BIOLOGY 1001
FALL 2004
LABORATORY 6
Sexual Reproduction in Plants: Flower Structure and Function
Seed Structure and Activity
Hormonal Stimuli in Shoot Development
As a multicellular organism grows, some of its cells and tissues become involved in sexual
reproduction. Two processes are important in sexual reproduction: (1) meiosis and (2)
fertilization.
Diploid cells contain two copies of each chromosome (e.g. humans somatic cells contain two
copies of each of 22 chromosomes plus either XX or XY). Haploid cells contain just one copy of
each chromosome (e.g. human gamete cells: egg and sperm with one copy of each of the 22
chromosomes plus either X or Y).
Meiosis and mitosis are both types of nuclear division (karyokinesis). Mitosis produces daughter
nuclei that are genetically identical to each other and to the mother nucleus. Both diploid and
haploid nuclei can undergo mitosis; diploid nuclei divide to produce identical diploid nuclei and
haploid nuclei divide to produce identical haploid nuclei. Only nuclei with an even number of
copies of chromosomes (diploid for example, with 2 copies) can undergo meiosis. Meiosis
produces 4 haploid cells from a single diploid cell.
Keep in mind that there are many organisms with more than two copies of each chromosome (a
condition called polyploidy), and that if these nuclei contain an even number of chromosome
copies, these cells can also undergo meiosis. For example, a nucleus that contains 8 copies of each
chromosome that undergoes mitosis will produce two nuclei genetically identical to each other
(and to the mother nuclei) that contain 8 copies of each chromosome. Following meiosis, this
same nucleus will divide twice to produce four nuclei that each have 4 copies of each
chromosome. For the sake of simplicity, we will use the terms “diploid” and “haploid” below, but
keep in mind that higher orders of ploidy can do the same things.
“Triploidy may be rare in wild plants; however, it is a favored condition in many
economically important plants. Plant breeders have learned that triploidy can produce
economically valuable traits in crops and ornamental plants. The only drawback is that the
plants cannot be perpetuated by the seeds of the triploid plant. Triploidy prevents normal
homologous chromosome separation, leaving the plants with abnormal seeds that rarely
develop. Thus, the plants must be propagated regularly by cross-breeding or vegetative
means such as cuttings.
Seedless plants are the major benefit of triploid plants. Watermelons are commonly raised
triploid to avoid undesirable seeds that affect the fruit's edibility. Each season, fresh
triploid seeds are sold to farmers because these watermelons cannot be propagated
Lab 6-1
naturally. Seeds that produce seedless watermelons are difficult to germinate and require
more exacting conditions to grow than standard diploid watermelon varieties. Currently,
the seeds cost six to 60 times more than seeded watermelons, thereby making them too
expensive for many small farmers and developing nations.
Grapes, mustards, oranges, radishes and many ornamental plants are sold as triploids.
The crops are valued for their seedless properties, and their triploid condition can also
result in extra pulp production. Ornamental flowers sometimes take on a larger size and
brighter colors when bred triploid. In addition, the seeds cannot be taken from the cut
flowers and grown illegally.” (from:
http://wps.prenhall.com/esm_graham_plantbio_1/0%2C6615%2C395706-%2C00.html)
When meiosis is accompanied by cytokinesis (as it often is), it produces four haploid cells from
one diploid cell. These haploid cells mature into gametes, the egg and sperm. Fertilization is the
fusion of egg and sperm and produces a diploid zygote. The driving force behind this reductionrestoration process is the creation of a diploid cell with a new mix of genetic information, half
from the female and half from the male.
Sexual reproduction has evolved over many millions of years. Early organisms were probably
haploid for most of their lives, with only very brief diploid stages. This is still found in many
present day organisms. In others, the diploid stage gradually became more prominent and the
haploid stage reduced. In animals, the organism is invariably diploid; only the gametes (eggs and
sperm) are haploid. Most flowering plants are also diploid but small haploid structures still exist
(female embryo sac and male pollen grain).
