script by liuqingyan

VIEWS: 35 PAGES: 3

									                             OUR FLOWERING WORLD
                                Script of Narration
Flowers - they’re one of nature’s greatest gifts to us. Can you imagine how dull our
world would look without them? How drab and colorless it would be? But as lovely as
they may be, flowers do much more than just bringing beauty to our lives. As we’ll see
today, flowers play a vital role in the natural world. They are, in fact, at the very heart of
the survival of the species on which they grow. And that’s because flowers are the
reproductive organs of the plants on which they bloom. No matter what kind they are,
or where they grow, the purpose of all flowers is the same: reproduction.

Now we know the purpose they serve, let’s look at how flowers are put together. Here’s
a lily, a typical flower. It, like many flowers, has both female and male parts. The rod-
like structure sticking out from the center is called a “pistil.” The pistil is a flower’s
female part. The structures around the pistil are called “stamens.” They’re the male
part. Together, as we’ll see, pistils and stamens make it possible for flowering plants to
reproduce. This is a close-up view of the ends of the flower’s stamens. Called
“anthers,” they produce special reproductive cells called “pollen grains.” And here are
some pollen grains as they look under a microscope. Pollen grains produce sperm, the
male sex cells. They carry one-half of the genetic material needed to produce a new
plant. As we’ll see later, pollen grains serve as a plant’s genetic messengers. It’s really
amazing just how much pollen some flowers turn out. Some common flowers produce
as many as a million pollen grains a day. All that pollen means a field such as this can
be really bad news for allergy sufferers. It’s fabulous, on the other hand, for bees and
other pollen eaters. We’ll talk about flowers as food shortly.

Right now, however, let’s take a look at our lily’s pistil, or female part. And here’s the
flower’s pistil, or female part. Its tip is called the “stigma” and the style is its neck. The
ovary is located here at the pistil’s base at the back of the flower. If we could look inside
the flower, we’d see that the ovary houses ovules. Ovules produce a plant’s eggs, or its
female cells. They contain the other half of the genetic material needed to produce a
new plant. For flowering plants to successfully produce seeds, pollen and the genetic
material it contains, must move from an anther to a pistil. When that happens, a long
tube grows down the pistil from a pollen grain to the ovary. Male genetic material from
the pollen then travels down the tube and fertilizes, or joins with, an ovule, or female sex
cell. The fertilized ovule then develops into a seed, and the ovary surrounding it
matures into a fruit. We’ll talk more about fruits shortly.

As you can see, the movement of pollen is critical for flowers. If pollen grains don’t
complete the all-important stamen-to-pistil journey, the waiting ovules can’t be fertilized.
And that would mean flowering plants couldn’t produce the seeds they need. That
would be a disaster. However, over the millions of years flowering plants have been
around, they’ve developed different ways of making sure their pollen makes the
essential stamen to pistil trip.




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Some plants, such as these grasses, rely on the wind to carry their pollen to waiting
pistils. That, however, is a very unsure way of getting the job done, and most flowers
use a far better way of transporting their pollen. They rely on insects and other critters,
such as birds, to carry their pollen from flower to flower. Both parties win under this
arrangement. The movers gain from the food the plants provide. This includes nectar,
a sugary solution on which many insects and other critters, such as hummingbirds, love
to dine, and pollen, an important food for many insects. Plants, on the other hand,
benefit by getting their pollen moved from flower to flower. Hungry butterflies provide a
great example of this ancient plant/animal partnership in action. Butterflies feed on
flower nectar. And as they feed at a flower, pollen rubs off its anthers onto the
butterfly’s bodies. Later, with any luck, some of the first flower’s pollen then rubs off the
butterfly’s body onto the pistil of another flower of the same species. Called
“pollination,” this transfer of pollen is one of nature’s most important processes. Without
it, flowering plants couldn’t produce the seeds needed to survive from generation to
generation.

Seeds come in many different shapes and sizes. No matter what kind they are,
however, they all have two key parts. All seeds have an embryo - or partially developed
plant - and a food supply to nourish it. Here we see some beans. Beans are typical
seeds. We’re soaking these so we can open them and see their parts. And here’s one
of the seeds after we soaked it for a couple of hours. The pointer shows its food supply,
which in beans is stored within parts called “cotyledons.” And here’s the tiny bean
embryo. It will feed on the food supply in the cotyledons until it matures to the point it
can produce food on its own.

Seeds of the bean family develop in protective capsules such as these called “pods.”
Such pods demonstrate an important characteristic of flowering plants. As their seeds -
in this case, the little green peas you see here - ripen within them, the ovaries of
flowering plants develop into the protective covering scientists call “fruits.” These fruits
come in many different forms. The fruit, or pods, of the beans at which we’ve been
looking are tough and leathery. Other fruits, however, such as these tomatoes and
oranges are soft and fleshy. But regardless of what kind they are all fruits do the same
important job. They all protect the seeds maturing within them.

Today we’ve explored the fascinating world of flowers. Not only do they add color and
beauty to our lives, most importantly, flowers are what allow the plants upon which they
grow to reproduce and to survive from generation to generation.

Now let’s take a few minutes to answer some questions. They’ll help to refresh your
memory about what we covered today. The directions are simple. When you hear this
tone, check either “true” or “false,” or fill in the blank with the correct word. Good luck,
and let’s get started.

True or False. Pollen grains transport genetic material from flower to flower.

True or False. Cotyledons contain a bean’s food supply.



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                    AGC/United Learning • 1560 Sherman Ave., Suite 100 • Evanston, IL 60201 • 800-323-9084
True or False. Pollen is produced at the tips of a flower’s pistils.

True or False. For a seed to be produced, pollen grains must travel from an ovary to an
anther.

Now let’s do things a little differently. Instead of checking “true” or “false,” complete the
following sentences with the correct words after you hear the tone.

Ovules develop within a plant’s ______.

Anthers produce specialized cells called ______.

A plant’s ovules produce its ______.

The partially developed plant within a seed is called an ______.

The ovaries of flowering plants develop into ______.

The structure considered the female part of a flower is called its ______.




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                    AGC/United Learning • 1560 Sherman Ave., Suite 100 • Evanston, IL 60201 • 800-323-9084

								
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