Land Plants, Evolution and Diversity
Outline of the Lecture (primarily from
Wallace)
What are Plants
Alternation of Generations
Plant
Divisions
Plant
Adaptations to Land
Special
Adaptations of Angiosperms
Morphology of a Typical
Angiosperm
Essential
vocabulary
What are Plants
Plants are multicellular photosynthetic organisms
that are believed to have evolved
from green algae. Both groups have
chlorophylls a and b and betacarotene as their photosynthetic
pigments, both store reserve food as starch, and both have cellulose
containing cell walls.
Distinguishing Features of all
plants
- multicellular
- photosynthetic (chlorophylls a and b and
carotenoids in thylokoid membranes.)
- food storage molecule - amylose
starch
- structural polysaccharide -
cellulose
- life cycle - sporic alternation of
generations
- diploid sporophyte
- haploid gametophyte
- produce multicellular embryo protected in
multicellular haploid (gametophyte -egg sac) tissue which differs
from green algae.
The plant life cycle is sporic. This involves an
alternation between a multicellular diploid sporophyte which, through
meiosis produces haploid spores which in turn undergo mitosis to form
a multicellular gametophyte which makes, what else,
gametes.
The multicellular diploid embryo is housed within
a multicellular gametophyte (in algae the embryo is separate from the
haploid tissue).
Plant Divisions
Plants are divided into two major groups based on
the presence (in vascular plants) or absence (in nonvascular plants)
of an internal vascular system for transporting water and dissolved
particles.
The nonvascular
plants -- bryophytes
-- require a constantly moist environment. They include:
Vascular
plants or tracheophytes include:
seedless vascular plants such
as -
seed- bearing forms. The success of
seed
plants (gynmosperms and angiosperms
may be attributed to:
- development of an extensive root
system
- an efficient vascular system (xylem
and phloem)
- reproductive structure in which the
gametophyte is protected inside sporophyte tissue (the
seed)
The seed plants include:
Plants
- Adaptations to Land
Some of the adaptations of plants to a terrestrial
existence include a waxy cuticle, surface pores (stomata) that enable
gas exchange, protected reproductive structures, and the retention of
the embryonic sporophyte within the female gametophyte. Review the
table below for more adaptations for life on dry land.
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Problems
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Solutions
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Spatial Segregation of
Resources
Minerals in the soil and
water
Light and CO2 in the
air
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Regional
Specialization
Roots
- lack wax and chlorophyll
- have large surfaces area (aided by
mycorrhiza
- have vascular tissue for conduction
of food and water
Leaves and Stems
- have waxy coverings to prevent water
loss
- chloroplasts can move to obtain the
best arrangement for absorption of light
- turgor pressure prevents
wilting
- stomata regulate water loss and
CO2 absorption
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Gravity
prevents vertical growth
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Lignin
reinforces cellulose. This
skeletal support allows turgor pressure to increase to
help maintain rigidity.
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Increase in
Height
requires mechanism to prevent
water loss and conduction of water more
effectively
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Vascular Transport
System
allows plants to grow taller in
order to reach light. The vascular system is made of
microscopic pipes, the xylem and phloem. Xylem is
lignified adding to the other supporting tissues.
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Adaptations to Dryness
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Seeds
help disperse plants and are
able to survive the harshest conditions. Some can survive
for years waiting for just the right conditions to
sprout.
Pollen
Pollen, the male gametophyte, is
designed to travel great distances by wind or
insect
Dominance of Sporophyte
Generation
The diploid condition exhibits
all the specialized tissues described above.
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Special Adaptations of
Angiosperms
Flowering plants are the most numerous of the
modern plants. Many of their flowers, designed to attract
pollinators, are the product of coevolution with insects (and
other animals) resulting in an efficient means of uniting sperm and
egg. Their fruits are often designed to aid in the dispersal of their
seeds.
Flower Specialization
How -
- colors, nectar, and
fragrances
- radial vs. bilateral
symmetry
- incomplete vs. complete
flowers
- perfect vs. imperfect
flowers
- single vs. composite flowers
Why -
- prevents self fertilization
- reduces or minimizes energy
needs
- attracts insect pollinators
Pollination
Pollen grains may be transferred from
anther to stigma without the help of moisture.
- wind and millions of tiny, buoyant
pollen grains
- coevolution between flowers and
pollinators such as bees
Double Fertilization
- A tube emerges from the coat of the pollen
grain (male gametophyte) and
- digests its way through the
style.
- Generative and tube nuclei become
sperm.
- The sperm enter egg sac via pollination
tube through micropyle
- one sperm unites with egg forming the
zygote
- the other sperm nucleus unites with a polar
cell containing two nuclei- endosperm is formed (triploid
tissue).
Morphology of a Typical Angiosperm
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The plant body is divided into a
root system and a shoot or stem system,
connected by vascular tissue that is continuous throughout
the plant. The root system of this dicot consists of a
taproot and several lateral roots. Shoots consist of stems,
leaves, and flowers. The blade, the expanded portion of a
leaf, is attached to a stem by a petiole. Nodes, the regions
of a stem where leaves attach, are separated by internodes.
At a shoot's tip is the terminal bud, the main growing point
of the shoot. Axillary buds are located in the upper angles
of leaves. Most of these axillary buds are dormant, but they
have the potential to develop into vegetative (leaf-bearing)
branched or flowers.
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