Kingdom Plantae: Angiosperms (Flowering
Plants )
Materials
1.
Large fresh flowers with superior ovaries
2.
Prepared slides of Lilium-cross sections
of ovary showing embryo sacs; Lilium-cross
sections of mature anthers
3.
Set of live flowers on display, including a composite, a grass, a primitive
flower (e.g., buttercup), an advanced flower with an inferior ovary (e.g.,
orchid), and an inflorescence
4. Model of a flower
Some
Suggested Learning Goals
1.
Know the parts of a complete flower, and
know the function of each part.
2.
Understand the basic variations in ovary position
and general structure of flowers.
3.
Understand the difference between a
compound ovary and a simple ovary.
4.
Learn the life cycle of a flowering plant, and understand how an immature ovule becomes a seed.
Introduction
The life cycle of
flowering plants exhibits the same alternation of diploid (2n) and haploid (n) generations
seen in the lower plants. However, the gametophyte phase is proportionately
greatly reduced and is confined within certain tissues of the sporophyte,
where it is entirely dependent on it. As in gymnosperms, two kinds of spores
and gametophytes are produced, the larger megaspores
giving rise to the female gametophytes, and the smaller microspores giving rise to the male
gametophytes.
With the aid of your
dissecting microscope, examine one of the flowers provided. Note the small
leaflike sepals comprising the calyx, and the usually colored or white petals, comprising the corolla. In some flowers, the sepals or
petals may be united into an undivided calyx or corolla. How many sepals and
petals d6es your flower have? Monocot flowers
generally have their parts (sepals, petals, stamens, stigma divisions) in
threes or multiples of three. Dicot flowers
usually have their parts in fours or fives. Is your flower a monocot or a
dicot?
Both
the calyx and the corolla are attached to the receptacle, the slightly to distinctly expanded tip of the flower
stalk (peduncle or pedicel). The male reproductive struc
tures
or stamens each consist of a slender
stalk, the filament, and a
pollen-bearing anther. In some
flowers, the filaments may be fused together or to the petals; they usually
surround the female reproductive structure, the pistil. The pistil consists of a stigma that may be knoblike, forked, feathery, or pointed; a
necklike style that can be long and
slender to short and stubby; and an ovary,
which is usually swollen.
There
are many variations of flower stucture. In lilies, for example, the sepals may
be of the same size and color as the petals. In the sunflower family, the
"flower" is actually an inflorescence
composed of many tiny flowers arranged so that they resemble a single
larger flower. Most of the tiny flowers have very small fused corollas and
stamens, but those around the margin each may have a large flattened,
petal-like extension of their corolla. In several families (e.g., the pumpkin
family), the stamens and the pistil usually are in separate flowers; in other
families, such as the buttercup family, there may be more than one pistil to a
flower.
Remove
the pistil from your flower, and note the swollen ovary at the base, the pollen-receiving stigma at the top, and the style
connecting the two parts. Is your stigma divided in any way, or is it
instead knoblike, or inconspicuous enough to be difficult to distinguish from
the style? Now cut the ovary longitudinally with a razor blade. Observe the
small whitish ovules. These
eventually become seeds as the ovary
matures into a fruit. Is your ovary
divided into two or more segments known as carpels?
Carpels represent individual pistils that have become united, and a pistil
with two or more carpels is said to be compound.
A simple pistil has only one carpel.
Before young anthers
mature, they usually contain four chambers in which microsporocytes undergo meiosis, producing quartets (sometimes
called tetrads) of microspores. After the nucleus of each
microspore has divided once by mitosis, the cells of each quartet separate, and
their walls often become sculptured or ornamented. These bodies are now called pollen grains. As the anther matures,
the wall between adjacent chambers usually breaks down, leaving just two pollen sacs, from which the pollen grains
are released through slits or pores.
After pollination (which is nothing more than
the transfer of pollen from an anther to a stigma and should not be confused
with fertilization), a pollen tube may
emerge from a pollen grain on the
stigma, and by following a gradient of chemicals diffusing from the embryo
sac, the pollen tube may grow down the style to the ovary and enter the ovule
through an opening or passage called the micropyle.
As the pollen tube grows, one of the two nuclei in the pollen grain, the tube nucleus, remains near the tip,
while the other nucleus (generative
nucleus) lags behind. Sometimes the generative nucleus divides in the
pollen grain, forming two male gametes or sperms,
but this particular mitotic division often takes place right in the pollen
tube while it is growing. The germinated pollen grain, with its pollen tube
containing two sperm nuclei, constitutes the mature male gametophyte.
A megasporocyte in each ovule undergoes meiosis, producing four
haploid megaspores. In most flowering
plants, three of the megaspores degenerate. The remaining megaspore becomes
larger as its nucleus undergoes three successive mitotic divisions. The three
successive divisions result in eight nuclei. This large, eight-nucleate cell
within the ovule constitutes the female
gametophyte.
One
of the eight nuclei, normally located toward the bottom of the female
gametophyte, functions as the female gamete or egg; the egg is flanked by two synergid
nuclei. If the egg is damaged, either of the synergids can substitute as
the egg. At the other end of the female gametophyte are three nonfunctional antipodal nuclei; the other two nuclei,
called central cell nuclei, usually
remain in the center of the female gametophyte where they sometimes fuse
together.
