Biology 1B - Lecture 3 - Ferns and Gymnosperms


Uploaded by UCBerkeley on 24.01.2011

Transcript:
Okay, I wanted to make an announcement just before the class begins, I have given the
wrong room number for the office hours and the review on Friday. The office hours are
going to be Monday, Wednesdays and Fridays in room 2013. I think he said it was going to
be from 9-11:00. He's giving his time. We usually have more students that want to go
to that session than there is space for.
This year there is a class which follows this one. I'm not sure where the room will be if
he decides to change the location if there are too many students. It just depends on
how many of you show up for his review session. Are there any questions?
Okay, last time I introduced you to the algae and they are the organisms that evolved with
the evolution of plants and they invented a very unique system. They invented an alteration
of generations. If we look at the eukaryote branch that we put on the board the very first
lecture, we had a
common ancestor, at this point. Then we had the branch. This is the eukaryote branch.
And what we said, here are humans, if we want to put this here. What we said, about, at
this point 1.5 billion years ago, the algae started to evolve. They evolved for about
a billion years, a billion years. And during that first billion years of their evolution,
they tried lots of different lifestyles. This is the time when he thinks that the alteration
of generations was invented. Many of the algae had it. During this time, when they are inventing
the alteration of generation, some of the algae moved to the ocean to lakes and bogs
that are associated with the land. We also have movement to lakes and bogs on the land.
This occurred during the first billion years or so.
Then at about 490 million years ago, something very spectacular happened. An alga living
in a lake moved from the water to the land. Now, let's think about this alga, which is
living in the water. Its lifestyle in the water involves alteration generation. It's
life structure involved gametes, because they can use the water for getting together. This
is the lifestyle that the organisms became getting used to. They moved on to the land.
What would have been some of the challenges that this organism would have faced moving
on to the land? Dehydration, okay, so this organism would have faced drying out. What
else? How about support, it wouldn't have any support because in the water it is buoyant.
What other things might it have faced?
Well, if it's live something the water, it's surrounding by all of the nutrients they would
use. They would fuse with all of the surfaces, because all of the surfaces would have been
in contact with the water. We had to worry about transport. And finally, we had to worry
about the fact that fertilization involved gametes. If you are moving on to the land
what about fertilization? How was this first organism on the land going to live in this
particular new environment? Today, we have some organisms that we think
are probably the living plants that are most closely related to the first group of land
plants. Those organisms are going to be the subject of the beginning of the lecture. They
are called the bryophytes. They are the most closely related current plant to the first
evolved plants. Land plants.
Now, you know of the bryophytes as the mosses and the liver warts. These are the common
names for them. These are organisms in which the lifestyle, I showed you some of them last
time; the lifestyle involves an organism that is now facing the challenges of living on
the land. Now, how is the bryophyte able to live on the land, if it has to face these
challenges? In regard to drying out, the bryophytes tend to live in areas that have moisture.
They are small, I will show you pictures of them, and tightly pressed to the soil, they
can absorb moisture throughout their entire surface. They have to be small, because they
don't have a transport system to move water throughout the entire plant. How about support
in if you remain small, you don't have to worry too much about support. You will see
in the lab, the bryophytes, because they are pressing on the land, they don't have to worry
about support. How about fertilization? They required water for fertilization. Their gametes
are motile. How are they going to overcome this? Occasionally
they
get water on them. There is water on the surface. During these periodic times, when there is
water available, they have an opportunity to swim and get together for the fertilization
to occur. The periods of time when the water comes and it is not very standard, it comes
and it may not come. It is possible that it would not produce quickly enough and the water
would dry out. What the bryophytes have done, their advance over the algae, they have decided
to go ahead and make their gametes, even though it is dry outside and put the gametes in a
structure and keep them alive until water is available. Then when water is available,
the gametes are ready to go and they can fertilize. What are the protective structures to protect
the gametes? The protective structure that bryophytes developed to protect the gametes?
What are they? They basically are like a little bag or a sack, which surrounds the gametes.
For the male gamete, or the sperm, the protective bag or structure is called an antheridium.
