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 ForestryHorticulture

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 ProvideEcosystem 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Wood has been replaced by other fuels.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

DrugsFragrancesInsecticidesHerbicidesfungicides.

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. They compare the fundamental morphological features of various green algae and green plants;

2. They analyze the fossil record of the lineage; and

3. 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. nonvascular plants2. seedless vascular plants3. seed plants

Seed plants There are five major lineages in

the group: 1. Cycads2. Ginkgoes3. Conifers4. Gnetophytes Collectively known as

gymnosperms5. 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 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).

(a)

Fossilizedsporophyte tissue. The spores were embedded in tissue that appears to be from plants.

(b)

Figure 29.6 a, b

Evaluating Molecular Phylogenies

Points From Phylogenetic Tree

1. Land plants probably evolved from green algae.

2. The green algal group called Charales is the sister group to land

• Charales are their closest living relative.

3. The green algae group is paraphyletic.

Points From Phylogenetic Tree

4. The land plants are monophyletic.5. The nonvascular plants are the most basal

groups among land plants.6. Morphological simplicity of the whisk ferns

is probably a derived trait.7. 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 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 AnthocerophytaMosses, phylum Bryophyta

Not monophyletic Represent an evolutionary grade. Mosses are most closely related to vascular plants

Bryophyte diversity

LIVERWORTS (PHYLUM HEPATOPHYTA)

HORNWORTS (PHYLUM ANTHOCEROPHYTA) MOSSES (PHYLUM BRYOPHYTA)

Gametophore offemale gametophyte

Marchantia polymorpha,a “thalloid” liverwort

Foot

Sporangium

Seta

500

µmMarchantia sporophyte (LM)

Plagiochiladeltoidea,a “leafy”liverwort

An Anthoceroshornwort species

Saprophyte

Gametophyte

Polytrichum commune,hairy-cap moss

Sporophyte

Gametophyte

Figure 29.9

The life cycles of Bryophytes

Gametophytes are larger and longer-living than sporophytes Bryophyte gametophytes

Produce flagellated sperm in antheridia Produce ova in archegonia Generally form ground-hugging carpets and are at most only a

few cells thick Some mosses

Have conducting tissues in the center of their “stems” and may grow vertically

Bryophyte sporophytes 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)

PTEROPHYTES (PHYLUM PTEROPHYTA)

WHISK FERNS AND RELATIVES HORSETAILS FERNS

Isoetesgunnii,a quillwort

Selaginella apoda,a spike moss

Diphasiastrum tristachyum, a club moss

Strobili(clusters ofsporophylls)

Psilotumnudum,a whiskfern

Equisetumarvense,fieldhorsetail

Vegetative stem

Strobilus onfertile stem

Athyrium filix-femina, lady fern

Figure 29.14

Transport in Xylem and Phloem

Xylem 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 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

Microphylls, such as those of lycophytes, may have originated as small stem outgrowths supported by single, unbranched strands of vascular tissue.

(a) Megaphylls, which have branched vascular systems, may have evolved by the fusion of branched stems.

(b)

Figure 29.13a, b

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, Fernsonly seedless vascular plants to have large, well-developed leaves