Unless
you have been provided with a specially selected slide, you probably will not
see all eight of the female gametophyte nuclei. Most laboratories for
introductory courses use Lilium (lily) ovary
cross sections to show female gametophytes. Such a prepared slide usually
contains several cross sections, and each section has parts of up to six
gametophytes present. However, because of the way in which the sections are
cut, complete gametophytes with all eight nuclei visible are seldom present.
You should first locate the most complete gametophyte sections with the low power of your compound microscope,
and then turn to high power to see details. Note that in a lily female
gametophyte, four of the nuclei are considerably larger than the other four
nuclei. This is because the lily gametophyte develops in a manner slightly
different from that of most other flowers.
By
the time it is mature, the diploid cells of the ovule surrounding it have
developed into two layers, the integuments,
which will later become the seed coat
of a seed. There/is normally a
gap or passageway, the micropyle, formed
between the integuments in the vicinity of the egg. This micropyle will later
allow access to the female gametophyte by a tube from a pollen grain.
Study the slide labeled "Lilium: mature anthers." Examine
one of the sections of the anthers. Locate a pollen
grain in which two
nuclei are visible. Note the relatively thick, sculptured outer wall. The
generative nucleus is more dense than the tube nucleus. How can you account for
some of the pollen grains on your slide apparently having only one nucleus?
(Hint: remember that you are looking at very thin slices of tissue.)
After pollination and
growth of the pollen tube has occurred, the contents of the pollen tube are
discharged into the embryo sac. The double
fertilization that follows involves the union of one sperm with the egg,
foiii~ing a zygote, and the union of
the other sperm with the polar nuclei, forming the endosperm nucleus. Since the endosperm nucleus is the product of
three haploid (n) cells all fusing
together, it ends up triploid (i.e., it
has 3n chromosomes)-a situation
unique to the flowering plants. The endosperm nucleus usually divides
repeatedly, producing 3n endosperm tissue,
which functions in food storage. The endosperm may become an extensive part of
the seed, or it may disappear soon after it is formed. When the endosperm
disappears early, part of the embryo, which
develops by repeated divisions of the zygote, may take over the food storage
function. The outer layers of the ovule (integuments) harden into a seed coat, which forms the outer covering
of a seed. While the seeds are maturing, the ovary undergoes transformation into
a fruit.
Drawings
to Be Submitted
1.
Label all the parts of a complete flower. Include PETALS (COROLLA), SEPALS
(CALYX), PEDUNCLE (or PEDICEL), RECEPTACLE, STIGMA, STYLE, OVARY, OVULE,
PISTIL, and STAMEN(S)-with ANTHER(S) and FILAMENT(S).
2.
Draw a section through an OVULE, using the prepared slide provided. Show the
EMBRYO SAC, with its ANTIPODALS, SYNERGIDS, and EGG, and the surrounding
tissues.
3. Draw a portion of a
cross section of a MATURE ANTHER from the prepared slide provided. Label POLLEN
GRAIN(S), TUBE NUCLEUS, and GENERATIVE NUCLEUS.
4.
Label the following on the drawings of the life cycle of a flowering plant
provided: SPOROPHYTE, FLOWER, FILAMENT, ANTHER, STAMEN, OVULE, MEGASPOROCYTE,
MEGASPORE, MITOTIC DIVISIONS OF MEGASPORE NUCLEUS, FEMALE GAMETOPHYTE,
ANTIPODALS, SYNERGIDS, EGG, INTEGUMENTS, PISTIL, OVARY, STYLE, STIGMA,
MICROSPOROCYI'E, MICROSPORES, POLLEN GRAINS, POLLEN TUBE, ZYGOTE, EMBRYO, SEED,
and FRUIT. Also indicate where MEIOSIS occurs and where FERTILIZATION takes
place.
Questions
1. What is the part of
the flower to which petals, sepals, and
stamens are attached?
2.
What is the part of the stamen to which an anther
usually is attached?________________________________
3.
Which part of a flower receives pollen?______________________________________________________
4. How many functional cells are usually produced
when a microsporocyte undergoes
meiosis? What are these cells called and what happens to them?_________________________________________________________________________________
5.
How many nuclei does a typical mature
pollen grain have before pollination?
How many nuclei does a
mature male gametophyte have just before fertilization?
6. How does a pollen tube enter an embryo sac?
7.
What is the difference between pollination
and fertilization?
8. What happens to antipodals after fertilization has
occurred?_______________________________________
9.
How is an endosperm nucleus formed?
What
becomes of an endosperm nucleus after it is formed?_________________________________________
10.
What structures become a seed coat?_______________________________________________________
1. What is the part of
the flower to which petals, sepals, and
stamens are attached?
2.
What is the part of the stamen to which an anther
usually is attached?________________________________
3.
Which part of a flower receives pollen?______________________________________________________
4. How many functional cells are usually produced
when a microsporocyte undergoes
meiosis? What are these cells called and what happens to them?_________________________________________________________________________________
5.
How many nuclei does a typical mature
pollen grain have before pollination?
How many nuclei does a
mature male gametophyte have just before fertilization?
6. How does a pollen tube enter an embryo sac?
7.
What is the difference between pollination
and fertilization?
8. What happens to antipodals after fertilization has
occurred?_______________________________________
9.
How is an endosperm nucleus formed?
What
becomes of an endosperm nucleus after it is formed?_________________________________________
10. What structures become a seed coat?