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1. Key Concepts
 The green plants include both the green algae and the
land plants. Green algae are an important source of
oxygen and provide food for aquatic organisms; land
plants hold soil and water in place, build soil, moderate
extreme temperatures and winds, and provide food for
other organisms.
 Land plants were the first multicellular organisms that
could live with most of their tissue exposed to the air.
A series of key adaptations allowed them to survive on
land. In terms of total mass, plants dominate today's
terrestrial environments.

2. Green Plants
• The green plants consist of the green algae and land plants.
• Green algae are important photosynthetic organisms in
freshwater habitats, while land plants are the key
photosynthesizers in terrestrial environments.
• Green algae have traditionally been considered protists, but
we study them along with land plants for two reasons: (1)
they are the closest living relatives to land plants, and (2)
the transition from aquatic to terrestrial life occurred when
land plants evolved from green algae.

3. Analyzing Morphological Traits
• Biologists have long hypothesized that green algae are
closely related to plants on the basis of several key
morphological traits, including their chloroplast and
cell wall structures.
• The green algae include species that are unicellular,
colonial, or multicellular and that live in marine or
freshwater habitats.
• Based on morphology, the major phyla of plants are
grouped into three categories: nonvascular plants,
seedless vascular plants, and seed plants.

4. Analyzing Morphological Traits
• Nonvascular plants lack vascular tissue—
specialized groups of cells that conduct water
or dissolved nutrients from one part of the
plant body to another.
• Seedless vascular plants have well-developed
vascular tissue but do not make seeds.

5. Morphological Diversity: Nonvascular and Seedless Vascular Plants
Nonvascular plants do not have vascular tissue to conduct water and provide support.
Bryophyta (mosses)
Hepaticophyta (liverworts) Anthocerophyta (hornworts)
Seedless vascular plants have vascular tissue but do not make seeds.
Lycophyta (lycophytes Psilotophyta Sphenophyta Pteridophyta (ferns)
or club mosses) (whisk ferns) (horsetails)

6. Morphological Diversity: Nonvascular and Seedless Vascular Plants
Nonvascular plants do not have vascular tissue to conduct water and provide support.
Bryophyta (mosses)
Hepaticophyta Anthocerophyta (hornworts)
(liverworts)

7. Morphological Diversity: Nonvascular and Seedless Vascular Plants
Seedless vascular plants have vascular tissue but do not make seeds.
Lycophyta (lycophytes Psilotophyta Sphenophyta Pteridophyta (ferns)
or club mosses) (whisk ferns) (horsetails)

8. Analyzing Morphological Traits
• A seed consists of an embryo and a store of
nutritive tissue, surrounded by a tough
protective layer.
• Seed plants have vascular tissue and make
seeds.

9. Morphological Diversity: Seed Plants
Seed plants have vascular tissue and make seeds.
Ginkgophyta (ginkgo)
Cycadophyta (cycads) Other conifers
(redwoods, junipers,
yews)
Gnetophyta (gnetophytes) Pinophyta (pines, spruces, firs) Anthophyta (angiosperms
or flowering plants)

10. Analyzing Morphological Traits
• Within the seed plants, gymnosperms
produce seeds that do not develop in an
enclosed structure.
• In the flowering plants, or angiosperms, seeds
develop inside a protective structure called a
carpel.

11. Why Do Biologists Study the Green
Plants?
• Biologists study plants not only because they
are fascinating organisms but also because we
could not live without them.
• Agriculture, forestry, and horticulture are
among the most important endeavors
supported by biological science.

12. Plants Provide Ecosystem Services
• An ecosystem consists of all the organisms in a
particular area, along with physical components
of the environment such as the atmosphere,
precipitation, surface water, sunlight, soil, and
nutrients.
• Plants provide ecosystem services because they
add to the quality of the atmosphere, surface
water, soil, and other physical components of an
ecosystem.

13. Plants Provide Ecosystem Services
• Plants alter the landscape in ways that benefit
other organisms:
– They produce oxygen via oxygenic photosynthesis,
– They build soil by providing food for decomposers,
– They hold soil and prevent nutrients from being
lost to erosion by wind and water,
– They hold water in the soil, and
– They moderate the local climate by providing
shade and reducing the impact of wind on
landscapes.

14. Plants Provide Ecosystem Services
• Perhaps the most important ecosystem service provided by
plants involves food.
• They are the dominant primary producers in terrestrial
ecosystems and provide the base of the food chain in the
vast majority of terrestrial habitats.
• Plants are eaten by herbivores, which are eaten by
carnivores, or meat eaters. Some organisms are
omnivores—those that eat both plants and animals.
• Finally, green plants are the key to the carbon cycle on the
land.

15. Providing Food, Fuel, Building Materials, and Medicines
• Plants provide most of our food supply as well
as a significant percentage of the fuel, fibers,
building materials, and medicines that we use.
• Agricultural research began with the initial
domestication of crop plants.
• Artificial selection for plants with certain
properties has lead to dramatic changes in plant
characteristics.

16. Providing Food, Fuel, Building Materials, and Medicines
• Humans have historically relied on plant-based
fuels such as wood and coal.
• Plants provide us with important sources of raw
material for clothing and household articles.
• Woody plants provide lumber to build houses
and furniture, and to make paper.
• Plants are a key source of medicines.

17. Humans Have Relied on Plant-Based
Fuels
Plant-based fuels COAL FORMATION
1. Dead plant material
Wood accumulates in marshy or
Petroleum and boggy habitats.
Coal natural gas
What energy
sources do you
think will be
2. If oxygen in water is
important in
scarce, the organic
the future?
matter decays only
partially, forming peat.
Peat
3. If the peat deposits
Pressure
Pressure
are later covered by
sediments and
compressed, the
Sediments
resulting pressure and
heat change them
Coal into coal.

18. Humans Have Relied on Plant-Based
Fuels
Plant-based fuels
Wood
Petroleum and
Coal natural gas
What energy
sources do you
think will be
important in
the future?

19. Humans Have Relied on Plant-Based
Fuels
COAL FORMATION
1. Dead plant material
accumulates in marshy
or boggy habitats.
2. If oxygen in water is
scarce, the organic
matter decays only
partially, forming peat.
Peat
3. If the peat deposits
Pressure
Pressure
are later covered by
sediments and
compressed, the
Sediments resulting pressure and
heat change them
Coal into coal.

20. 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 DNA
sequences from homologous genes to estimate
phylogenetic trees.
• Let’s consider each of these complementary
strategies.