AP Biology Vascular Plants Lab: Flowers, Fruits and Seeds by kxq14559

VIEWS: 1,450 PAGES: 7

More Info
									                                        AP Biology Vascular Plants Lab: Flowers, Fruits and Seeds
                                        Objectives: At the end of this lab activity, students will be able to
                                           • Identify the reproductive organs of flowering plants
                                           • Explain the evolutionary significance of flowering plants
                                           • Describe the difference between pollination, fertilization and seed dispersal
                                           • Distinguish between monocot and dicot plant structures such as roots,
                                               stems and leaves

Flowering plants (also known as angiosperms) are economically important to humans for many reasons. The number of
products we use each day that come from flowering plants is staggering. They produce much of our food and large
amounts of natural fibers for our housing and clothing. These products include your morning paper, the wooden chair you
may be sitting on, or the watermelon you had for lunch. In this lab we will examine the reproductive structures of flowering
plants—flowers, fruits and seeds. We will also examine the structural differences between the categories of flowering
plants—the monocots and the dicots.

Monocots and Dicots: Differentiation of Angiosperms
There are two major groups of flowering plants, the dicots (dicotyledons) and the monocots (monocotyledons). Dicots
are those plants having two cotyledons in the seed, whereas monocots only have one. Cotyledons are food storage
organs in the seed. If present, the cotyledons are the first leaves of a plant and are modified for food storage and may
constitute the largest part of the embryo within the seed. The cotyledons are the first appendages of the stem above the
root system.

Monocots and dicots differ greatly in the structure of their leaves, stems and roots. The vascular tissue bundles are
arranged differently in each type of plant as shown in Figure 1.

The major differences between these plant types that you should be familiar with are outlined below:
    •   Number of cotyledons -- The number of cotyledons found in the embryo is the actual basis for distinguishing the
        two classes of angiosperms, and is the source of the names Monocotyledonae ("one cotyledon") and
        Dicotyledonae ("two cotyledons"). The cotyledons are the "seed leaves" produced by the embryo. They serve to
        absorb nutrients packaged in the seed, until the seedling is able to produce its first true leaves and begin
    •   Number of flower parts -- If you count the number of petals, stamens, or other floral parts, you will find that
        monocot flowers tend to have a number of parts that is divisible by three, usually three or six. Dicot flowers on the
        other hand, tend to have parts in multiples of four or five (four, five, ten, etc.). This character is not always reliable,
        however, and is not easy to use in some flowers with reduced or numerous parts.
    •   Leaf veins -- In monocots, there are usually a number of major leaf veins which run parallel the length of the leaf;
        in dicots, there are usually numerous auxillary veins which reticulate between the major ones, forming a net-like
        pattern. As with the number of floral parts, this character is not always reliable, as there are many monocots with
        reticulate venation.
    •   Stem vascular arrangement -- Vascular tissue occurs in long strands called vascular bundles. These bundles
        are arranged within the stem of dicots to form a cylinder, appearing as a ring of spots when you cut across the
        stem. In monocots, these bundles appear scattered through the stem, with more of the bundles located toward
        the stem periphery than in the center. This arrangement is unique to monocots and some of their closest relatives
        among the dicots.
    •   Root development -- In most dicots (and in most seed plants) the root develops from the lower end of the
        embryo, from a region known as the radicle. The radicle gives rise to an apical meristem which continues to
        produce root tissue for much of the plant's life. In dicot plants, a thick, enlarged taproot is often present. By
        contrast, the radicle aborts in monocots, and new roots arise adventitiously from nodes in the stem. Because of
        this, monocot roots tend to be fibrous in nature. These roots may be called prop roots when they are clustered
        near the bottom of the stem.
    •   Secondary growth -- Most seed plants increase their diameter through secondary growth, producing wood and
        bark. Monocots (and some dicots) have lost this ability, and so do not produce wood. Some monocots can
        produce a substitute however, as in the palms and agaves.
(above information modified from http://www.ucmp.berkeley.edu/glossary/gloss8/monocotdicot.html)

AP Bio Flowers and Fruits Lab--The Angiosperms.doc
Page 1
                                    Figure 1. Differences between monocot and dicot plants.

Specimen Observation:
You will observe various roots, stems and leaves of monocot and dicot plants. Some of these may be microscopic
observations. Choose 4 whole plant specimens from those available and do the following:
   1. Sketch each specimen in your notebook. Identify it as either monocot or dicot based on the characteristics you
         are able to observe. Provide evidence for your assertion.
   2. On high power (400x), sketch a cross-section of a monocot and dicot root, stem and leaf. Label the vascular
         bundles, taking care to label the xylem and phloem.

