Transcript Spore-Forming Plants

Spore-Forming Plants
The Lower Plants
Classification of Life
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If we go back to ancient Greek times,
Aristotle (350 BC) was the first and
most influential classifier of the world.
He classified living things into plants
and animals. Three sub-groups for
each: for plants: its size, and for
animals, where it primarily lives.
This matches our naïve view of the
world: living things come in two basic
varieties: those that move around
(animals) and those that are rooted to
the ground (plants).
– he wasn’t thinking about evolution
or phylogeny!
Aristotle’s views were considered
definitive, about this and everything
else in the physical world, up until the
development of science in the 1600’s.
Linnaeus
• Carolus Linnaeus (1707-1778) a.k.a. Carl Linne
– Swedish naturalist: Latinized name
• Like Aristotle, he classified the world into plants and
animals, but divided these groups up by the presence of
various traits.
• Plants were grouped into genera and given multi-word
Latin names. Linnaeus shortened this to the binomial:
genus followed by species. (like Homo sapiens)
• He also grouped them into larger groups (classes) based
on sexual characteristics: the Sexual System of
Classification
– For example: "Nine men in the same bride's chamber,
with one woman“. This meant 9 stamens with 1 pistil in
the same flower. (All in Latin, of course)
• although this system was invented for convenience, it fit
evolutionary reality fairly well, and we still basically use
it (with modifications and corrections)
• He also invented the kingdom-phylum-class-orderfamily-genus-species hierarchy.
Five Kingdom Model
• New things kept being discovered, and
several things became obvious:
– fungi aren’t plants,
– bacteria and other prokaryotes are
fundamentally different from plants and
other eukaryotes
– single celled eukaryotes (protists) are not
easily classified as plants or animals or
fungi
• This led to the Five Kingdom model of R.
H. Whittaker, in 1969.
– The Monera are the prokaryotes: the
domains of Bacteria and Archaea. Monera
is not a term used today.
– Plants evolved from protists, but where is
the boundary between them?
Three Domain System
• The Three Domain system was invented
Carl Woese, based on sequencing
ribosomal RNA.
• It shows a more realistic view of the world
of life:
– two very different types of prokaryote
– Most eukaryotes are protists
– Protists aren’t a monophyletic group.
• Classification today is based on DNA
sequencing.
– Relatively easy to do
– DNA is what is actually inherited between
organisms
– We understand the mechanisms of change
(mutation) in DNA
• DNA-based phylogeny matches traditional
phylogeny reasonably well, but lots of
changed details.
– Especially in the less familiar groups and in
very ancient branchings.
What is a Plant?
• Somewhere there is a boundary between photosynthetic
algae (protists) and plants, a logical place to divide them.
• We want to distinguish between plants and algae on the basis
of two things:
– Plants should be a monophyletic group (a single common ancestor and
all descendants) as shown by solid DNA evidence.
– Plants should match our naïve view of the world as primarily
photosynthetic land-dwelling organisms rooted to the ground.
• Not that we can't have some exceptions, but they need good
explanations.
Some Possible Divisions
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Start with the protist group the Archaeplastida.
This group came from the primary endosymbiosis
of a cyanobacterium that created the plastid
(chloroplast).
– We could call this whole group “plants”: anything
containing a plastid that is not the result of
secondary endosymbiosis. (recall that many protists
developed a secondary endosymbiosis with
eukaryotic red or green algae).
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Quite early, this groups split into the red and green
algae.
– Red algae contain phycoerythrin, a red pigment that
helps gather light deep in the water.
– Green algae contain chlorophyll b and in many ways
is very similar to land plants. Green algae have long
been thought to be ancestral to plants, and DNA
evidence has confirmed this.
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Some would like to include green algae and plants
in a monophyletic group called Viridiplantae:
“green plants”.
– Probably not widely accepted due to the tradition of
keeping algae as a separate group.
