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Today’s topic is the exciting process of plant reproduction in angiosperms. But before
we get into that, we want to tell you a little story that completely relates---and always
reminds us of how fascinating plant reproduction really is.
Now we are sisters that live in different areas of Texas, with our own separate families.
One day, in the Pinky household, I happened to notice a few bees flying by our window.
I happen to be pretty enthralled by most insects so my family watched these bees for a while.
It didn’t take long to realize that these bees had made a nest---a beehive so to speak---in
the insulation material in our house. They had found a small opening through the window
sill. Some people may not enjoy this, we realize- but we were a little fascinated. It’s not every
day that you can observe a beehive so close. But over the weeks, we found a lot of friends
suddenly started having some weird plans and things that came up that always seemed to
keep them from being able to visit our house. One day, we had a little family barbecue and
learned a very scientific lesson. (Not really).
Bees do not like barbeque!
Or maybe they don’t like the smoke from the grill, we're not really sure. And maybe it was
just that our particular bees had a distaste for barbecue---we have not done a controlled
study on this. But on this day, it became apparent to us that the bees had to go. But
we didn’t want to call an exterminator. Why? Well that’s where we get into this
amazing story of plant reproduction. We’ll come back to this story in just a minute and
how it ties into plant reproduction. Angiosperms are flowering plants, and they
are some of the most successful and diverse plants around.
When you think of flowering plants, you may not realize that a lot of plants actually
produce flowers. Many grasses for example. Many crops. All plants that produce fruit.
In fact, flowering plants all bear fruit---just maybe not the type of fruit you are thinking of.
Not all fruit ---if going by the biological definition---are sweet or even edible.
By the biological definition, a fruit develops from the ovary of a plant. Which doesn’t
exactly sound appetizing.
Pumpkins, green beans, tomatoes, squash---these
all developed from the ovary of a flowering plant. These are all fruits.
The term “vegetable” really isn’t
used as a true biology definition. A lot of what grocery stores classify to be vegetables
are other structures of plants----like leaves (think of lettuce) or stems (think of celery).
And sometimes grocery stores just classify vegetables as plant structures that are generally not as sweet.
So angiosperms have flowers and these are the reproductive structures. Many flowers
contain both male and female parts. Other flowers only contain one part.
Let’s talk about flower parts of an angiosperm. First, we’ll start with male parts. The
male parts make up the stamen. Stamen has “men” in it so that helps me remember
male. The stamen includes the filament and the anther. The filament is a stalk that supports
the anther---the anther is a rather fluffy structure that produces pollen.
Now on to female parts. The pistil. The pistil is made of three major parts. The stigma---I
like to say “sticky stigma” because pollen is supposed to stick onto this sticky structure.
The style, which is the stalk that supports the stigma. And then the ovary at the base of
this structure. The ovary is where fertilization will occur inside and seeds are formed within it.
Also remember that the ovary will ripen into a fruit. There are some parts of the flower that are
neither male nor female. Sepals for example. Sepals protect a developing flower bud. Or
petals for example. Fancy structures that help attract pollinators. Which will take
us to my bee story and also the steps of angiosperm (aka flowering plants) reproduction.
First, let’s understand a pollinator. Pollinators can include many types of organisms. Hummingbirds, butterflies, and bees are all examples.
While pollen contains amino acids that some pollinators may feed on, and bees do certainly use the pollen as a food source (especially for their young),
another great attracting factor for many pollinators is nectar.
Nectar is a sugary sweet substance produced by many flowering plants and it is a great source of carbohydrates- meaning a great source of energy!
As a pollinator drinks
some of the sweet nectar, they tend to brush against the anthers of a flower. Remember that anthers produce
pollen. Have you ever gotten a really good look at a bee? Many times you will see a yellow
dust---pollen---all over their bodies. They just can’t help themselves…
The idea is if this pollen can be brushed from a pollinator onto the sticky stigma, the flower
can be what we consider pollinated. But it’s not done yet. Pollination is not enough. The
sperm cell must join an egg to be fertilized. And if it's just sitting there on the stigma, it hasn't gotten there yet.
Just a disclaimer, we are going to
simplify this amazingly complex process of fertilization in angiosperms.
But understand the main function is that this process is going to help the plant develop seeds.
Which can then grow into new baby plants.
First, you have to understand that pollen grains- well mature ones anyway- consist of two types of cells.
A tube cell and a generative cell.
So basically this pollen grain, once landing on the sticky stigma
uses the tube cell to burrow down a pollen tube from the stigma through the style
to the inside area of the ovary. The generative cell will divide to form two sperm cells.
So, what do these two sperm cells want to do?
Well, they are actually going to target an ovule, which is inside the ovary.
There can be multiple ovules inside a plant ovary.
But each ovule has the potential to develop into a seed if fertilized.
Now the ovules have two major key players to mention.
An egg cell and something else called two polar nuclei, which I'll get to in a second.
One of the sperm cells fertilizes the egg.
This forms a zygote
which is basically a fertilized egg. The second sperm cell joins with the two polar nuclei.
The function of combining with these two polar nuclei is that this will develop into the endosperm. The
endosperm is full of nutrients for a developing baby plant.
Now if you are wondering, how did the egg cell develop and where did the polar nuclei come from?
I highly encourage doing a little research! It's a process we need another video clip to cover.
Because the fertilization process involves sperm cells joining two different things (the egg and the polar nuclei)---we
call this double fertilization. Double fertilization is an important event in angiosperms.
Angiosperms also have an added benefit that the surrounding ovary will ripen and develop into a fruit
(remember: not necessarily an edible fruit) that can help seeds travel far from the parent plant.
You don't want baby plants to have to compete with their parent plant.
For the nonedible fruits, some have the ability to stick to animal fur or be carried by wind or water.
For the edible fruits, when an animal eats them, the seeds may pass through the digestive system unharmed but in this
way, they can travel a great distance from the parent plant.
One thing to point out that we didn’t mention in our example--- many species of angiosperms
are picky and will only accept pollen from other plants and not accept pollen from their
own anthers. (Basically not allowing self pollination). Which would make the travel
of pollinators even more important. For crops all over the world----what would we do without
our pollinators? Wind and other methods can only take you so far. Because pollinators are so important, we found a beekeeper who happily
relocated our bees and took them off to a bee farm where they could happily pollinate
flowers and let us barbecue at our house in peace.
Well that's it for The Amoeba Sisters, and we remind you to stay curious!