The flower is an innovation of the angiosperms. A typical
flower has a variable number of stamens, the male portions,
and a pistil, the female portion. Each stamen is characterized
by the thin filament capped by a bulbous anther. The anther
contains the male reproductive cells, which at maturity are
called pollen. The female reproductive parts are more
complex. The swollen base of the pistil, termed the ovary,
contains one or more ovules that will develop as seeds with
the ovary wall developing as fruit tissue (pericarp). Rising
above the ovary is the rod-like style, topped by the stigma,
which often has a brush-like appearance.

Pistils are derived from carpels, the fundamental female
reproductive unit of flowering plants. Pistils may be derived
from single carpels, termed a simple pistil, or multiple carpels,
termed a compound pistil. In general, the carpels
contributing to a compound pistil are fused through the style
and ovary; however, the stigmas are frequently unfused.

                                                                                         Figure 2. A typical flower.
AP Bio Flowers and Fruits Lab--The Angiosperms.doc
Page 2
Floral characteristics are routinely used for plant identification, both at the species level and at higher levels of
organization. For example, monocots and dicots, the two primary classes of flowering plants, can be readily
distinguished based on floral characteristics. Monocot flowers’ floral organs tend to be produced in groups of three. In
contrast, dicot flowers tend to have their floral organs being produced in multiples of four or five. For example, a flower
with six stamens, three petals, and three sepals would be an example of a monocot flower. In contrast, a flower with four
petals and eight stamens is a dicot flower.

Examination of the flower specimens
You will be given a specimen of flower to examine. For the flower you examine, you will do the following:
   • Sketch each flower before dissecting it and label it with the appropriate flower organs that are visible. Be sure
   your drawing takes up at least a half page and is done in pencil, in COLOR.
   • Using your hands, carefully tape the following structures into your lab notebook and label them:
             o Anther and filament
             o Stigma/Style/Ovary
             o Cross section of ovary containing ovules (you may need a scalpel for this part; ask your teacher for one)
             o Petals
             o Sepals (if your flower contains them)

Gametophytes in Angiosperms
The life cycle of flowering plants may be confusing because sexual qualities are often mistakenly attributed to the
sporophyte generation and because the gametophyte is usually not known at all. The life cycle of the flowering plants is
similar to the life cycle of the mosses and the ferns, but with some major modifications.

There are many different types of flowers that can appear on sporophyte plants. Some flowers contain both stamens and
a carpel, while others have just one or the other. Both the stamen and carpel contain tissue where spores are produced
by meiosis. Angiosperms can produce two types of spores within the flower. The smaller of the two kinds of spores
(microspores) develop into the male gametophyte and the larger spores (megaspores) develop into the female

Special cells within the anther portion of the stamen produce four cells called microspores from one microspore mother
cell. The single nucleus of each microspore divides mitotically and produces a cell with a generative nucleus and a tube
nucleus. This microscopic cell (pollen grain) contains two nuclei and is the male gametophyte plant. The generative
nucleus will divide and produce two sperm nuclei.

How do these sperm reach the egg? The entire male gametophyte plant (pollen grain) is carried by the wind, insects,
birds, or bats to the carpel containing the egg. This process is called pollination. The base of the carpel of the flower
contains the ovary. Within the ovary one or more ovules are found, containing a specialized cell called a megaspore
mother cell. This diploid cell produces four haploid cells by meiosis. Three of these cells atrophy and the remaining one
develops into a megaspore. As you have learned previously, spores germinate and grow, but in this instance the
megaspore develops within the ovule, which is within the ovary.

The development proceeds as follows: The single nucleus produces two, then four, then eight nuclei by mitosis. One
nucleus from each set of four migrates to the center of the embryo sac (female gametophyte) and becomes the two polar
nuclei. Cytokinesis then occurs, walling off three cells at one end of the developing female gametophyte and three at the
opposite end. Thus the microscopic female gametophyte contains eight nuclei in seven cells.