Green Algae
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Green algae come in many
varieties: mostly unicellular, but
some are colonial, and a few are
genuinely multicellular.
All green algae have:
– two membranes surrounding the
chloroplast (i.e. primary
endosymbiosis)
– Chlorophyll a and b, along with
carotene (yellow) and xanthophylls
(orange-brown)
– Cellulose and pectin in the cell
wall.
– Starch for food storage (something
to live on when it’s dark). Red
algae use a different form of starch
called “Floridean starch”.
More Green Algae
• Most are aquatic (marine or fresh
water), but some live on land.
– There’s a type of algae that lives on snow
fields. Has a red pigment to block UV light.
• Some land algae are symbionts with
fungi (lichens).
• Most have both sexual and asexual
reproduction, with gametes having 2
flagella.
• Most are primarily haploid, although a
few have a diploid phase as well.
• Possible use of green algae for biofuels.
They are very efficient at converting
sunlight into chemical energy, and easy
to grow.
Charophytes
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The green algae can be split into two
main groups: the chlorophytes and the
charophytes.
– Most are chlorophytes, including
single celled forms and colonial forms.
– The charophytes, especially a group
called the stoneworts (Charales), are
the green algae group most closely
related to plants. Based on DNA
evidence.
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They have whorls of short branches
surrounding a central stem. No leaves,
just photosynthetic branches.
They live in shallow water and lake
margins: occasionally dry out.
Charophytes are the most structurally
complex of the green algae.
Charophytes as the Sister Group to Plants
• Some characters shared by charophytes
and plants, and no other groups: shared
derived characters, used to define
monophyletic groups.
– Charophytes protect their embryos
with a layer of sporopollenin, a
tough polymer whose composition
isn’t precisely known. Land plants
use it to cover spores and pollen
grains.
– Phragmoplast: A structure used in
cell division, it is the scaffold on
which the new dividing wall between
the 2 daughter cells forms.
Embryophyte = Land Plant
• The current most common definition
of “plant” excludes all green algae.
The group is called the Embryophyta.
• The defining characteristic of an
embryophyte is that they carry the
multicellular embryo within the
mother’s body. That is, the female
gamete is fertilized inside the mother
plant’s body, and it continues to
develop there, using nutrients from
the mother (a placenta).
• Embryophyte and land plant are
synonyms: they refer to the exact
same group of organisms.
Evolutionary Trends
• By moving onto the land, plants had to deal with 2 big issues:
gravity ( or lack of buoyancy) and dryness.
• Major trends:
1. development of roots, shoots, vascular system. Roots needs to
absorb nutrients, not just hold onto the surface. Shoots need to
support photosynthetic system off the ground. Vascular system to
transport materials between parts of the plant.
2. Waxy cuticle on the plant surface to prevent desiccation.
3. Increasing the diploid phase of the life cycle, and decreasing the
haploid phase. Diploid gives a backup copy of each gene, as a defense
against random mutations. Allows a larger, more complex body.
4. Spore, seed and pollen protection and dispersal. How can they be
protected, how can the male gametes find the females, and how can
new individuals disperse to new locations.
Major Plant Groups
• Four groups, some of which have more
than one phylum:
– Bryophytes. Mosses, liverworts and
hornworts. No vascular system. The most
primitive plants.
– Seedless vascular plants. Ferns of various
types. Have vascular system but use spores
to reproduce.
– Gymnosperms. Conifers, cycads, ginkos.
Have seeds, a major innovation (so the next
set of lecture notes starts here.
– Angiosperms. Flowering plants: most of
the common plants. Seeds develop in an
ovary.
• Note that some groups aren’t
monophyletic. They are probably
independent branches off the main
evolutionary line. We will largely ignore
this, however.
Plant Phylogeny
Alternation of Generations
• The sexual cycle in eukaryotes has a diploid phase
and a haploid phase.
