Download Lecture III.6. Plants.

Lecture III.6. Plants.
Simplified plant phylogeny. The word “plants” is here taken
to mean “embryophytes” (aka “land plants”), i.e., multicellular photoautotrophs in which one of the life cycle stages is
an embryo. That excludes green algae, which can be multicellular, but which lack such a stage.
Introduction.
 “Plants” = “embryophytes”
= “land plants”.
1. Multicellular.
2. Chlorophyll a and b capture energy from light.
An even simpler phylogeny.
3. Store carbohydrates.
4. Develop from an embryo protected by parental tissue.
 Charophyte algae the sister group.
1. Use CaCo3 for support.
2. Multicellular. Differentiated into
rhizoids, thalli, branchlets.
3. Male (antheridia), female reproductive structures (oogonia).
4. Spores protected from desic-
cation by sporopollenin, an A stonewort. Note
impt. coat component of plant the orange male sex
organs (antheridia).
spores and pollen grains.
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 Embryophytes can be categorized as follows:
1. All Embryophytes:
a. Non-tracheophytes. Vascular (water / nutrient
conducting) tissue absent: Liverworts, hornworts, mosses.
b. Tracheophytes. Vascular tissue present.
2. Tracheophytes:
a. Seeds absent: Clubmosses, ferns and related.
b. Seeds present: Gymnosperms, angiosperms
and related.
3. Angiosperms (flowering plants):
a. Monocots. Grasses, palm trees,
etc. One seed leaf.
b. Dicots. Two seed leaves. The
majority of plant species.
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A more detailed plant phylogeny. Note the principal synapomorphies: Ability to live on land; vascular tissue; seeds and
flowers. If one defines “plants” as embryophytes, the presence
of chloroplasts with chlorophyll a and b is a shared ancestral
character.
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Alternation of Generations – Review.
 Multicellular haploid
diploid individuals.
and
 Multicellular sporophyte (2n)
1. Develops mitotically from
a diploid zygote.
2. Produces haploid spores
Alternation of generations.
by meiosis.
Compare with figure on page
3. Spores contained in struc- 18 of Lecture 3.2.
tures called sporangia.
 Multicellular gametophyte (n)
1. Develops mitotically from a haploid spore.
2. Produces haploid gametes – also by mitosis.
 Fusion of two gametes results in the formation of a diploid
zygote from which the sporophyte develops, etc.
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Alternation of Generations in Plants.
 In most plants, gametes produced by structures called
gametangia. Two kinds:
1. Antheridia produce sperm.
2. Archegonia produce eggs (one per archegonium).
Gametangia (singular, gametangium) protect the gametes from
desiccation and mechanical damage. The embryo develops in, and
is nourished by, the archegonium (plural archegonia).
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 Archegonial Zygote Retention.
1. Archegonium the site of fertilization.
2. Functions as a womb, in
which sporophyte develops.
3. Specialized transfer cells
function as a placenta.
Micrograph of a liverwort
gametophyte showing two
4. In flowering plants (angio- developing sporophytes and
sperms), these functions surrounding transfer cells.
performed by other structures.
 Charophyte algae (stoneworts) manifest archegonial retention of the zygote.
1. But these algae haplontic
– only the zygote is diploid.
Charophyte life cycle. The
2. Therefore, if charophytes
zygote is retained in the feembryophyte sister group,
male gametangium, which is
multicellular sporophyte a
called an oogonium.
derived trait.
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Moss and fern life cycles. In these groups, sperm cells swim to the egg.

In mosses,
the gametophyte dominates; in ferns, the sporophyte.
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 In non-tracheophytes, such as mosses, the gametophyte is the “plant.” The sporophyte remains attached to
the gametophyte on which it is nutritionally dependent.
 In tracheophytes, the sporophyte is the “plant”.
 Embryophyte evolution marked by gametophyte reduction and sporophyte enlargement.
Progressive reduction of the gametophyte and enlargement of the
sporophyte. a. In hornworts (non-tracheophytes), the sporophyte
sits atop, and is nourished by, the photosynthetic gametophyte. b.
In horsetails (non-seed tracheophytes), the gametophyte (left) is
much smaller than the sporophyte (right).
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Evolutionary Time Line.
 Cambrian. Molecular phylogenies suggest Cambrian /
Precambrian origins.
 Ordovician. Non-tracheophytes.
 Silurian. Possible tracheophytes.
 Devonian.
1. Tracheophytes for sure.
2. Trees / forests.
3. Seed plants.
 Carboniferous. Conifers.
 Cretaceous. Angiosperms.
 Cenozoic. Angiosperms dominate. Grasslands extensive.
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Left. Cooksonia (Silurian). Some species may have had xylem
and phloem. Note the terminal sporangia (spore-bearing structures. Living plant was about 1½ inches tall. Right. Thursophyton
(Devonian) with scale-like leaflets reminiscent of living clubmosses (primitive tracheophytes)
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A Devonian forest. Acanthostega, a labyrinthodont amphibian, is shown in the foreground.