Huge quantities of pollen must be produced to ensure that pollen arrives at a carpel of another flower of the same
species. Once the pollen reaches the carpel, the tube nucleus of the pollen grain causes a pollen tube to grow through the
tissue of the carpel and enter through the micropyle (an opening in the ovule) to reach the egg within the female
gametophyte. One sperm fertilizes the egg and produces a diploid (2n) zygote, which grows into an embryo sporophyte.
The second sperm fertilizes the two already fused polar nuclei, producing a triploid (3n) endosperm nucleus. This
endosperm cell will produce a large number of cells by mitosis to serve as food for the sporophyte embryo. Portions of the
ovule, the embryo, and the endosperm tissue make up the resulting seed. When the seed is planted it germinates, and
the embryo sporophyte grows rapidly into a new mature sporophyte.

AP Bio Flowers and Fruits Lab--The Angiosperms.doc
Page 3
Angiosperm Male Gametophyte
Get a microscope and examine the prepared slide of a cross section through the stamens of Lilium. The slide shows six
anthers and may include a centrally located ovary that contains ovules.

Observe a single anther, which is composed of four anther sacs (microsporangia). Note the formation of microspores
(with a single nucleus) from a diploid microspore mother cells. You may also see mature pollen grains with two nuclei.

Label the following parts on the diagram below: anther, filament, microspore mother cell, microspore, mature pollen

Angiosperm Female Gametophyte, Pollination and Fertilization
Examine the prepared slide of the Lilium ovary and locate the developing ovules. Each ovule, composed of the
megasporangium and other tissues, contains a megaspore mother cell (diploid), which produces megaspores (haploid),
only one of which survives. The megaspore will divide three times by mitosis to produce the eight nuclei in the embryo
sac, which is the greatly reduced female gametophyte. Note that angiosperms do not even produce an archegonium.

The slide will not contain all stages of development, and it is almost impossible to find a section that includes all eight
nuclei. Locate the three nuclei near the opening to the ovule. One of these is called the egg cell. The two nuclei in the
center are the polar nuclei.

When pollen grains are mature, the anthers split and the pollen is released. When pollen reaches the stigma, it
germinates to produce a pollen tube, which grows down the style and eventually comes into contact with the opening to
the ovule (the micropyle). During this growth, one pollen nucleus divides into two sperm nuclei. One sperm nucleus
fuses with the egg to form the zygote, and the second fuses with the two polar nuclei to form the triploid endosperm,
which will develop into a rich nutritive material for the support and development of the embryo. The fusion of the two
sperm nuclei with nuclei of the embryo sac is referred to as double fertilization. Formation of triploid endosperm and
double fertilization are unique to angiosperms.

Label the following parts on the diagram below: pollen grain, pollen tube, sperm nuclei, ovary, ovule, polar nuclei,
egg cell, micropyle

AP Bio Flowers and Fruits Lab--The Angiosperms.doc
Page 4
So, what’s next?: Fruits
The zygote formed at fertilization undergoes rapid mitotic divisions, forming the embryo. The endosperm also divides; the
mature ovule forms a seed. At the same time, the surrounding ovary and other floral tissues form the fruit.

The seeds of flowering plants are surrounded by a tissue called the fruit, which may be fleshy or dry. The culinary
designation of “vegetable” is based on the use of the plant part (eaten as part of the main course in a meal). Vegetables
are actually various plant parts; some are fruits (e.g., tomatoes and peppers), leaf stalks (celery), leaf blades (spinach),
lateral buds (Brussels sprouts), young shoot (asparagus), massive flowering structure in bud stage (broccoli), root (sweet
potato), underground storage stem (white potato).

Functions of fruit
Although fruits come in all shapes and sizes, they all function in protecting the seeds inside and in aiding seed dispersal.
Protection may be afforded by hardening of the fruit to make accessing the seeds more difficult, or by accumulation of
acids or other toxins. Fleshy colored fruit attract birds and animals; seeds pass through the gut unharmed. Some types
of seeds cannot germinate unless they have first passed through the digestive tract of an animal. Many fruits promote
wind dispersal. Other fruits have hooks, spines, and bristles that readily cling to fur and clothing—just walk your dog in
an old field in autumn and see! Fruits called pods dry out as they mature and rip open, flinging out the seeds.

                                             Figure 3. Flower and Fruit Structures.

Ovary: female reproductive structure that usually develops into the fruit.
Ovule: egg-bearing structure of the flower that develops into a seed.
Pericarp: layers of fruit derived from the ovary and surrounding the seeds.
Seeds: develop from the ovules within the ovary.
In some flowers, other parts of the flower may also develop into parts of a fruit.

The arrangement of the ovules in the chambers (locules) of the ovary determines how the seeds are arranged in the fruit.