– Diploid: 2 copies of each chromosome, one from
each parent
– Haploid: only 1 copy of each chromosome.
– Fertilization: two haploid cells (the gametes)
combine to form a new diploid cell, the zygote.
– Meiosis: a diploid cell undergoes a special form of
cell division that results in 4 haploid cells.
• Alternation of generations means that both the
diploid and haploid phases are multicellular.
– Humans do not have alternation of generations:
haploid phase is 1 cell only
– Most fungi do not have alternation of generations:
diploid phase in 1 cell only.
– Plants do have alternation of generations
Alternation of Generations in Plants
• The diploid phase is called the sporophyte. Some cells in the sporophyte
undergo meiosis, which produces haploid spores. This occurs in a
multicellular structure called a sporangium.
– Spores are unicellular and packaged to survive harsh conditions.
• Spores germinate into a new haploid plant, the gametophyte. Some cells
in the gametophyte develop into haploid reproductive cells, the gametes.
The gametes are the equivalent of human sperm and eggs. Production of
gametes occurs in a multicellular gametangium.
• Two gametes fuse together during fertilization, producing a zygote, a
single diploid cell that is the first cell of the new sporophyte.
• The zygote develops into an embryo (multicellular diploid) attached to the
mother. The embryo is then released from the mother. It starts growing
as an independent sporophyte.
Male and Female
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In the fungi (and many protists), the gametes are almost identical. There is no “male” or
“female”, just different mating types.
Plants (and animals) have distinctly different male and female gametes.
– The male gamete (sperm) is dispersed out into the world, and must find the female.
– The female gamete (egg) stays inside the mother’s body, and is fertilized there. It is usually
larger than the male gamete.
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The gametes are made by the gametophyte, the haploid plant. In some plant groups,
separate male and female gametophytes are produced, and in other species, a single
gametophyte produces both male and female gametes.
The structures that produce gametes:
– Male gametes (sperm) are produced in antheridia.
– Female gametes (eggs) are produced in archegonia.
Examples
• In this fern, the sporophyte produces just one kind of
gametophyte, which is bisexual. A single gametophyte plant
produces both antheridia and archegonia.
Another Example
• In this moss, a
single kind of
sporangium
produces
gametophytes that
are unisexual:
either male or
female. The female
gametophyte
produces
archegonia, which
make the eggs.
The male
gametophyte
produces
antheridia, which
make sperm.
M+F continued
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In the seed plants, the gametophytes are differentiated into male and female:
the male gametophyte is the pollen grain and the female gametophyte is the
ovule. These plants have two different kinds of sporangia, one for each sex.
“micro-” refers to the male and “mega-” to the female.
– The microsporangium produces microspores (microgametophytes),
which then produce antheridia, which make sperm (male gametes).
– The megasporangium produces megaspores (megagametophytes), which
then produce archegonia, which make eggs (female gametophytes).
In seed plants, the antheridium and archegonium have been reduced to very
small structures that are not identified as separate from the rest of the
gametophyte. The gametophyte as a whole generates the sperm and eggs.
Example
• In this pine tree (a gymnosperm),
the sporophyte plant produces
separate megaspores and
microspores. The megaspores
develop into female
megagametophtyes and the
microspores develop into male
microgametophytes. The
antheridium and archegonium
have been reduced to very small
structures that are not identified
as separate from the rest of the
gametophyte. The gametophyte as
a whole generates the sperm and
eggs.
Move to Sporophyte Dominance
• A major change over the evolutionary history of
plants is a move from a dominant haploid
gametophyte to a dominant diploid sporophyte.
– Bryophytes: mostly haploid, with small diploid
sporophyte growing out of the gametophyte.
– Ferns: the main plant body is a diploid sporophyte, but
there is a small free-living haploid plant
– Flowering plants: the plant is diploid, and the
gametophyte is reduced to 3 cells for the male and 8
cells for the female.
