Lab 12
Reproduction in Seed Plants

    Spermatophytes are tracheophytes that produce seeds.  Seed-bearing plants are divided into gymnosperms and angiosperms.  Gymnosperms include the coniferous (cone-bearing) plants and their relatives (non-flowering seed plants).  Angisperms include flowering, seed plants.

    The dominant generation in seed plants is the sporophyte generation.  Sporophylls (sporangia producing leaves) are highly modified.  In gymnosperms the sporophylls make up the cone, and in angiosperms the sporophylls are found within the flower. 

Question 1:  What is meant by the gametophyte and sporophyte?

Question 2:  What do the terms angiosperm and gymnosperm mean?

LAB OBJECTIVES:
    1.  To identify the structures and functions of the seed bearing plants reproductive organs.
    2.  To observe the reproductive pattern in seed bearing plants from gamete formation through seed germination.

I.  Gymnosperms
    Gymnosperms for the most part are evergreen trees and shrubs.  Several of the plant groups included among the gymnosperms are now either wholly extinct or represented by a few living species.  The conifers - pine, spruce, fir, sequoia, and hemlocks as examples, have survived for several million years and still form extensive forests.  The oldest living plants known are gymnosperms; there is a living bristlecone pine almost 5000 years old.  Coniferous trees are important sources of lumber and wood and produce pulp, pine tar, resins, turpentine, many other commericially used products.  The pine tree will be utilized as a model of gymnosperm reproduction.

    A.  Pine Life Cycle:
The sporophyte generation of the pine produces spores in reproductive organs known as cones or strobili.  Two types of cones are produced: staminate (male) cones produce spores which develop into male gametophytes (pollen grains); ovulate (female)  cones produce spores which develop into female gametophytes containing egg cells.  Pollination (by wind) is followed by fertilization which produces an embryo contained in a seed.  Fertilization, formation of the embryo, and the development of the seed take place within the ovulate cone.  The entire process from cone formation to seed production may take up to 21/2 years in some species.

Question 3:  What is pollination?

Question 4:  What is fertilization?

ACTIVITY:
    B. Staminate Cones:
Male or staminate cones are very small and fragile.  They are produced in clusters at the end of certain branches.
Because of our familiarity with the features of "pine cones" which are the female cones, it is not easy to perceive male cones  without biological investigations.

        1.  Obtain a staminate cone and examine it with a hand lens or dissecting scope and a teasing needle.  Note the cone is compopsed of individual segments.  Tease these apart with the needle.  Each of these segments is a tiny modified leaf called a microsporophyll.  In a mature cone they are papery in texture.  See figure 1.

        2.  Obtain a microscope slide of a staminate pine cone, locate the structures your instructor points out.

        3.  Your instructor may demonstrate pine pollen sacs on a dissecting scope, and pine pollen grains on a microscope slide.  See figures 2 & 3.

Question 5:  Where are microspore cells located?

Question 6:  What is the function of the sperm nuclei?

Question 7:  What purpose does the "wing-like" structures of the pollen grains have?

    C.  Ovulate Cone
The larger, more conspicuous pine cones are the ovulate or female cones.  They are green and hard when young, but become brown and woody as they mature.  Refer to figure 4 & 5.

ACTIVITIES:
        1.  Examine a prepared slide showing a longitudinal section of an ovulate cone.  The cone contains a rather thick central axis.  On either side of the axis you can see modified leaves or scales; these are megasporophylls.  The bulge of the megasporophyll near the point of attachment to the axis is the megasporangium or ovule; the female gametophyte develops here.  Refer to figure 6.  The megaspore develops into the megagametophyte containing the egg.

Question 8:  By what cellular processes are megaspores produced?

Question 9:  What generation is in the ovule at the completion of megasporogenesis?

        2.  Obtain an immature ovulate cone.  Make careful observations of this green, hard and compact female cone.  The female cone grows on the tree in early spring.  In early spring of the second year of development pollination takes place.  Pollen rests on the sharp points of the scales and is held in place by a sticky substance produced by the ovulate.

Question 10:  Are the scales loose or closed?