Flowers evolved as organs for the production of haploid cells and gametes, retention of the egg,
efficient transfer of the sperm (pollen) and fertilization. Following fertilization, changes occur
which retain, nourish and protect the zygote as it grows into the embryo. A second fertilization
produces an endosperm and the ovule with embryo and endosperm grows into the seed.
Simultaneously, the ovary portion of the carpel develops into the fruit, which protects and
disperses the seeds. Evolution has produced an amazing diversity of flowers and fruits that permit
these processes to occur with efficiency and precision, often with the help of animals.
Seeds contain an embryo and a source of nourishment that is either inside the cotyledons of the
embryo or is a separate tissue, the endosperm. Mature seeds are dry and allow dispersal away
from the mother plant and survival through dry and/or cold periods. The seed breaks dormancy
and germinates when conditions are right for it to begin growing into the young plant. These
conditions always include water necessary for the hydration of the seed and growth of the embryo.
Other conditions may include warmth, cold periods, fire, or even a trip through the digestive tract
of a particular species of bird. During germination, the seed must mobilize the stored materials to
provide nutrients to a growing plant that has not yet developed a shoot system for photosynthesis.
As the young plant grows, it must send shoots up and make leaves to conduct photosynthesis, and
it must send roots down to reach water and mineral nutrients required for growth.
Lab 6-2
In this laboratory, we will investigate the flowering plant as a case study in sexual reproduction,
looking at flowers and fruits. In addition, we will study the structure of seeds and then one process
of seed formation, the formation of starch from glucose precursors. We will also begin an
experiment exploring the role of a plant hormone, gibberellic acid, in controlling internodal length
in stems by affecting cell expansion.
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** Remember that Lab 7 begins with a LAB PRACTICAL EXAMINATION on
Laboratories 1 to 6. The exam will consist of timed stations, each asking multiple short
answer questions.**
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NOTE: Bring calculators to laboratory next week
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I. Floral structure and function
A. The angiosperm flower: basic parts
B. A closer look at sexual reproduction
C. Variation in floral structure
II. Angiosperm seeds and fruits
III. Monocots and dicots
IV. The effect of gibberellic acid on stem growth in genetic dwarfs (start)
V. OBSERVATION OF TOBACCO CULTURES AND CLONES
Your tobacco leaf cultures are two weeks old today. Be sure to observe them carefully and
to record your observations on the appropriate data sheets.
=========================================
Textbook reading prior to coming to lab: Tobin and Dusheck. Third Edition: p. 630.pp. 646-653;
and pp. 669-673.
Lab 6-3
I. FLOWERING PLANTS: FLORAL STRUCTURE AND FUNCTION
The flowering plants are members of the Kingdom Plantae, Division Tracheophyta, Class
Angiospermae. They are the dominant members of the world's flora, consisting of at least
250,000 species. They are of fairly recent origin, their fossil remains probably not being older
than 125 million years.
The success of flowering plants lies in their flowers, reproductive structures that carry the
gametes (eggs and sperm) and allow great precision in pollination. Pollination is the transfer of
sperm cell-bearing pollen from the anther of a stamen to the stigma of a carpel that bears the egg.
It may occur within a single flower or between flowers on the same plant (self-pollination) or it
may occur between flowers on different plants, usually of the same species (cross-pollination).
Pollination depends on agents such as the wind, water, insects, birds, and even some mammals.
Flowers are composed of four sets of organs that are really modified leaves--sepals, petals,
stamens and carpels, each with its own function. Many flowers have evolved dramatic changes
in composition and shape to accommodate the pollinating mechanism. Pollinating animals have
often co-evolved with their source of nourishment.
Today, we will study the basic flower and then examine some of the diversity seen in modern
flowers. Much of this diversity can be correlated with the type of pollinator the flower attracts.
