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New Topic: Introduction to
Plant Biology
Highlights
in Plant
Evolution
• Land plants
evolved
from green
algae
1
Overview of Plant Classification
I. Bryophytes: non-vascular plants with protected embryos;
lack phloem & xylem; includes mosses, liverworts,
hornworts
II. Seedless, Vascular Plants: possess cells joined into tubes
that transport water (xylem) or nutrients (phloem);
includes ferns and horsetails
III. Seed Plants: major advance in the occupation of
terrestrial habitats; the seed is a sporophyte embryo
packaged along with a protective coat
A. Gymnosperms: “naked seeds;” Lack enclosed
chambers (ovaries) in which angiosperm seeds
develop; includes cycads, ginkos, conifers: pines, firs,
redwoods
B. Angiosperms: flowering plants; most diverse: 250,000
species compared to 720 gymnosperms
Overview of Plant Classification
I.
Bryophytes:
non-vascular
plants with
protected
embryos; lack
phloem &
xylem;
includes
mosses,
liverworts,
hornworts
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Overview of Plant Classification
II. Seedless,
Vascular Plants:
possess cells
joined into tubes
that transport
water (xylem) or
nutrients
(phloem);
includes
lycophytes, ferns
and horsetails
Overview of Plant Classification
III. Seed Plants: major advance in the occupation of terrestrial habitats; the
seed is a sporophyte embryo packaged along with a protective coat
A. Gymnosperms: “naked seeds;” Lack enclosed chambers (ovaries) in
which angiosperm seeds develop; includes cycads, ginkos, conifers:
pines, firs, redwoods
3
Overview of Plant Classification
III. Seed Plants: major
advance in the occupation
of terrestrial habitats; the
seed is a sporophyte
embryo packaged along
with a protective coat
B. Angiosperms:
flowering plants; most
diverse: 250,000
species compared to
720 gymnosperms
Plant Life Cycle: Alternation of Generations
• Gametophyte: haploid
generation (n)
• Sporophyte: diploid
generation (2n)
• Two generations alternate
• Gametophytes form gametes
by mitosis
• Sporophytes produce spores
by meiosis
• Spore: develops directly into
adult
• Gamete: must combine with
other gamete to form zygote
4
Variations on Alternation of Generations
• a) Sporophyte dependent on gametophyte (bryophyte)
• b) Large sporophyte, small, independent gametophyte e.g., fern
• c) Reduced gametophyte dependent on sporophyte (seed plants)
Plants: Origin, Evolution, and
Diversity
• Diversity: major lineages existing today
– Liverworts, hornworts, mosses: non-vascular;
18,600 species
– Lycopods, horsetails, ferns: vascular, but
seedless; 13, 015 species
– Gymnosperms: seed plants: 721 species
– Angiosperms: seed plants with the unique
invention of the flower: 230,000 species
5
Transition to Land
• Advantages and challenges of living on
land
–
–
–
–
Sunlight and CO2 are more available
Threat of desiccation
Transporting water through the plant body
Establishing an upright body to compete for
sunlight
– Transporting gametes without water
Evolutionary adaptations to
terrestrial life
Challenges:
Adaptations:
Threat of desiccation
Transporting water
through the plant
body
Establishing an
upright body to
compete for sunlight
Transporting gametes
without water
– Cuticle and stomata
– Vascular tissue
– Lignin in cell walls and
an erect growth habit
– Reduced
gametophytes: male
gametophyte travels to
female as pollen
– Specialized methods of
photosynthesis: C4 and
CAM
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• Evolutionary adaptations to terrestrial life
–
–
–
–
–
Cuticle (wax) and stomata (pores)
Vascular tissue
Lignin in cell walls and an erect growth habit
Reduced gametophytes: male gametophyte travels to female as pollen
Specialized methods of photosynthesis: C4 and CAM
The first stomata
Plants from the Rhynie
Chert in Scotland
(385 million years
ago)
First evidence of
stoma:
• Pore: opening
• Guard cells: change
shape to open and
close pore
Guard cells
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Water Conducting Cells (of the
Xylem)
Water conducting cells are dead at
functional maturity; living interior
disintegrates and cell’s thickened
wall remains behind forming a nonliving conduit
• Elongated cells of two types:
• Tracheids:
– Long thin cells with tapered ends
– Water moves from cell to cell
through pits
• Vessel elements:
– Angiosperms
– Wider, shorter, thinner walled and
less tapered than tracheids
– Aligned end to end to form
micropipes
Wood: ligin in cell walls
LYCOPODS
During the Carboniferous
period, Lepidodendron trees
reached heights of 120 feet.
HORSETAILS
This extinct tree, called
Calamites, reached
heights of 80 feet.
SEED PLANTS
Some conifers, such as this
California redwood, can
reach heights of 300 feet.
8
Pollination & Fertilization in
Angiosperms
Angiosperm Life Cycle
9
Angiosperms: Development of
Endosperm
Angiosperms: Fruit Development
10
Plant/Animal Comparisons I
• Animals: most animals exhibit determinate
growth, i.e., growth of the individual ceases when
adult stage is reached
• Plants: growth is indeterminate; i.e., the
individual continues to grow after it reaches the
adult stage
• Growth is generated by specific regions of active
cell division called meristem (groups of cells that
retain the ability to produce new organs
indefinitely
• All plant organs arise from cell divisions in
meristem
Plant/Animal Comparisons II
Animals:
• Germ-line is sequestered early in development in most
animal groups (exceptions include cnidarians)
• Cells lose totipotency early due to differentiation
• Mutations in somatic cells are not transmitted to offspring
Plants:
• The meristem is perpetually embryonic tissue (retains
totipotency)
• Mutations that occur in meristem during development of
an individual can be transmitted to offspring
• Restricted case of the evolution of acquired characters
11
Plant Sensory Systems
• Plants possess surprisingly sophisticated
systems for responding to environmental
conditions, including:
– Light
– Gravity
– Touch
Plant responses involve receptors and
hormone-mediated signal transduction
12
Hormones Are Regulatory
Signals
• Hormones affect growth and development
and enable plants to deal with
environmental changes.
Figure 35.1a
Charles and Francis Darwin’s phototropism experiments
Light
Control:
Tip removed:
Tip covered:
Bends toward
light
No bending
No bending
Coleoptile
covered:
Bends
toward
light
Conclusion:
Light is sensed
at the tip of the
shoot.
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Figure 35.1b
DARWIN’S PHOTOTROPISM HYPOTHESIS
Sensing
tissue
Light
(stimulus)
Hormonal
signal
1. Cells at the
tip of the shoot
sense light.
2. Hormone
travels from the
tip down the stem.
Responding
tissue
3. Cells in the stem
respond to hormone.
Bending results.
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