I will show you pictures. It is a bag inside of which are the male gametes or the sperm.
For the female gamete, the egg, it is also protected by a bag or protective structure.
It called the archegonium. The gametes are able to produce a structure that allowed them
to live on the land, even though they require water for fertilization to protect the gametes
from drying out. The first thing that you should keep in mind
is in the bryophytes,
just like in the algae, they have alternation of generation. That is they alternate between
a gametophyte and a sporophyte. In the algae, we said, that this alternation of generations
resulted in different kinds of themes. Sometimes the gametophyte and the sporophyte is the
same size, sometimes the gametophyte is tiny, sometimes they are the same size. We don't
know what this evolved. We can say in the gametophytes, the gametophyte generation,
is the larger generation.
Not only is it larger, but it is the photosynthetic living generation. Recall in the algae, both the sporophyte and
the gametophyte were free living. In
the sporophyte is tiny and not free living. Let's look now at an example of the lifecycle
by looks at the gametophytes. I will draw a moss plant. This may be a couple of inches
high. The gametophytes are smaller, as I said. Here's the female gametophyte. It is free
living. On the top it has an Arco gone yum with an egg inside of it. Now, let's make
the male gametophytes, also free living. This is the male gametophyte. It has now an antheridium with the sperm.
How do these two get together? How does fertilization occur? There are lots of ways, what we think
is happening, either water comes in and rain drops with hit the spore, the part which contains
the sperm and they can splash over to the female. In any event, let's assume that we
have fertilization occurring.
In fertilization, we would have the female plant or the female gametophyte. We would
now have the new embryo. 2N developing on the top, inside the archegonium. Everyone
clear on this? Now, if we waited for some time, what we would see is a remarkable thing
happening. Here's the female gametophyte, it's fertilized. If we look at the top, we would see a structure
growing out on the top of the moss. This is the sporophyte, it is growing out of the archegonium. In this particular one, the sporophyte is
not free living. Not free living. It totally depends on the gametophyte for food. Totally
depends on the gametophyte for its food, it's a parasite. Why this occurred? Why this particular relationship
developed? We don't know. In the cases of the gametophytes, the dominant generation
is the gametophyte. Dominant generation is gametophyte. Here we have a group of organisms,
which are very successful, they have been around for a long time and they were able
to evolve and spread to many areas on the planet.
They don't have a transport system to allow for their surface to move away from the soil
and allow for the nutrients to be spread to the entire organism. This is the pressure
on the plants at that time. Why can we not get bigger and move to different
environments? The next step in the evolution of plants is based upon these two evolutionary
pressures and addressed in the next step of the evolution is the transport system. The
next step in the evolution of plants we think occurred to 403 to 387 million years ago.
This next step was the development of a transport system. Or as we call it, as we refer to it in your
book, the development of a vascular system. With the development of this transport system,
plants had the opportunity to move away from the water and spread into environments that
are not always wet and plants had the opportunity to become very large. Today we would say there
are about 250,000 vascular plants. Now, the cell types that make up the vascular system,
we are going to discuss later on. I want to begin by making an artificial division in
the plants. This has to divide the vascular plants into two groups. It helps us to consider
these two groups. The first group of vascular plants that we are going to talk about are
plants that are vascular but without seeds versus plants with seeds. And I would like
to begin by discussing the plants without seeds, that's the seedless, vascular plants.
The seedless vascular plants are a group of organisms. The first is the ferns. The fern
is a seedless, vascular plant. There are about 12,000 species of ferns right now. I brought into the lecture here, three different
types of ferns, three different examples of ferns. You are going to have them in the lab.
The distinguishing feature of ferns is the leathery leaf on the fern. It is called a
frond. The leathery leaf is called a frond. If we look closely on the under side of the
leaves, you are going to see other structures. They appear, from where you are as dots. These
structures are involved in reproduction. These are made up of spores. So the frond is a leaf
for reproduction and has spores on it. We call this leaf, which has spores attached
to it called a sporophyll. A fern is made up of sporophylls on which are attached spores.