• Being diploid means there is a second copy of
each gene. So, if one copy gets mutated
(mutation happens all the time), the other copy
can continue to fill its role, so the cell lives on.
Otherwise, organisms just can’t get too complex.
Animals are also diploid for the same reason.
Photosynthesis
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Photosynthesis uses energy from light to
convert carbon dioxide (CO2) into sugar.
• Occurs in the chloroplasts, which were once
free-living bacteria that got swallowed up by
endosymbiosis.
– In other parts of the plant, chloroplasts get
used for storage of food or other pigments
(like in flowers). Called plastids.
• Two parts to photosynthesis: light reactions
(occur only in the light) and the Calvin cycle
(occurs in both light and dark).
– Light reactions: Light energy is captured by
chlorophyll and used to extract high energy
electrons from water, which converts it to
oxygen.
– Calvin cycle: The high energy electrons are
used to convert carbon dioxide into sugar.
This is called carbon fixation.
Cell Walls
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The cell wall is mostly made of cellulose.
– Cellulose is a molecule made of many glucose
sugar molecules linked in long chains
– Starch is also made of many glucose units, but
the linkages between the glucoses is different in
cellulose and starch. This gives them different
chemical properties.
– Notably, almost all organisms can easily digest
starch, but very few can digest cellulose.
• Mostly just some types of bacteria and
protists
– Cellulose is probably the most common organic
compound on Earth.
In cells needed for support or water conduction, the
cell wall is thickened and strengthened by lignin, a
complex organic compound that is even harder to
digest than cellulose.
Cells, Tissues, and Organs
• A big difference between plants and their protist ancestors is that plants are
multicellular and have different organs.
– Multicellular organisms have many different types of cell.
– Tissue: a group of cells with a common structure
– Organ: a group of different tissues organized for a common purpose.
• The main plant organs: leaves, stems, roots. Plus various reproductive
organs.
• All the cells in an individual have the same genes. Different cell types occur
because different sets of genes are active.
• An example: the stem of a plant has two main functions: to support the
upper parts of the plant, and to conduct fluids between the roots and the
leaves. Stems have several tissues in them: epidermis (the outer covering),
xylem and phloem (vascular tissues), and fibers (for support) and general
body cells. In turn, the vascular tissues are composed of several different
cell types: tracheid and vessel elements for the xylem, and sieve tubes and
companion cells for the phloem.
Meristems
• Meristems are special regions in the plant
where cell division occurs. Cells in other
parts of the plant don’t divide. Meristems
produce all of the new cells; once a cell
leaves the meristem, it can enlarge but
not divide.
– Apical meristem: at the tip of the plant
shoots and at the tip of the roots. This is
where growth occurs, producing new
leaves, branches, flowers, etc.
– Lateral meristem: in the stems of woody
plants: they produce lateral growth. Also
called cambium layers.
– Once a cell has been produced in a
meristem, it goes through a process of
differentiation, which turns it into some
particular type of cell.
– Xylem, phloem, epidermis, etc.
– Meristems are the equivalent of stem
cells in animals.
Vascular Tissue
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Two basic types: xylem and phloem
Xylem conducts water and mineral nutrients up from the roots.
– Xylem cells are dead and hollowed out.
– Wood is made of xylem, but even non-woody plants have xylem.
– Water is pulled up by transpiration: water molecules evaporating from the leaves pull other
water molecules up the tubes, because water molecules stick together.
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Phloem cells carry organic matter (mostly sugar) from the leaves to other parts of the
plant.
– Unlike xylem, phloem cells are alive.
– The cells are connected by many pores, so material flows easily between the cells.
– Flow of material in both directions
Xylem and Phloem
• Xylem and phloem occur together, in
bundles that also include supporting cells.
• Xylem in the inside, phloem on the
outside, with the meristem (cambium)
between them.
