants
seed plants
Seed plants

There are five major lineages in
the group:
1.
2.
3.
4.

5.

Cycads
Ginkgoes
Conifers
Gnetophytes
Collectively known as gymnosperms
angiosperms
Flowering plants
Fossil Record
The fossil record for land plants began 476
million years ago

It
is massive and is broken up into five
segments, each of which encompasses a major
event in the diversification of land plants
Fossil Record
Fossil Record

Fossilized spores and tissues

Have been extracted from 475-million-year-old
rocks
(a) Fossilized spores.
Unlike the spores of
most living plants,
which are single
grains, these spores
found in Oman are
in groups of four
(left; one hidden)
and two (right).
(b) Fossilized
Figure 29.6 a, b
sporophyte tissue.
The spores were
embedded in tissue
that appears to be
from plants.
Evaluating Molecular Phylogenies
Points From Phylogenetic Tree
Land plants probably evolved from green
algae.
The green algal group called Charales is the
sister group to land
1.
2.
•
3.
Charales are their closest living relative.
The green algae group is paraphyletic.
Points From Phylogenetic Tree
4.
5.
6.
7.
The land plants are monophyletic.
The nonvascular plants are the most basal
groups among land plants.
Morphological simplicity of the whisk ferns is
probably a derived trait.
Seeds and flowers evolved only once.
New Territory

For more than the first 3 billion years of
Earth’s history


The terrestrial surface was lifeless
Since colonizing land

Plants have diversified into roughly 290,000 living
species
Plants Evolved from Green Algae
Green algae have traditionally been considered
protists, but we study them along with land
plants for two reasons

they are the closest living relatives to land plants
 the transition from aquatic to terrestrial life
occurred when land plants evolved from green algae.
 charophyceans closest relatives

Green Algae
The green algae are a paraphyletic
group that totals about 7000
species. They have a double
membrane and chlorophylls a and b,
but relatively few accessory
pigments.

Adaptations Enabling the
Move to Land

In charophyceans


A layer of a durable polymer called sporopollenin
prevents exposed zygotes from drying out
The accumulation of traits that facilitated
survival on land

May have opened the way to its colonization by plants
Derived Traits of Plants

Five key traits appear in nearly all land plants
but are absent in the charophyceans

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Apical meristems
Alternation of generations
Walled spores produced in sporangia
Multicellular gametangia
Multicellular dependent embryos
Key Lineages of Green Plants
Green Algae
 Ulvobionta
 Coleochaetales
 Charales (Stoneworts)
Nonvascular Plants (“Bryophytes”)
 Hepaticophyta (Liverworts)
 Anthocerophyta (Hornworts)
 Bryophyta (Mosses)
Seedless Vascular Plants
 Lycophyta (Lycophytes, or Club Mosses)
 Psilotophyta (Whisk Ferns)
 Sphenophyta (or Equisetophyta) (Horsetails)
 Pteridophyta (Ferns)
Seed Plants
 Gnetophyta (Gnetophytes)
 Cycadophyta (Cycads)
 Ginkgophyta (Ginkgoes)
 Coniferophyta (Conifers)
 Anthophyta (Angiosperms)
Nonvascular Plants, Bryophytes


The most basal lineages of land plants
Three lineages with living representatives
Liverworts, phylum Hepatophyta
 Hornworts, phylum Anthocerophyta
Mosses, phylum Bryophyta

Not monophyletic
 Represent an evolutionary grade.
 Mosses are most closely related to vascular
plants

Bryophyte diversity
Gametophore of
female gametophyte
LIVERWORTS (PHYLUM HEPATOPHYTA)
Foot
Plagiochila
deltoidea,
a “leafy”
liverwort
Seta
Sporangium
Marchantia polymorpha,
a “thalloid” liverwort
500 µm
Marchantia sporophyte (LM)
HORNWORTS (PHYLUM ANTHOCEROPHYTA)
An Anthoceros
hornwort species
Saprophyte
MOSSES (PHYLUM BRYOPHYTA)
Polytrichum commune,
hairy-cap moss
Sporophyte
Gametophyte
Figure 29.9
Gametophyte
The life cycles of Bryophytes


Gametophytes are larger and longer-living than sporophytes
Bryophyte gametophytes




Some mosses


Have conducting tissues in the center of their “stems” and may grow
vertically
Bryophyte sporophytes

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Produce flagellated sperm in antheridia
Produce ova in archegonia
Generally form ground-hugging carpets and are at most only a few cells
thick
Grow out of archegonia
Are the smallest and simplest of all extant plant groups
Consist of a foot, a seta, and a sporangium
Hornwort and moss sporophytes

Have stomata
Hepaticophyta, Liverworts

Liver-shaped leaves
can grow on bare rock or tree bark, which
helps in soil formation

Anthocerophyta, Hornworts
The sporophytes look like horns
and have stomata

Bryophyta, Mosses
May be abundant in extreme
environments
 Can become dormant
 Sphagnum

species
are among the most profuse
Seedless Vascular Plants
Paraphyletic group
 Forms a grade between the nonvascular
plants and the seed plants
 Have conducting tissues with cells that
are reinforced with lignin, forming vascular
tissue.


The general groups of seedless vascular
plants
LYCOPHYTES (PHYLUM LYCOPHYTA)
Strobili
(clusters of
sporophylls)
Isoetes
gunnii,
a quillwort
Selaginella apoda,
a spike moss
Diphasiastrum tristachyum, a club moss
PTEROPHYTES (PHYLUM PTEROPHYTA)
Psilotum
nudum,
a whisk
fern
Equisetum
arvense,
field
horsetail
Athyrium
filix-femina,
lady fern
Vegetative stem
Strobilus on
fertile stem
Figure 29.14
WHISK FERNS AND RELATIVES
HORSETAILS
FERNS
Transport in Xylem and Phloem
Xylem

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Conducts most of the water and minerals
Includes dead cells called tracheids
Phloem


Distributes sugars, amino acids, and other organic
products
Consists of living cells
Evolution of Roots

Roots

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
Are organs that anchor vascular plants
Enable vascular plants to absorb water and
nutrients from the soil
May have evolved from subterranean stems
Evolution of Leaves

Leaves


Are organs that increase the surface area of
vascular plants, thereby capturing more solar
energy for photosynthesis
Leaves are categorized by two types


Microphylls, leaves with a single vein
Megaphylls, leaves with a highly branched vascular
system
One Model of Evolution

Microphylls evolved first, as outgrowths of
stems
Vascular tissue
Figure 29.13a, b
(a) Microphylls, such as those of lycophytes, may have
originated as small stem outgrowths supported by
single, unbranched strands of vascular tissue.
(b) Megaphylls, which have branched vascular
systems, may have evolved by the fusion of
branched stems.
Phylum Lycophyta: Club Mosses, Spike
Mosses, and Quillworts

Modern species of lycophytes


Are relics from a far more eminent past
Are small herbaceous plants
Lycophyta, Club Mosses

Most ancient plant lineage with roots

Tree-sized
dominated the coal-forming forests of the
Carboniferous period.

Psilotophyta, Whisk Ferns


Restricted to tropical regions
Have no fossil record
Sphenophyta (or
Equisetophyta), Horsetails
Flourish in waterlogged soils by allowing
oxygen to diffuse down their hollow stems

Pteridophyta, Ferns
only
seedless vascular plants to
have large, well-developed leaves