Now, the spores that are just sitting here on the underside, waiting for something to
come and knock them off. If you look at this in the microscope, the spores are not just
sitting on the leaf, they are inside another structure. We won't have much for terminology
today guys, I promise. It is called a sporangium. Structure containing -- so if this structure produces spores, which
generation are we looking at? The sporophyte or the gametophyte? The sporophyte, that's
exactly right. When you look at a sporophyte outside, the generations that you see is the
sporophyte, this is the 2N diploid generation. In the bryophytes, the dominant, the big generation
is the gametophyte. Why has it switched to the sporophyte? I have a lot of these tiny
plants, I'm going to show you a picture in a moment. These are the gametophyte of the
fern, they are much smaller and they are needed to complete the lifecycle.
Now, let's talk about the lifecycle of the fern and point some of the features of this
lifecycle in relation to what has come before in the algae and the gametophytes. These two plants are necessary
for this fern to complete the lifecycle. Most of you have never realized, if you look under
ferns, you may find gametophytes grows and you may also not. Let's look at the lifecycle
of the fern and what are the changes, the advances they have made over the gametophytes
and the algae? Let's begin with the sporophyte. The 2N generation, the one in which we have
sits up here in front. The sporophyte now makes what? Spores! Okay, great. We have meiosis
and we have spores, the spores are N. What do spores grow into gametophyte. The exact
same thing with the algae.
What does the gametophyte look like in a fern? Well, the example that I'm going to show you,
this is not 2 A typical. It is a gametophyte that is .3 to maybe 1 centimeter in size.
This gametophyte is free living, that is photosynthetic. But it is the smaller generation. So, let's
draw one of these gametophytes that comes in the fern. We are going to draw it over
here. Often they look heart-shaped and you are going to see it in the lab, I will show
you pictures in a moment. You will find there are structures in two different portions of
the gametophyte. In this structure are the archegonium and in this region of the gametophyte
we can have the antheridia.
You can say, if you are going to produce sperm and it is going to fertilize its cell, there
is no chance for genetics. Even though it produces both sexes, it may not result in -- after the egg is fertilized, I will
draw this up in a moment, the sperm are reproduced and they will go off somewhere else. Let's
take the gametophyte and put it into your lifecycle. Let's pretend, for you are discussion
here, we are going to self-fertilize. We are going to fertilize with the egg and sperm
with the same genetic material. We are going to produce the egg and the sperm, this is
all N, all haploid. We now get fertilization occurring and following fertilization we get
an embryo. This is 2N and grows into a sporophyte. The same lifecycle that you saw before. Nothing
is different, it is the same elements on the lifecycle. Let's look more closely at this
gametophyte after fertilization occurs. Here's an archegonium. Here is the new embryo. The new 2N generation.
Anybody confused with what I'm doing here? Good, the embryo is fertilized and this is
going to become the new sporophyte.
If we wait for some time, to go on, we look at only the archegonium, what we see from
the archegonium is the new fern, the new sporophyte growing out of the archegonium. Basically,
for a short period of time, the sporophyte is a parasite on the gametophyte. It's getting
its nutrition from this gametophyte. Once it produces sporophylls it can become a new
generation. Sporophylls are short-lived and very transitory and they probably get absorbed
and used by the developing gametophyte. You are not very likely to find the sporophytes
around the tern. Only when they are fresh and fertilization can occurred.
What is different with the ferns compared to the algae and the gametophytes? First of
all, in the ferns we have retained alternation of generations. Just as what we saw before, but
now the sporophyte is dominant but both the sporophyte and gametophyte are independent. That is they
can each make their own food, independent. This differs from what we saw in the bryophytes.
These two things differ from what we saw on the bryophytes. Finally, there is a characteristic
which the ferns have retained from the bryophytes and the algae. When I talk about fertilization
occurring in the archegonium, how does the male gamete get over to fertilize the egg?
In this particular case, the fern gametes still need water. So the gametes still retain
flagella. So water is needed for fertilization. There are some differences but some things
have been retained, most specifically alternation of generations and the need for this particular
gametes to occur. Now, what I would like to mention to you,
before I show you the slides of these organisms and we move on to the next group. As the gametophyte
becomes smaller, there are fewer and fewer cells, which are available to protect the gametes
in the Arco rid yum. This is evolved in the next evolution step. Let's take a look now
at some pictures.