– So, in a tree, the xylem becomes wood,
and the phloem is the layer just under the
bark. Removing the bark from a circle
around the tree kills it because the
phloem has been disrupted: the roots are
not connected to the leaves.
• A meristem layer (called vascular
cambium) lies between the xylem and
phloem, and generates new cells.
Leaves
• Leaves are the main site of photosynthesis.
• Photosynthesis mostly occurs in the layer of cells just below the
epidermis. (palisade layer)
• The sugars are then transported to other parts of the plant
through the vascular system.
– The spongy tissue below the palisade layer carries the sugar (dissolved in
water) to the veins of the leaf, which are part of the vascular system.
• Leaves are coated with a waxy layer called the cuticle. The leaf
epidermis cells secrete the cuticle, which helps prevent the leaf
from drying out.
Stomata in the Leaves
• A big development in bryophytes is
stomata: openings in the leaves that
open and close in response to
conditions. (singular=stoma)
• The leaves are covered with the waxy
cuticle, which is impermeable to gases
• Photosynthesis needs CO2 from the
atmosphere, and it releases oxygen
• Transpiration needs water vapor to
evaporate out of the leaves, but in
hot, dry climates, too much
evaporation would kill the plant.
• Stomata can open and close in
response to the need for carbon
dioxide and the need to avoid drying
out.
• All plants except liverworts have
stomata.
Evolution of Leaves
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The basic structure of having a flat surface to
maximize exposure to light for photosynthesis
is found in many algae as well as in the plants.
However, these are not considered “true
leaves”, because they have no vascular tissue.
Microphylls are leaf-like structures found in
some of the seedless vascular plants: lycopods
and horsetails. Microphylls have a single
strand of vascular tissue down the middle.
Seem to have evolved as outgrowths from the
stem.
Megaphylls are the type of leaf seen in ferns
and seed plants. Megaphylls have a branched
system of veins (vascular tissue). They seem to
have evolved independently from microphylls,
in response to a drop in atmospheric carbon
dioxide in the late Paleozoic. Maybe by filling
in the spaces between small branches
(webbing).
Bryophytes
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Bryophytes are also called the non-vascular plants. They
do not have xylem and phloem to conduct fluids and
nutrients between different parts of the plant body.
The bryophytes are not a monophyletic group (clade).
Instead, we use the name for 3 phyla: the liverworts, the
hornworts, and the mosses.
All three groups spend most of their life cycle as haploid
gametophytes.
The sperm are produced in antheridia. They have flagella
(several), and they have to swim through drops of water
to find the eggs inside the archegonia of another plant.
No vascular system means that the plants must be small
and low to the ground, and that they are mostly found in
moist environments.
Roots are called rhizoids: they just hold the plant down
and don’t extract water and nutrients for the rest of the
plant. Each one is a single elongated cell.
Liverwort pore: always open,
in contrast to stomata, which
open and close.
More on Bryophytes
• The fertilized egg is contained within the
archegonium of the gametophyte. This
zygote grows into a sporophyte without
leaving. The sporophyte grows out of the
gametophyte.
– The sporophyte is composed of a foot that
anchors it to the gametophyte, a stalk, and
the sporangium, where meiosis and spore
production occurs.
– The sporophyte is not photosynthetic, and it
is completely dependent on the gametophyte
fro survival.
Liverworts
• Liverworts are probably the earliest branching
plant lineage.
– The name come from an old and very incorrect idea
called the Doctrine of Signatures: a plant resembles
the organ it can heal. Liverworts somewhat
resemble the liver, and so they were thought to cure
liver ailments.
• Liverworts are small and low to the ground.
They produce a flattened stem that looks like
a leaf, but it lacks the different cell and tissue
types found in real leaves.
• Unlike all other plant groups, liverworts don’t
have stomata that open and close in response
to different environmental conditions.
Instead, liverworts have pores that are always
open. (Hornworts and mosses have stomata.)
Hornworts
• Another bryophyte group. The
sporophyte grows out of the
gametophyte, but the sporophyte is
photosynthetic.