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Plants have Coevolved with Other Organisms.
 Fungi – mycorrhizal associations provide nutrients.
 Herbivores.
1. Defense, e.g., phytolyths.
2. Chemical
defenses
–
“secondary compounds.”
 Pollinators.
1. Flowers attract pollinators.
2. Examples:
a. Bee flowers – yellow,
strong scent, concentrated nectar.
b. Hummingbird flowers –
red, tubular
c. Hawkmoth flowers –
long, white tubes.
Root tip, root hairs (arrows)
and mycorrhizae.
 Seed dispersers (animals) –
e.g. via fruit ingestion / elimiDatura and hawkmoth.
nation.
 Mixed interactions – Hawkmoth larvae eat Datura leaves.
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 Yucca–Yucca Moth. An Obligate Mutualism.
1. Fertilization requires services of the moth.
a. Pollen in pollinia – agglutinated masses of
pollen.
b. Ovary superior.
2. Female moth.
a. Collects pollinia with Yucca moth gathering pollen.
tentacles (males don’t
have them).
b. Chews them up to free the pollen grains
c. Pushes masticated pollinia down stigmatic canal. See
https://vimeo.com/7048122.
d. Injects eggs into the plant’s ovary
with specialized ovipositor.
3. Developing larvae eat the developing
seeds, exit through fruit wall and pupate in the soil near base of plant.
4. Yucca plants selectively abort fruits
with too many developing larvae.
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Transition to Land.
 Ancestors of plants lived in fresh water.
 Terrestrial existence offers abundant resources:
1. Light.
2. CO2.
 And it presents problems:
1. Desiccation / gas exchange.
2. Support / transport.
3. Acquiring water / nutrients from the soil.
4. Reproduction.
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Desiccation / Gas Exchange.
 Waxy cuticle – prevent evaporative loss.
 Stomata are pores that permit gas exchange.
1. Each pore surrounded by a pair of guard cells.
2. When the guard cells swell, the pore opens.
a. Normally, pores open during the day; close at night.
b. When the plant is water-stressed, stomata remain
closed during the day.
Cross section of a leaf. Cuticle (upper and lower surface) prevents water loss. Stomata (lower surface) permit gas exchange
via guard cells that swell when exposed to light if the leaf is adequately hydrated.
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Stomatal opening and closing. In response to light and water availability, potassium ions, 𝑲+ , are pumped into the guard cells. Water
follows. This causes the guard cells to bow outward, thereby opening the pore. The structural basis of this response is two-fold: (1)
Radially oriented cellulose microfibrils prevent guard cell diameter
from increasing. As a result, the response to water entry is stretching. (2) The thinner outer guard cell walls stretch more easily than
the thicker inner walls. This causes the cells bow away from each
as shown in the figure.
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Transport and Support.
1. Vascular plants have specialized structures to transport
water, minerals and sugars throughout the plant.
2. Xylem conducts water / minerals from roots to leaves.
a. Series of continuous “pipes” formed from dead cells.
b. Developed gradually in the course of plant evolution.
3. Xylem also provides support via secondary cell wall.
Evolution of water transport system (xylem) in embryophytes. Secondary cell wall formed of lignin provides strength. Tracheids (cells
with pores in the secondary cell wall) and vessel elements (cells with
larger openings penetrating both primary and secondary cell walls)
enhance transport. Phylogenetic note: Vessel elements evolved independently in gymnosperms and angiosperms.
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 Water Transport / Support in Non-Tracheophytes.
1. Some have simple conducting cells.
2. Many
a. Grow in dense moist masses through which water
moves by capillary action.
b. Have leaf-like structures that catch and hold water.
c. Small enough for nutrients to circulate by diffusion.
3. No support, they just sprawl.
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 Water Transport in Tracheophytes.
1. In aquatic environment, plant surrounded by water.
a. Cytosol ~ isotonic with seawater.
b. Cell wall prevents membrane rupture in freshwater
where the flow of water is into the cell.
2. On land, water acquired
from the soil and transported to the leaves.
a. Root hairs absorb water
from the soil.
b. Xylem transports the water upwards.
3. Transport driven by evapotranspiration.
a. Like sucking soda through
a straw.
b. Evaporation provides the
“suck” – tension.
c. Water molecules “stick”
together (cohesion).
d. Result is a column of water traveling up the xylem.
e. When the column gets too
long, it breaks. Limits tree
height.
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Top. Water transport from
soil to leaves. Bottom. Redwood leaves get smaller as
one goes up the tree.
 A Really Cool Experiment.
1. Observation: Leaf size
decreases as one goes up
a redwood tree.
2. Alternative Hypotheses:
a. Lack of water.
b. Increased sunlight (“sun”
vs. “shade” leaves).
3. Suggestive observation:
At 95 m., leaves of epiphytic1 redwoods larger
than those of host.