There are three major types of fruits produced by angiosperms: simple fruits, aggregate fruits, and multiple fruits. A
simple fruit develops from a single pistil. A peanut and an apple are considered simple fruits. Simple fruits may be either
dry or fleshy. An aggregate fruit develops from one flower with multiple pistils. An aggregate fruit is composed of
numerous small, fleshy fruits that develop together on a common flowering plant. Strawberries are a common example of
an aggregate fruit. A multiple fruit forms when ovaries from numerous flowers are clustered together. We eat a multiple
fruit when we eat a pineapple.

You will observe the following fruits:
Apple                        Grapes                             Strawberry                     Peach/Plum
Corn on the cob              Orange                             Raspberry

For each of the above fruits, do the following:
    1. Make a drawing of each fruit that accurately depicts the fruit’s structure, wholly and in cross section.
    2. Write a brief description of how this fruit’s seeds are likely dispersed.
    3. Provide evidence of ways this specimen is adapted for survival on land.

The hallmark of the angiosperms is the production of a fruit that bears a seed or seeds. The seeds are also one way to
distinguish monocot (single seed leaf) plants from dicot plants (two seed leaves).

AP Bio Flowers and Fruits Lab--The Angiosperms.doc
Page 5
Most seeds have the same basic structure: a tough, outer seed coat made of cellulose; cotyledons, which are the
embryonic leaves; an inner nutritive endosperm, which provides the developing plant embryo with a starchy food source
during its early development; a radicle, which is the rapidly growing embryonic root, and a plumule, which is the
embryonic shoot.

In order for seeds to germinate, one important factor must be present: water. If water is not present, the enzymes stored
in the seed will not be activated, allowing the seed to become metabolically active. Seeds can lie dormant for years and
years as long as they are dry, however, once moistened they become metabolically active and will begin to germinate.
During germination, the radicle rapidly emerges and begins to push its way out of the seed. The tender tip of the radicle
is covered by a protective cap that protects the rapidly growing cells from the roughness of the soil. As the radicle grows,
root hairs may also begin to grow as offshoots of the root. These root hairs provide the developing plant with a large
surface area for the absorption of water.

Take a dicot seed that has been soaked in water for four hours and gently ease it open without disturbing the inside
structures. Observe these structures using a binocular microscope. Do the same with the monocot seeds provided. You
may need a scalpel to open the monocot seed. Use the diagram given below to guide you as you draw and label the
structures of the seeds in the beakers provided.

                                     Figure 4. Characteristics of monocot and dicot seeds.

Lab Questions: Answer in your lab notebook.

Answer these questions using complete sentences in your notebook.

    1. What is the purpose of the cotyledon?

    2. Note the location of xylem and phloem tissues (vascular bundles) in the monocot and dicot stems. How are the
       vascular bundles similar in both plant types? How are they different?

    3. Dicot plants often have taproots. What function does a taproot have that fibrous roots likely do not?

    4. Is your flower a monocot or a dicot? Explain how you know which category your flower falls into.

    5. Does your flower have both petals and sepals? If so, how many petals and/or sepals does your flower have?
       What purpose do the sepals serve? What purpose do the petals serve?

AP Bio Flowers and Fruits Lab--The Angiosperms.doc
Page 6
    6. Why do you think the anthers are positioned where they are?

    7. Why do you think the ovary is positioned where it is?

    8. How do you think this flower is pollinated? Think about the possible ways this flower could be pollinated and
       describe the most likely way this flower is pollinated.

    9. What is the evolutionary advantage of having a flower as a reproductive structure?

    10. Which does your flower produce in greater numbers: ovules or pollen grains? Explain why this would be
        important in terms of reproductive success.

    11. What are some adaptations of flowers to help attract pollinators?

    12. How is the stigma of your flower adapted to capture and hold pollen?

    13. Sometimes, pollen from a different species lands on the stigma of a flower. Based on your knowledge of cell
        communication, suggest a mechanism that would ensure that only the correct species of pollen germinates on the
        stigma of a particular type of flower.
    14. What is the difference between pollination, germination and seed dispersal?
    15. Explain the conditions needed for the germination of a typical seed.
    16. Why is the embryonic root the first organ to emerge?
    17. Outline the metabolic processes during germination of a starchy seed. Be specific.
    18. How do monocot seeds and dicot seeds differ from one another? How are they similar?
    19. Name at least three adaptations seeds have for dispersal.

AP Bio Flowers and Fruits Lab--The Angiosperms.doc
Page 7

To top