A. THE ANGIOSPERM FLOWER: BASIC PARTS
A typical flower consists of four sets of appendages:
i.
The outer sepals (usually green) form the calyx, which protects the flower when it is a bud.
In some flowers, like the gladiolus, the sepals are the same color as the petals. Some flowers
also have special leaves or bracts that further protect the flower and may also be brightly
colored. The red of the poinsettia "flower" and the showy orange spike of the
bird-of-paradise "flower" are examples of bracts. The bracts of the gladiolus are green, as are
most bracts.
ii.
The colored petals form the corolla, which protects the inner reproductive parts and attracts
insects and birds. Glands secreting nectar, a sweetish liquid, are found in the base of the
petals of some flowers.
iii. The stamens are the male reproductive structures. Each stamen consists of a filament (stalk)
that holds up the anther (pouch) containing the microsporangia. Here, microspores are
formed by meiosis and develop into pollen grains.
Lab 6-4
iv. The carpel is the female reproductive structure. Each carpel consists of the stigma (where
the pollen grains land), the style (stalk), and the basal ovary, which contains the ovules. An
ovary may be simple or compound. Do not confuse the ovary of a plant with the ovary of an
animal; the processes going on in each are different. The number of ovules within a carpel
can vary from one to many hundred. The space within the carpel and surrounding the ovule
is called the locule.
v.
The entire flower is supported by a stalk (peduncle). The apex of the peduncle is the
receptacle, which supports all of the floral appendages
FIGURE 6-A: BASIC ANATOMY OF AN ANGIOSPERM FLOWER
1. Examine a gladiolus flowers and identify the four sets of appendages and the stalk. Record
your observations in Table 6-1. Make a labeled drawing on your worksheet.
2. With a razor blade gently make a thin cross section of the ovary. Note the number and
arrangement of the carpels, locules, ovules, and the placenta, the site of attachment of the
ovules. See Figure 6-B. Make a labeled drawing on your worksheet.
B. SEXUAL REPRODUCTION: A CLOSER LOOK
In flowers, each ovule consists of a megasporangium surrounded by one or two integuments,
with an opening, the micropyle. In the megasporangium, only one cell divides by meiosis,
producing four haploid megaspores. Three of these usually degenerate. In the remaining
megaspore, the nucleus divides mitotically three times, producing eight haploid nuclei. Three of
Lab 6-5
the nuclei migrate to one end and form three cells (the antipodals) and three move to the other
end and form three cells (one egg and two synergids). The two remaining nuclei in the center
(the polar nuclei) rest in the central cell. The final structure is the embryo sac, a small, eightnucleate, seven-celled, haploid organism. Look at Figure 6-B.
In the anther, the mass of cells is the microsporangium, and each cell divides by meiosis to
generate four haploid microspores. Each microspore divides mitotically to produce first two
nuclei (the tube nucleus and the generative nucleus). See Figure 6-C. The cell develops thick
distinctive wall thickenings and matures as the pollen grain, which is then shed when the anther
splits open.
FIGURE 6-B. STAGES IN THE DEVELOPMENT OF A MEGASPORE
INTO A FEMALE GAMETOPHYTE (EMBRYO SAC)
Lab 6-6
FIGURE 6-C. STAGES IN THE DEVELOPMENT OF A MICROSPORE
INTO A MALE GAMETOPHYTE (POLLEN GRAIN)
After the pollen is transferred (pollination) to the stigma, which is covered with a sticky fluid, the
tube nucleus directs the growth of the pollen tube. As the tube nucleus leads the way, the
generative nucleus follows and divides mitotically to produce two sperm cells
Lab 6-7
When the pollen tube reaches the ovule, the tube nucleus disintegrates and the pollen tube breaks
open releasing the two sperm cells. A double fertilization takes place: one sperm unites with
the egg to form the diploid zygote (2n), while the other sperm fuses with the two polar nuclei to
form the triploid endosperm (3n). The triploid endosperm nucleus divides many times to form a
triploid endosperm tissue, which serves to nourish the zygote. The zygote can then divide to form
the multicellular embryo. The endosperm, embryo and integuments comprise the seed.