What does it mean for one generation to be dominant over the other? It means it is larger.
Here is a pot of moss and the moss itself is about a couple centimeters high. It is
the gametophyte generation and here is the sporophyte generation growing out from the
moss. If we look at it closely, here is the photosynthetic generation and this is the
sporophyte, growing independently, free living. They will produce gametophytes. Here is a
liver wart around it is pressed directly to the soil and absorbs it's waters a nutrients
and doesn't have to have transport systems. This is relatively thin and water transport
can occur pretty easily. Can we lower the lights a little more? Here
is showing you the different types of structures produced on the male gametophytes and bryophytes.
Here is the same structure producing the sperm and here is the structure producing the egg.
When it rains or there is water, the sperm swim or are splashed on to where the eggs
are. If you look at this structure, you can actually see the sporophyte developing. It
remains attached for its entire life under the life of this sporophyte. It makes an arm
and makes the lifecycle again. This is moss and this is used commercially for a long time.
Here are the ferns now. When we are looking at the ferns we are looking at the sporophytes
and alternation generation. We can see the dots; they are made up of many, many sporangia.
There are hundreds of spores, it is like nested dolls inside. If the cells are allowed to
germinate. These are the gametophytes and they are half a centimeter to one-centimeter
in size. They are free living. This tube is about 2 centimeters wide and you can see these
gametophytes, if you come up after lecture, you can see the gametophytes. Water is needed
because these sperm have flagellula on them and they swim over to the female, it is located
in region. Here is the egg. Here is the sperm. Fertilization will occur, when it occurs,
out of the archegonium, the new sporophyte will occur. This is the material that I would
like you to understand about the ferns. Now, as I mentioned to you, the gametophyte
is getting smaller, as a result of getting smaller, the sperm and the egg have less protection.
Therefore, any advantage that the plant may have, any structure that the plant may have
to provide protection would be favored. So, the next evolutionary step is the development
of a new structure which provides extra protection for the developing gametes. The new structure
is the seed. We believe that the seed started evolving about 360 million years ago. This
is when the gametes were going strong, they didn't have seeds. 200 million years ago plants
having seeds appeared to be at an selective advantage. That's because, there was a long,
dry period on the earth and there was glaciation. Any organisms that allowed them to pass through
periods of drought would be favored. This is when the seed plants involved and they
are the majority of the plants outside. They occupy the deserts and these are the seed
plants. I want to talk about the first group of seed
plants, the important first group that evolved, they are known as the gamner or conifers.
We are going to talk seed plants and the first one that evolved in the seed plant evolution
and it is known as the gamiest sperms. These are also known as the conifers or the cone-bearing
plants. Examples of cone-bearing plants that all of you are particular with are the pine
trees, spruce trees, redwood. These are all trees which belong to the conifers or the gymnast sperms. Where does the gymnast
sperm come from? What does it mean? Gymnast sperm, if we translate it means naked seed.
What do we mean by naked seed? I want you all to take a second to watch what I'm going
to do here. Here's a gymnosperm cone, maybe a pinecone? If I take this gymnast sperm cone,
I turn it upside down and the seeds come out. They are naked, there is nothing protecting
them when they are exposed to the environment. The gymnast sperms are a very successful group
of plants. They have spread to many environments. They tend to be in high elevations, if you
go to the mountains, you notice that the cone bearing plants a dominant part. Where does
the seed come from? How does it form? Where does it originate from? In order to give you
an understanding of where the seed comes from, I want to take you bake. Each of the fern
leaves is a sporophyll. If you can imagine taking this fern with the individual sporophylls
and collapsing it down to a cone, each results in a sporophyll, each scale is a sporophyll.
What do sporophylls do? It has spores. The same thing goes on in the cone-bearing plants.
Each of these sporophylls have a cone attached to it. This is the beginning of understanding
where the seed comes from. More importantly, in the gymnast sperm, they develop two types
of cones. A large cone, this is what you are particular and a smaller cone, it is transitory,
it doesn't last for a long time. They produce two types of spores. I want to begin by diagramming
the cone and gives you the name of the two different types of spores.
