Lab Exercise
26
Flowering Plant Reproduction
Objectives
Introduction
- To be able to identify the parts of a flower
- Be able to distinguish between dicots and monocots
based on flower morphology
- Become familiar with the structure of the ovule, seed and
plant embryo
- Understand how flowers can be arranged in an
inflorescence
- To be able to identify some of the principle types of fruits
One of the most important functions for biological organisms is reproduction. As a consequence, some of the
structures that are most distinguishable for a group of
organisms are those structures associated with the reproductive process. The flowering plants are no exception.
In these lab activities, use the material in the Reproduction
simulation within the Plant section of the BiologyOne DVD.
- Become familiar with the seed structure
- Become familiar with the different patterns of
germination
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Activity 26.1
Floral Structures
Flowers are reproductive organs composed of what are
thought to be highly modified leaves. These organs function either directly in the reproductive process, function to
promote pollination, or function to protect the flower.
vantageous for pollination. This third region is called the
stigma. It is frequently hairy or covered with a sticky fluid
to help capture pollen grains.
Go to the Reproduction simulation within the Plant section of the BiologyOne DVD. Review the structures of the
flower shown there.
In the center of the flower is found the structure(s) which is
responsible for the production of egg cells. This structure
is called the pistil and may be composed of one or more
subparts called carpels. The number of carpels may be
used to distinguish between the monocots and dicots.
The pistils of monocots usually have 3 (or multiples of 3)
carpels. Dicots usually have 4 or 5 carpels or multiples of
4 or 5 carpels. The pistil can also be divided into functional
regions. At the base of the pistil is a swollen portion, the
ovary, which contains the egg producing structures, the
ovules. Above this is a thin, elongate portion, called the
style, which elevates the third region to a position ad-
Outside the pistil(s) are the structures specialized to produce the pollen. These are the stamens. Like the carpels,
in monocots the stamens are in 3s or multiples of 3. In
dicots, the stamens are in 4s or 5s or multiples of 4 or 5.
The stamens are divided into two regions. At their apex is
swollen tip called the anther which is where the pollen is
produced. The filament elevates the anther to a position
where the pollen can be effectively dispersed.
Surrounding the stamens are the petals. The petals are
collectively referred to as the corolla. As with the above
flower parts, 3 or multiples of 3 in the corolla indicate a
monocot; 4 or 5 or multiples of 4 or 5 indicate a dicot. In
some flowers the petals may be highly reduced and not
evident. In others, the petals are very bright and showy.
Many petals become highly modified to attract specific pollinators to the flower. After pollination, petals are usually
shed.
Floral Structures
stigma
petal
pistil
style
stamen
{
anther
ovary
filament
ovules
sepal
receptacle
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Activity 26.2
Anthers & Pollen
The outer most structures of the flower are the sepals,
collectively referred to as the calyx. Again, the number of
sepals is characteristic for monocots and dicots. The calyx
covers and protects the flower during floral development.
In some plants, the sepals of the calyx will become petallike and help to attract pollinators.
One last feature to note: if the ovary is located above the
other flower parts, it is said to be a superior ovary. If the
lower portion of the stamen, petals, or sepals are fused to
the ovary, the ovary is said to be inferior.
In the Results Section, label the illustration of a flower.
The male functions of the flower occur in structures at the
tips of the stamen called anthers. Commonly, each anther
consists of four chambers. Inside each chamber, microspore mother cells undergo meiosis to produce haploid
pollen grains. In flowering plants, each pollen grain initially
contains just two cells. On germination of the pollen grain,
of these cells will produce the pollen tube while the second cell will divide once to produce two sperm nuclei.
When pollen grains reach the stigma of a flower (pollination) the first event to occur is the germination of that pollen grain. This produces the pollen tube that will grow and
digest its way down through the style. Eventually the pollen tube reaches the ovaries where it deposits two sperm
nuclei to fertilize the egg and to form the endosperm.
In the plant reproduction simulation on the BiologyOne
DVD, examine the structure of the anther and pollen
grains. Make your own drawing of these structures in the
Results Section.
Pollen Grain & Tube
pollen grain
sperm nuclei
tube nucleus
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Activity 26.3
Ovary & Ovule
Activity 26.4
Inflorescence
Pollen tubes deliver two sperm nuclei to a structure called
the embryo sac. This structure is surrounded by layers of
tissue (the integuments) that protect the sac and developing embryo. The sac and these tissue layers are called an
ovule. The ovules are located in the swollen basal part of
the pistil called and ovary. As these structures mature, the
ovules develop into seeds and the ovary develops into the
fruit.
