Download Plant Diversity I Notes

College Biology – Plant Diversity I
Overview:
Earliest plant-like life on earth was
cyanobacteria 1.2 billion years ago BYA
Fungi, plants on land 500 million years ago MYA
Tall plants and first forests 385 MYA
Today there are over 290,000 plant species
Plants inhabit most environments (except the most harsh
such as mountain tops, the poles, some deserts)
The presence of plants provided O2 and land plants
provided a food source for animals.
Land plants evolved from green algae
Green algae are a photosynthetic, eukaryotic,
unicellular organism in the domain Protista
(Protists).
Green algae (Charophytes) are the closest relative
to land plants. This is supported by morphological
and biochemical evidence.
1. Many key characteristics of land plants are also
commonly seen in many protists, including
 Multicellular, eukaryotic, photosynthetic
 Cell walls made of cellulose
 Chloroplasts containing chlorophyll a and b
2. Green algae are the only protists that share the
following traits with plants:
 Circular rings of proteins in the plasma
membrane that synthesize cellulose fibers for
the cell wall
 Peroxisomes containing enzymes that reduce
carbon loss during photorespiration
 Flagellated sperm (seen in some plants)
 Formation of a group of microtubules called
the phragmoplast that is involved in the
formation of the cell plate during cell division.
3. Genetic evidence shows that the Charophycean
green algaes are the closest relatives to plants.
Adaptions that allowed plants to move to land:
1. Ability to withstand periods of desiccation (drying
out). Over time natural selection favoured plants
that could survive not being submerged in water.
2. Charophyceans have a layer of durable polymer
called sporopollenin that prevents zygotes from
drying out.
Plants have sporopollenin around their spores,
allowing spores to withstand dry conditions.
Phylum Plantae
All plants are embrophytes – they produce embryos
(Whether or not green algae should be included in
Protista or Plantae is still being debated by
scientists).
Five key traits that show in modern plants but not in
algae:
(Note that not all plants show these traits, but
Charophyte algae do NOT show these traits)
1. Alternation of generations
In their life cycles, plants alternate between two
generations of multicellular organisms.
a. Gametophyte
b. Sporophyte
Each generation is multicellular and gives rise to the
other:
- The multicellular, haploid gametophyte
produces gametes (eggs and sperm),
through mitosis.
- The eggs and sperm fuse (fertilization) and
produce a diploid zygote.
- The diploid zygote develops into a
multicellular, diploid sporophyte.
- The sporophyte undergoes meiosis to
produce haploid spores, that then develop
into a haploid multicellular gametophyte .
The gametophyte and sporophyte generations often
look very different even though they are the same
plant.
2. Multicellular, dependent embryos
(the basis of the term embryophyte to describe land
plants)
 A multicellular embryo develops from the
zygote and remains attached to the female
parent, from which it receives water and
nutrients.
3. Walled spores produced in a Sporangium
 The sporophyte has organs called sporangia.
Cells within the sporangia undergo meiosis to
produce 1N (haploid) spores.
 The plant spores are haploid reproductive
cells that grow into haploid gametophytes
through mitosis
 The polymer sporopollenin makes the walls
strong and resistant to harsh environmental
conditions.
 Spores are often disperse through air
4. Multicellular Gametangia
Gametangia are structures that produce gametes
(egg and sperm)
 The female structure is called an archegonia
It is a single-pear shaped organ that produces
a single, non-motile egg.
 The male structure is called the antheridae,
and produces sperm that are released to the
environment.
5. Apical meristems
Apical meristems are localized regions at the tips of
stems and roots where much cell division occurs,
allowing for the elongation of roots and stems.
Plants evolved other traits that helped them live on land:
- Formation of a cuticle layer on the
epidermis: The cuticle is a layer of polymer
or wax that reduces water loss as well as
protects against too much water. It also
provides some protection.
- Development of symbiotic relationships
with fungi (mycorrhizae) that assisted
plants in obtaining nutrients from the soil.
- Production of chemical compound in
leaves that act to protect the plant from
herbivores or pathogens. We use many of
these as spices or medicines.
Origin and Diversification of Plants
1. Fossil tissue and spores that are similar to modern
plants were found in Omam and date to 475 MYA.
2. Ancestral plants have diversified greatly, producing
a huge variety of modern plants.
3. Land plants can be informally grouped based on the
presence or absence of vascular tissue.
Vascular tissue are groups of cells that are joined
into tubes to transport water and nutrients
throughout the plant.
Non-vascular Plants: (Bryophyta)
Includes mosses and liverworts
1. Basic characteristics :
 Do not have complex vascular systems.
 Have multicellular embryos and apical
meristems
 Lack true roots and true leaves
2. Bryophytes spend most of their life cycle in the
haploid gametophyte form, which is also larger
than the shorter lived sporophyte form.
3. The Bryophyte gametophytes grow low to the
ground in green, branched, one-cell thick
filaments. Like fungi, this form makes them very
efficient at absorbing nutrient from the soil.
The gametophyte is anchored to the ground by a
single celled, filament like rhizoid. The rhizoid is
not like a true root in that it is a single cell and
does not play an active part in absorbing water
and nutrients.
4. They are small and low to the ground because
the lack of vascular tissue that causes them to
rely on and be limited by the ability of diffusion to
move water and substances around the
organism.