FIGURE 6-D. DOUBLE FERTILIZATION
Lab 6-8
C. VARIATION IN FLOWER STRUCTURE
1.
You will study four different flowers in lab today. Perform a complete examination on
one flower before moving onto the next.
2.
Examine each flower first as it occurs on the plant. Record the scientific and common
name of the flower in Table 6-1. Record also whether the flowers occur singly or in
inflorescences. A tulip is an example of a flower that occurs singly, while gladiolus
flowers occur in an inflorescence group. BEWARE: Plants can be tricky – some
structures that look like “flowers” are actually inflorescences (composites such as daisies,
dandelions, and sunflowers are actually inflorescences)!
3.
Only if you are specifically instructed to do so, remove a flower and take it to your lab
bench. (Some of the flowers are unfortunately for demonstration purposes only. If you
were to dissect all of the flowers, we wouldn’t have enough for students in all the lab
sections.) Examine the flower first with the naked eye. Then, place it under the
dissecting microscope and, with forceps or a needle, gently separate the floral parts.
4.
Identify the floral parts in your assigned flower. Record your conclusions in Table 6-1.
As you are working, make a drawing of the flower, clearly labeling its features.
Floral group: How many flowers are there per peduncle? If there is more than one, this is
called an inflorescence.
Floral symmetry: What kind of symmetry does the flower have? Radial like a dinner plate,
bilateral like a person, or no symmetry?
Petal/sepal color: What color are the sepals? What color are the petals? Are they the same or
different?
Sepal number: How many sepals are there?
Petal number: How many petals are there? Are there the same number as sepals?
Stamen number: How many stamens are there? Are there the same number as sepals? As
petals?
Ovary type: How many carpels/ovaries are there? The same as sepals? petals? stamens? If
there are multiple ovaries, are they separate or fused together?
Ovary location: are the sepals attached above the ovary (in which case the ovary is “inferior” or
“buried”) or attached below the ovary (in which case the ovary is “superior” or “exposed”)
Floral sex: Does the flower have both male and female parts? If the flower has only male OR
female parts, do separate male and female flowers occur on the same plant?
Lab 6-9
II. ANGIOSPERM SEEDS AND FRUITS
Within the fruit are ripened or ripening seeds. The seed is a resting stage in plant development
which can carry the embryo or young plant through unfavorable conditions. When provided with
suitable growing conditions, the seed, if viable, will germinate and produce another plant.
After fertilization, the endosperm nucleus (3n) divides rapidly, building up endosperm tissue and
displacing the old megagametophyte and megasporangial tissue. The zygote then divides many
times to produce a multicellular sporophyte (2n) or embryo while still in the ovule.
The embryo consists of:
a radicle which will give rise to the root
an epicotyl which will give rise to the shoot (stems and leaves)
one or two cotyledons (seed leaves)
the hypocotyl which connects the radicle, epicotyl and cotyledons.
Examine a soaked bean. Note the hilum, the scar left from the attachment to the placenta in the
ovary, and the micropyle, the small opening through which the pollen tube entered. Break open
the seed by separating the two cotyledons. Examine the young embryo and note epicotyl and
plumule (the embryonic true leaves at the tip of the epicotyl). Next, locate the hypocotyl and
radicle. Notice there is no endosperm in these seeds; it has all been converted to two cotyledons.
Label Figure 6-E.
Obtain a soaked corn kernel. Note that it is not simply a seed, but a complete fruit covered by the
ovary wall, as are all grains. Lay the kernel flat and, with a razor blade, make a lengthwise cut
from top to bottom. The embryo is a relatively small part of the kernel, popularly known as the
"germ". It is usually removed during the process of making meal or flour. The endosperm is not
converted into cotyledons in these plants, but remains present throughout the life of the "seed."