4. Experiment: Branches re- Top. Epiphyte and host tree
leaves. Bottom. Host tree
moved from the host tree at
branches grown in pots.
90 m. above the ground and
grown in pots expand. [Photos from Koch et al. 2004.
Nature. 428: 851-854.]
5. Conclusion: leaf size depends on water availability and
the degree to which leaves are expanded.
1
Obtain water from soil accumulated on host tree, i.e., in crooks & crannies.
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 Sucrose Transported by Phloem.
1. Sucrose actively transported
from source cells into the
phloem sieve tube cells.
2. Water taken up from nearby
xylem by osmosis.
3. Water pressure drives sap
down the sieve tube to
companion cells in roots.
4. Water moves back to the
xylem.
 Water transport in xylem does
not require energy expenditure
(water flows down concentration gradient). Sucrose transport, does.
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Plant phylogeny with support / transport synapomorphies. Note the independent evolution of vessel elements in angiosperms and gnetophytes (gymnosperms).
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Reproduction.
 Retention of embryo in archegonia reduces need for water.
1. Embryo develops in moist, internal environment. But,
2. In non-seed plants motile sperm must swim to the egg.
3. Complete independence of water not achieved until the
evolution of seeds and pollen.
 In non-seed plants, bisexual gametophytes produce both
antheridia and archegonia (homospory).
 In seed plants, two types of spores, and gametophytes
are male or female.
1. This condition called heterospory.
2. Microsporangia produce microspores that develop
into ♂♂ gametophytes that produce sperm.
3. Megasporangia produce megaspores that develop
into ♀♀ gametophytes that produce eggs.
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4. Micro- or megasporangia can be on the same or different individuals – i.e., sporophytes can be male, female or hermaphrodite.
 Pollen Grains
1. Male gametophytes, each
surrounded by a protective
outer coat of sporopollenin
(exine) and an inner coat of cellulose (intine).
2. Dispersed by wind or animal pollinators.
Pollen grain schematic.
3. Produce sperm that is delivered to the female gametophyte.
4. Result is fertilization, a diploid embryo and a seed.
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5. Non-angiosperm seeds contain
tissue from three generations.
a. Megasporangium (2n) – becomes the seedcoat.
b. Female gametophyte (n) – nutritive tissue.
c. Embryo (2n) – the new sporophyte. Development arrested until seed germinates.
6. In angiosperms (flowering plants),
a. ♀ gametophyte reduced to 7
cells and no longer nourishes Above right. Simpliembryo – i.e., there is no arche- fied schematic of an
angiosperm seed. a.
gonium.
seed coat; b. triploid
b. Nutritive tissue develops from trip- endosperm; c. seed
loid (3n) endosperm via a process leaf; d. embryonic
stem.
called double fertilization.
c. Seeds protected by ovary / fruit tissue.
7. In gymnosperms (conifers),
a. ♀ gametophyte reduced but still nourishes embryo.
b. No endosperm. Naked seeds (no ovary).
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Angiosperm Life Cycle.
Angiosperm life cycle. Pollen grain (male gametophytes surrounded
by protective coat) delivers sperm to female gametophytes via pollen
tube that grows through the stigma and eventually penetrates the megasporangium. Two sperm nuclei traverse the pollen tube. One fuses with
the egg to form the first cell of the diploid embryo. The other gives rise
to the triploid endosperm (next figure) that will nourish the developing
seedling. When the seed is fully developed, embryonic development ceases and does not resume until the seed germinates.
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Double Fertilization in Angiosperms.
In angiosperms, one sperm nucleus fuses with the egg producing
the embryo. A second sperm nucleus fuses with the two polar nuclei producing the triploid endosperm that nourishes the developing
plant after the seed germinates.
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Flowers.
 Key (along with seeds) to angiosperm success – 250,000+
species.
Flower structure. Pistil and stamens are respectively the female and
male parts. Ovaries contain ovules; ovules, megasporangia; and
megasporangia, megagametophytes. Likewise, anthers contain
microsporangia, etc. Petals / sepals are often brightly colored to attract
pollinators. Stamens, pistils, sepals and petals all evolved from leaves.
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Plant phylogeny with reproductive synapomorphies.
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Questions.
1. (6 pts) Plants transitioned from freshwater to land in the
Ordovician; vertebrates, in the Devonian. List and compare the adaptations with regard to gas exchange and
support that enabled each group to make the switch.
2. (6 pts) List and compare the adaptations regarding reproduction that enabled vertebrates and plants to transition
from freshwater to land.
3. (6 pts) Compare dispersal in plants and animals.
4. (6 pts) Physical constraints – inability to transport water to
a height of greater than about 120 m – limit the maximum
height of trees. Why should trees be tall in the first place?
Why don’t they allocate some of the energy that goes into
trunk tissue to making more leaves?
5. (4 pts) What are phytoliths? In what kinds of plants are
they found? How do they help the plant defend against
herbivores? Requires outside reading.
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