5. Some mosses, however have primitive
conducting tissue in their “stems” that allows
them to grow taller, about 2 meters. Although
this tissue transports nutrients like vascular
tissue, it has a different structure and most likely
evolved independently from true vascular tissue
by convergent evolution.
Convergent evolution is the
independent evolution of similar features in species
of different lineages. Convergent evolution creates
analogous structures that have similar form or
function, but that were not present in the last common
ancestor of those groups.
6. The gametophytes are either male or female,
meaning the produce either archegonia or
antheridium, but not both.
The archegonia produce one egg each
The antheridium produces sperm with flagella
 The flagellated sperm swim through thin films
of water and into the archegonia to fertilize the
egg. The embryo develops within the
archegonia.
7. A dependent sporophyte:
The sporophyte cannot grow independently and
remains attached to the archegonium.
The sporophyte has three parts:
A foot that remains embedded in the
archegonium. The foot absorbs
nutrients from the parent gametophyte.
A seta, a stalk that moves nutrients and water
to the sporangium
A sporangium, a structure that produces
spores through meiosis. One sporangium
may produce 50,000 spores.
Ecological Importance of Mosses
Found all over the world
Spores are easily dispersed by wind
Common in moist forests and wetlands
Some are pioneer species
Some live in extreme environments like the
tundra and desert because they have
evolved ways to rehydrate after losing
water.
Economic Importance of Mosses
Sphagnum moss (aka peatmoss) is a
used in potting mixtures and gardening.
Peat is partially decayed organic matter found
in boggy regions. It forms deep layers,
and due to low oxygen, low temp, and
low pH, it decays very slowly.
Corpses 1000’s of years old have been
found very well preserved in peat bogs.
Peat is harvested and burned as a fuel.
Peat deposits contain about 30% of Earth’s
stored carbon. Burning of the peat returns the
carbon to the atmosphere, increasing the CO2
levels and adding to global warming.
Vascular Plants:
Vascular plants differ from non-vascular plants in that
they have complex vascular systems made up of two
types of cells (xylem and phloem) that transport nutrients
through out the plant, allowing them to grow tall.
Vascular plants make up the majority of plants.
Vascular Plant Characteristics:
1. Can be subdivided into two groups:
Seedless vascular plants: which lack seeds and
Include ferns and club mosses
Seed plants : which produce seeds.
Seed: and embryo packaged up with a food
source and protected by a seed coat.
- Seed plants have 2 divisions:
A. Gymnosperms
Greek gymno means “naked” and
sperm means seed.
The seeds of gymnosperms are not
enclosed in a container.
They include conifers (cone bearing
plants such as pine, fir, spruce)
B. Angiosperms
Greek angion means “container”
Seeds develop inside chambers called
ovaries, which are found in flowers and
develop into fruits.
90% of all plants are Angiosperms.
2. The sporophyte generation (2N) is the larger and
more complex plant, and the dominant form in the
alternation of generations.
 Ex: The leafy ferns we know are the
sporophyte. A fern gametophyte is a tiny
plant that grows at or below the soil surface.
3. Complex vascular tissue
Xylem conducts water and minerals
It is made up of dead, hollow tube like cells
called tracheids. During their development
the cytoplasm is lost so the cells can act as
pipes.
Phloem : living cells connected into tubes that
transport sugars, amino acids, and other
organic products.
Lignin – a strengthening polymer that stiffens the
cell walls of vascular tissue. Its presence
gives the plant support , and has added to the
ability of vascular plants to grow tall.
Added height allowed vascular plants to
disperse their seeds more effectively, and
disperse farther than shorter plants.
4. Evolution of Roots
Roots – organs that absorb water and nutrients from
soil and anchor plants to the ground.
Root tissue resembles fossil stem tissue,
suggesting that roots may have evolved from
underground stems.
5. Evolution of Leaves
Leaves increase the surface area of a plant
and act as the primary photosynthetic organ.
 Microphylls – small spine shaped leaves with
a single vein
(thought to be the first type of leaf to
develop, evolving from an outgrowth of
stems)
 Megaphylls – leaves with highly branched
veins
6. Sporophylls and Spore Variation
Sporophylls are modified leaves that produce
Sporangia. Ferns produce clusters of
sporangia (called sori) on the underside of
their sporophylls.
 Most seedless vascular plants are
homosporous, meaning they form one kind of
sporangium that produces only 1 kind of spore
that develops into a bisexual gametophyte
 All seed plants and some seedless plants are
heterosporous, meaning they form two types
of sporangium (archegonium and
anthergonium) which produce 2 types of
spores , male and female.
Classification of Seedless Vascular Plants
1. Lycophyta : club mosses
 most ancient form of vascular plant
2. Pterophyta: ferns and horsetails
 Most widespread seedless vascular plant
 More closely related to seed plants than
lycophyta
Significance of Seedless Vascular Plants
1. Grew to great heights and produced the first forests
in the Devonian and early Carboniferous periods.
2. The huge amounts of photosynthesis occurring in
those ancient forests removed much CO2 from the
air and could have contributed to the following ice
age.
3. An accumulation of dead organic matter from these
forests were eventually buried and formed coal
deposits.