Since a basic source of food in the world is flour or meal from grains such as corn, it can be seen
that humans live largely on triploid tissue. Examine the corn carefully, and label Figure 7-B with
the following structures: the seed coat, endosperm, cotyledon, epicotyl, epicotyl, hypocotyls,
radicle and indicate the embryo.
Next, add a drop of I2KI solution to the cut surfaces of both seeds. Thinking back to lab 2, what
does I2KI staining indicate? Where do the corn and bean seed store this energy source?
Indicate on the Figures 6-E and 6-F where the I2KI reacted, making sure your labels are
clear.
Lab 6-10
FIGURE 6-E. BEAN SEED
FIGURE 6-F. CORN SEED
Lab 6-11
Following double fertilization of the flower, the ovule develops into the seed and the ovary portion
of the carpel increases in size to become the fruit. Thus, we may think of a fruit as the ripened
ovary of a flower bearing one or more seeds. Squash, cucumbers, and tomatoes are all botanical
fruits, as are acorns, coconuts and maple fruits (often called maple seeds).
The role of young fruits is to protect developing seeds. But when the seeds are mature, the role of
fruits changes dramatically to aid seed dispersal. As in the case of flowers, evolution has produced
a great diversity of fruits to accommodate different ways of seed protection and dispersal, the latter
often with the help of animals. See Figure 6-G for a summary of fruit types.
When mature, fruits fall into two categories, fleshy and dry. In fleshy fruits, the outer fruit layers
turn soft and may attract an animal or simply rot, dispersing the seeds. In dry fruits, the outer
layers lose water and dry out.
FIGURE 6-G. SUMMARY OF FRUIT TYPES
Examine several of the fruits available in the lab. See if you can see the remnants of the floral
parts.
Lab 6-12
III. MONOCOTS AND DICOTS
When one examines various flowering plants, they fall into two groups, each with distinctive
features: the subclass Dicotyledonae (dicotyledons or dicots) and the subclass Monocotyledonae
(monocotyledons or monocots). The following indicates some generalities. As is usual in biology,
there are exceptions.
FIGURE 6-G. MONOCOTS VS DICOTS
Examine three of the plants available in the lab and determine whether they are monocots or dicots.
Record your observations on your data sheet.
Lab 6-13
IV. EFFECT OF GIBBERELLIC ACID ON STEM GROWTH (begin experiment today)
Gibberellic acid is a member of a class of plant growth hormones called the gibberellins. Like the
other four classes of plant growth hormones, the gibberellins affect a range of responses in plants.
The response it is most closely associated with is the stimulation of stem growth. Spray gibberellic
acid on a plant and the stem tissue between the leaves (the internodal tissue) will lengthen and thin.
The distance between the leaves will increase and the plant will become taller. Gibberellic acid is
particularly effective when sprayed on plants that are genetically dwarf. With the right
concentration and under the right conditions, the GA-treated plant will grow as tall as the
genetically tall plants.
We know that gibberellins are produced by the young leaves of a shoot and are slowly transported
down the young stem, where they cause cell expansion and stem elongation. The response to GA
treatment in genetically-dwarf plants suggests that the allele for dwarfness operates by producing a
lower level of gibberellic acid naturally in the plant. By spraying the plant, we supplement that
insufficient GA supply and allow the cells to expand normally.
In this exercise, you will ask, as a class, several questions: (1) Can you confirm reports that GA
causes stem elongation in genetic dwarfs and, if it does, will they be as tall as the genetically tall
plants? (2) Will GA stimulate stem elongation in genetically tall plants?
You and your partner will be assigned a particular gibberellic acid concentration to test. Both
dwarf and tall plants will be treated with that particular concentration. Others will act as controls
and will not receive GA. You will measure the height of each plant at the beginning and end of the
experiment.
PROCEDURE:
1. Claim one of the numbered stakes on a horizontal row of genetic dwarf and genetic tall
pea plants growing in trays.