To the layman, a flower consists of a collection of petals
and other structures at the tip of a stem. Included in this
category are tulips, geraniums, and roses. These are
simple flowers. However, for many plants closer examination reveals that what may at a casual glance appear
to be a single flower is actually an arrangement of many
small flowers in a cluster or group on a flower axis or
on branches of such an axis. This cluster is termed an
inflorescence and many types have been recognized
and given distinctive names. The type of inflorescence is
useful in the identification of plants, since it is constant for
a species. The advantage of this type of floral grouping is
that the small flowers are more easily available to insect
pollinators.
Inside the embryo sac, meiotic and mitotic cell divisions
have produced eight nuclei (the cell membranes between
nuclei are only weakly developed). These nuclei are usually arranged so that 3 are located at each end of the sac
and two are in the center. The egg nucleus is the center
nucleus at the end of the sac nearest where the pollen
tube will enter. One sperm will fertilize the egg to produce
the embryo. The two central nuclei (the polar nuclei) will
fuse with the second sperm nucleus delivered by the pollen tube. This creates tissue that is triploid (3n), having
three sets of chromosomes. This tissue is called the endosperm. Thus, in flowering plants, two fertilizations occur,
a process called double fertilization. Double fertilization is
unique to the flowering plants. The endosperm serves as
a food reserve for the developing embryo.
Observe the ovary and ovules in the plant reproduction
simulation on the BiologyOne DVD. When you are able to
identify the structures discussed above, make your own
drawing of these in the Results Section.
In the simulation, click on the names for some of the different types of inflorescences to see that arrangement of
flowers.
Ovule
polar nuclei
embryo sac
egg nucleus
integument layers
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Activity 26.5
Fruits
After pollination, the flower will develop fruit. The fruit’s
primarily function to aid in the dispersal of the seeds.
Some are fleshy and eaten by animals that later defecate
the seeds. Others produce special adaptations to cling to
passing animals, some to be blown by the wind, or others
to explosively expel their seeds.
The fruit develops from the wall of the ovary. Thus, you
can define a fruit as a mature ovary. The nature of the fruit
is quite varied, from very fleshy to very hard and woody. In
addition, other parts of the flower and even other parts of
the plant may be incorporated into the fruit as it matures.
In some plants, the fruits from separate flowers will fuse
that have inferior ovaries, and the receptacle or hypanthium becomes a part of the fruit. Accessory fruits can be
simple, aggregate or multiple fruits. A strawberry is a good
example of an accessory fruit in which most of the flesh of
the strawberry is from the flower’s receptacle.
The specific characteristics of fruits allow us to identify
(and of course name) different types. Use the key on the
next page to determine the botanical term for the five
simple fruits (A through E) shown in the simulation. The
following are some features about these fruits you may not
be able to determine from the pictures.
Fruit A: While this hickory nut is open now, the fruit is
considered indehiscent because it does not open until well
after it has be released from the parent plant. The wall of
the fruit has a very woody texture.
together to form a single, large fruit structure.
All fruits may be classified into three major groups on the
basis of the number of ovaries and the number of flowers
involved in their formation. These are:
Fruit B: This fruit is entirely derived from the ovary wall, no
other flower part are involved in this fruit’s tissues.
Simple Fruits - Simple fruits develop from a single matured ovary in a single flower.
Fruit C: This fruit does not open to release its seed.
Aggregate Fruits - Aggregate fruits consist of a number of
matured, separate ovaries formed in a single flower and
arranged over the surface of a single receptacle. Individual ovaries are called fruitlets.
Multiple Fruits - Multiple fruits consist of the matured ovaries of several to many flowers more or less united into a
mass. Multiple fruits are almost invariably accessory fruits.
What does it mean to say a fruit is accessory? An accessory fruit is one that develops from tissues surrounding
the ovary. Accessory fruits generally develop from flowers
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Fruit D: Derived only from the ovary, this fruit develops a
hard rind.
Fruit E: Derived only from the ovary, the inner part of the
ovary wall become very hard while the outer portion of the
wall is fleshy.
In the Results Section, record the path you follow in the
key for each fruit (i.e. 1b, 9b, 10a, etc.) and the botanical
term for the fruit type.