So, the larger of the cones produces a larger cone, which you know of as the pine cone,
the pine cone, let's say. Produces a spore on a scale. We give this spore a special name,
we call it a megaspore. The smaller cone
produces spores and these are called microspores. But you know them by another name, the name
that you know these microspores by is pollen. This is what pollen is. For those of you that
are allergic to pine, it is going to be produced relatively soon and you will see large puddles
of pollen on the ground. How does the pollen get from here to here? By the wind, no longer
do we need to have flagellated sperm. In the gymnast sperm, flagellated sperm have disappeared,
by and large, a couple of exceptions and we use the wind to carry the male gamete to where
the female gamete is. The wind is a unreliable messenger. When you have something pollenated
by wind, you have to make sure you get enough for fertilization. In the case of pine, you
have to make a million pollen grains for each successful fertilization.
Let's look at the pollenization of the pine. I want to begin on the right here, with the
male cone and see what goes on in the male cone and then we will put the female cone
on. So, here is the male cone, these are
microsporophylls. So if we look at a microsporaphylls, it has on it a sporangium, like we have seen
in the ferns, inside are the microspores, which are haploid. When the pollen is to be
shed, depending on the species, the microspores take on a pollenistic feature. You can see
the cells and these things that look like Mickey Mouse ears. They are pollen grain ready
to be shed. Now, let's look at what goes on in the female cone. Here's the female cone
made up of these scales, these megasporophylls now. So, here's a mega sporophyll. One of these
scales, you told me these scales produce spores. Here we have a sporangium and inside we have
a megaspore, which is N. This mega spore, what does a spore develop into, forget the
word "mega" a gametophyte. This gametophyte produces the egg. So the stage is set. We
now have the female gamete ready. And we now have something that is related to the male
ready. Pollen grain travels through the air, comes over and lands on the scale of a cone
and sits down, maybe from the wind to the inside. From there something happens from
a pollen grain. A pollen grain matures into a male gametophyte. It may have five or 6
cell, a very small number of cells. Now the gametophyte generation is reduced down to
six or seven cells, tiny, really tiny. How about in the female? The gametophyte is also
reduced, maybe a couple thousand cells but still relatively small. How do we compare
what goes on in the gymnast sperm to the ferns? The point that you should get from this particular
example are the following in the gymnosperm, the gametophytes are never free living. Never
free living, they are always dependent on the sporophyte. Now how about the question
that I raised for you in the beginning, which is the seed? Where does the seed come from?
Here we have the sporangium and
we call this sporangium the mega sporangium. Now we have, fertilization has occurred and
on this scale, this I'm just going to draw again for you, here is the mega sporophyll.
We have a mega sporangium. Maybe we have an embryo. And now the seed coat, or the seed
develops. A new structure develops outside. This is a great mystery. A new structure develops
out of the sporangium. This is the seed coat. We are not quite sure what is the original
structure that developed into this seed coat. You can find a seed equal to a mega sporangium
surrounded by a seed coat. And now, if we're lucky, I want to just show you a couple of
pictures in the last three minutes that we have of the gymnast sperms.
Let's turn the lights down again, sorry, maybe I won't show it to you? Let's see here --
if we don't show it here, well, hang on a second guys, let's see -- All right, hang
on. Okay, here we go, maybe we don't go? All right, we still have two minutes left, I want
to show you the pictures of the gymnosperm. Here is the female cone and behind it you
can see the male cone with its scales. If we look more closely, you can see the scales,
on each of these scales are megaspores and if you look closely, you can find the impression
of where the seeds where. Here you can see the developing seed. This is the embryo. This
is the megagametophyte. Here is the megasporangium and on the outside is the seed coat. The megasporangium
is surrounded by the seed coat. If we make a cut down the center of a male cone. You
can see the microsporangium. Inside are the pollen grains, the last two pictures show
you the pollen grains. These Mickey Mouse-type ears are involved in transporting with the
wind. Okay, see you on Wednesday then.