• Another small, low-growing plant.
Distinguished by the horn-shaped
sporophyte.
Mosses
• The mosses are the largest group of
bryophytes.
• There are a number of plants called “moss”
that are really not bryophytes: reindeer
moss is an example
• Some mosses have a strand of vascular
tissue, but not a full xylem-phloem
combination as is found in all non-bryophyte
plants.
• Mosses have a use for humans: they absorb
water very well, so peat moss is a common
soil additive in gardening. It is also used for
fuel in some parts of the world. It is also
burned to produce the smoky taste of Scotch
whiskey.
• Peat moss bogs are quite acidic, and this
preserves dead organisms.
Sphagnum mosses cover 1% of the earth’s surface
Seedless Vascular Plants
• Two phyla: the lycophytes (club mosses) and the pterophytes (ferns and
horse tails)
• Sporophyte is dominant, with small gametophyte. The eggs are in
archegonia, and the sporophyte grows right out of the gametophyte, just
like in the bryophytes. Sperm have flagella and move through drops of
water to the eggs.
• Roots and shoots both form branches (not true in bryophytes).
Protrachaeophytes and early vascular plants
• All are extinct. Known only from
fossils, which date to 420 mya.
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Sporophytes (as shown in the
figure) are branched and grow
independently of gametophytes.
• Some had vascular tissues. Up to 50
cm tall, which is HUGE compared
to bryophytes!
• Consisted of stem tissues only
(photosynthetic vertical stems and
horizontal rhizomes).
Aglaophyton [29.12/29.11]
• NO leaves
• NO roots for absorbing water (but
rhizoids for anchoring).
Lycophytes
• Lycophytes are club mosses (plus spike mosses
and quillworts).
– Not closely related to the “true” mosses,
which are bryophytes with no vascular
system.
• Leaves are microphylls.
• Some called ground pines: they resemble small
pine trees, with the sporangia at the tips looking
somewhat like pine cones.
• Many are epiphytes: they grow off the ground,
supported by trees but just using them for
support, not as parasites.
• Flash powder, used by photographers before
electricity-powered flash bulbs, was Lycopodium
club moss spores: they are very small and burn
very rapidly.
Paleozoic Club Mosses
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Club mosses were a dominant form in
the Carboniferous period of the
Paleozoic. Sometimes called “scale
trees” because the bark had scales on
it.
• They were the first plants to grow into
trees, up to 40 meters tall.
• They grew in swamps, and most of
our coal in Illinois comes from
Carboniferous club mosses (plus ferns
and horsetails).
• These big lycophytes are long extinct,
but smaller ones survived and exist
today.
Pterophytes
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Pterophytes are ferns, horsetails, and whisk ferns.
Have megaphyll leaves.
Gametophyte is a small multicellular structure called a
prothallus that is often bisexual. The large sporophyte
grows right out of it.
Ferns have roots that can branch at any point. In
contrast, lycophyte roots can only branch at the
growing tip, by forming a Y with two equal branches.
Azolla (duckweed), a very small fern, grows in rice
paddies. It has a cyanobacterium symbiote, which fixes
nitrogen, useful to fertilize the rice.
Whisk ferns are just 2 small genera, mostly tropical
epiphytes. No true roots, just rhizoids to hold them
down. (but roots may have been lost secondarily).
Dichotomous branching: Y shaped, 2 equal branches:
considered a primitive trait, with unequal branches
evolving later.
Horsetails
• Horsetails have silica in their stems that
makes them good for scouring pots:
"scouring rushes".
• Horsetails were very common, up to 15
meters tall in the Carboniferous period.
Today only a few species exist.
• Sporangium at the top of the plant.
• Photosynthesis in the stems.
• When the stems branch, a whorl of smaller
branches appear (i.e. the whorls are
branches, not leaves).
Carboniferous Swamp