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Classification o Fruits
Fruit formed from several flowers which have fused to form what looks like a single fruit. ............... Multiple fruit
Fruit formed from several separate pistils within a single flower. .................................................. Aggregate fruit
Fruit formed from a single pistil within a single flower. ......................................................................... Simple fruit
Conspicuous part of fruit formed from receptacle of flower, not from the pistil. ............................ Accessory fruit
Key to some simple fruits:
1a. Fruit dry at maturity ....................................................................................................... 2
2a. Fruit indehiscent (does not break open) ......................................................... 3
3a. Fruit with wings ........................................................................ Samara
3b. Fruit without wings ............................................................................ 4
4a. Ovary wall thick and woody ............................................. Nut
4b. Ovary wall thin and papery .................................................. 5
5a. Seeds not attached to ovary wall ................ Achene
5b. Seeds attached to ovary wall ................. Caryopsis
2b. Fruit dehiscent (breaks open to disperse seeds) ............................................. 6
6a. Pistil made from one carpel ............................................................... 7
7a. Fruit breaks open lengthwise on one side ..................Follicle
7b. Fruit breaks open lengthwise on two sides ...............Legume
6b. Pistil made from more than one carpel .............................................. 8
8a. Fruit dehiscing to release free seeds by splitting
where the carpel are fused ....................... Septicidal Capsule
8b. Fruit dehiscing to release free seeds by forming a
hole near the top of the fruit ........................ Poricidal Capsule
1b. Fruit fleshy or pulpy at maturity ....................................................................................... 9
9a. Flesh of fruit formed from flower parts other than the ovary wall .............. Pome
9b. Flesh of fruit formed entirely from the ovary wall ............................................ 10
10a. Inner portion of ovary wall becomes hard and stony ............... Drupe
10b. Inner portion of ovary wall remains fleshy ...................................... 11
11a. Texture of fruit homogeneous, covered by a skin ....... Berry
11b. Texture not homogeneous, covered by leathery rind ....... 12
12a. Fruit divided into section, contains
juice sacs ............................................. Hisperidium
12b. Fruit not divided, no juice sacs ..................... Pepo
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Activity 26.6
Seeds
When the two sperm from a pollen grain enter the embryo
sac, one will unite with the egg nucleus to form the embryo while the second sperm will unite with the two polar
nuclei to produce the triploid (3 sets of chromosomes)
endosperm. As the embryo and endosperm mature, the
integument layers also mature. These layers eventually
become the seed coat. The seed coat varies between
species. In some, the seed coat has layers that are thick
and ‘woody’, in others, the seed coat is thin and ‘papery’.
At this point, the ovule is now called a seed. A mature
ovule is one way to define the seed.
In the dicots, there are 2 cotyledons which quickly replace
the endosperm as the site of food storage in the seed. In
the monocots there is only a single cotyledon. Here, the
cotyledon serves as an organ to absorb nutrients from
the long-lasting endosperm. The cotyledon is sometimes
called the scutelium in the monocots. Two additional tissue
layers occur in the monocots. These are the coleoptile and
the coleorhiza which surround the embryonic shoot and
root. These structures protect the emerging shoot and root
as they push through the soil.
After studying the dicot and monocot seeds, label the illustrations of these in the Results Section.
Examine some external features of a seed in the simulation.
The mature embryo has a number of distinguishable
features. Along the axis of the embryonic plant you can
find the apical meristem at the ‘shoot’ end and the radicle
at the ‘root’ end. These are the embryonic shoot and root
respectively. Just below the apical meristem are 1 or 2
cotyledons. These are the ‘seed leaves’. The region of the
embryonic stem below the point of cotyledon attachment
is called the hypocotyl and the region above the cotyledon
attachment point is called the epicotyl.
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Activity 26.7
Germination
Once a seed has been dispersed, hopefully it will germinate to produce a new plant, thus completing the
reproductive cycle. Just as variation occurs in the other
reproductive functions and structures of the flowering
plants, the characteristics of germination also vary.
Examine the illustration of germination patterns below.
Note the position of the cotyledons as the seedling grows.
Answer the questions concerning seedling development
in the Results Section.
Germination Patterns
Pea
Bean
Corn
cotyledon
seed coat
seed coat
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Lab Exercise
26
Name _______________________
Results Section
Activity 26.1
Floral Structures
4. ____________________
1. ____________________
2. ____________________
5. ____________________
3. ____________________
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6. ____________________
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Activity 26.2
Anthers & Pollen
_______________________
object
_______________________
object
_______________________
object
_______________________
object
Activity 26.3
Ovary & Ovule
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Activity 26.5
Fruits
unknown
fruit
path through key
fruit
type
A
B
C
D
E
Activity 26.6
Seeds
5. ______________
6. ______________
1. ______________
2. ______________
7. ______________
3. ______________
8. ______________
4. ______________
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Activity 26.7
Germination
1. What part of the seedling emerges from the seed first?
2. What are the differences between the cotyledon of a corn plant and a bean plant?
Do they function differently?
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