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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Wood has been replaced by other fuels.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