2. Measure the height of each plant in your row. Record your data in the appropriate
columns of Table 6-2. (The plants will be about 1.5 to 2 cm.)
CAUTION: Be sure to measure the height of your plants from the surface of the soil
to the tip of the shoot growth (to the apical meristem), not to the tip of the leaves.
3. Your instructor will spray the first pair of plants with a mixture of gibberellic acid and
Tween. The second set of plants will be sprayed with Tween only. Tween is a
detergent that helps ensure that the gibberellic acid adheres to the leaves.
4. At the next lab meeting, you will measure the height of your plants. Then, everyone's
results will be pooled and statistical tests will be performed to see if there are
statistically significant differences between the treated and control plants.
Lab 6-14
BIOLOGY 1001, Fall 2003
NAME________________________________
DATA SHEET: LABORATORY 6—Flowers, Seeds, and Fruits
I. Floral Structure and Function
A. Basic parts of an angiosperm flower
1. Sketch and label the parts of the gladiolus flower as directed by the text in your lab handout.
B. Sexual Reproduction
1. Diagram and briefly explain the process of pollen formation in angiosperms.
Lab 6-15
2. Diagram and briefly explain the process of egg and embryo sac and egg formation in
angiosperms.
3. Please diagram and summarize the events that occur during double fertilization.
Lab 6-16
C. Variation in Floral Structure
1. A landscape version of this table will be available in lab and will provide you with more space.
TABLE 6-1
VARIATION
IN FLORAL
STRUCURE
Trait
Scientific Name:
Common Name:
A
B
Hypothetical examples
of possible variations
Floral
Group
Single
Infloresce
nce
Floral
Symmetry
Radial
Bilateral
Petal/Sepal
Color
red sepals,
yellowpetal
Same, all
green
Sepal
Number
4
6
Petal
Number
4
6
Stamen
Number
20
6
Ovary Type
Several
Separate
One
Fused
Ovary
Location
Superior
(exposed)
Inferior
(buried)
Floral Sex
Perfect
(both)
Imperfect
(one sex)
Lab 6-17
C
D
2. Labeled flower drawings:
A
B
C
D
3. Choose two of the four flowers you examined and speculate on how their variations in floral
structure may be related to pollination and seed dispersal strategies.
Lab 6-18
BIOLOGY 1001
NAME_________________________________
DATA SHEETS: LABORATORY 6
II.
Angiosperm Seeds
1. Attach your labeled diagrams of the seeds examined (Figures 6-E and 6-F).
2. Compare where the bean and corn seeds store their energy.
3. Choose one of the fruits available in the lab room. Sketch the fruit and label it’s original
floral parts and what each developed into.
Lab 6-19
III. Monocots and Dicots—Complete the following table:
Plant name
flower parts in multiples of 3, 4, 5
or other? (if no flowers, write “no
flowers”)
leaf veins parallel or
netted/branched?
secondary growth absent or
present? (only if you can tell; if
you can’t tell, say so)
other observations?
conclusion: monocot or dicot?
Lab 6-20
BIOLOGY 1001, Fall 2003
NAME________________________________
DATA SHEET: LABORATORIES 6 and 7--GA
TABLE 6-2. DATA SHEET FOR GA EXPERIMENT
Group Data
OVERALL PLANT HEIGHT MEASUREMENTS (in cm)
Treatment
Day 1
Day 8
Control Dwarf (tt)
Control Tall (TT)
Treated Dwarf (tt)
Treated Tall (TT)
TABLE 6-3. DATA SHEET FOR GA EXPERIMENT
Group Data
INTERNODE LENGTH MEASUREMENTS (in cm) – Day 8 only
Note: Record the internode distances measuring from the bottom up.
Treatment
Bottom
Top
Control Dwarf (tt)
Control Tall (TT)
Treated Dwarf (tt)
Treated Tall (TT)
Lab 6-21