Question 11:  How do the pollen grain nuclei reach the egg located at the base of the megasporophyll?

        3.  Obtain a mature ovulate cone.  Note the woody nature of the cone and that the scales are now opened.  Fertilization has taken place and seeds have developed on the upper surface of the megasporophyll.  Holding the cone upside down, shake it to see if it contains any seeds. 

Question 12:  Why did the cone open its scales?

Question 13:  What is the approximate age of the mature female cone?

    D.  Pollination and Fertilization
Fertilization takes place about a year after pollination.  A basic description of pollination and fertilization in the pine is found in the text, or will be stated by your instructor.

    E.  The Seed and Germination
Hormones produced by the developing embryo cause the seed to develop.  A seed develops from each of the two ovules located on the upper surface of the megasporophyll.  The seed consists of the embryo, endosperm and the seed coat, part of which is modified into a wing-like papery projection.  When conditions are favorable, the seed germinates or sprouts to produce the sporophyte generation.  See figure 6.

II.  Angiosperms
    The most abundant plants today are the angiosperms.  These plants have "covered seeds", or seeds enclosed within a fruit.  Angiosperms are flowering plants. 

    Between 100 and 200 angiosperms fit into the human economy and direct ecology.  About 10% of them are important in world trade, and 15 species provide the bulk of the world's food crops.  (It is substantiated that 12 plants stand between man and starvation).  The importance of the use of angiosperms for medicinal and cosmetic purposes is of great magnitude.  Aside from the biological role of this plants, their role in the beauty and pleasure of human life is immeasurable.

    This section of the lab exercises will introduce you to the evnets in the life cycle of the flowering plants.
   
    A.  Life Cycle of Flowering Plants
The sporophyte generation in angiosperms produces the flower.  Microsporangio and megasporangia are found within the flower.  Following gamete formation, pollination takes place.  Fertilization produces the zygote which undergoes mitosis to develop into the embryo.  The embryo becomes a part of the seed, and the seed produces hormones that cause the fruit to form.

Question 14:  What is the function of the microsporangia?

Question 15:  What is the function of the megasporangia?

    B.  The Flower
The flower functions primarily in the reproduction of the plant and is the precursor of fruits and seeds.  Flowers vary greatly in size, shape, number of parts and colors.  These and other characteristics of flowers are important in the identification and classification of plants.      

ACTIVITY:
        1.  Carefully examine a living flower.  It is a cluster of several different structures, all of which are regarded as modified leaves.  As you examine your flower, refer to your text and Photo Atlas.  Note that the typical leaf parts are arranged in whorls.  The structure to which the flower is attached is a receptacle.  The outer most whorl consists of sepals.  Sepals are usually green or dull colored, but may be colored or transparent.  The next whorl consists of petals.  Petals are usually bright colored.  Stamens, the male reproductive organs, make up the third whorl of the flower cluster.  Stamens are identified by an enlarged structure atop a long slender stalk.  The female reproductive organ makes up the fourth whorl or the center of the cluster; it is called the pistil (carpel).  A typical flower having all four structures (sepals, petals, stamens, pistil) is termed a complete flower.  An incomplete flower is a flower with one or more of these parts missing.  Flowers are also regarded as perfect or imperfect.  Perfect flowers have both male and female structures located in the same flower; imperfect flowers have either male or female organs but not both (staminate or pistillate flowers).  The number of each whorl of flower parts constitutes the floral formula.  For instance, a flower formula of "5-5-10-2" indicates 5 sepals, 5 petals, 10 stamens, and 2 pistils.

        2.  Label figure 7 - structures of a typical flower.

Question 16:  Is the flower you examined perfect or imperfect?  complete or incomplete?

        3.  Remove a stamen from your flower and note the filament, a rather slender stalk supporting the enlarged anther located on the tip of the filament.  The anther contains pollen sacs.  Refer to figure 8.  Microsporangia located inside of the pollen sacs produce pollen grains, the male gametophyte.  The formation of spores and the physical appearance of angiosperm pollen is similar to gymnosperm pollen.  When pollen grains are mature, the pollen sacs will burst releasing pollen to be carried to the pistil for fertilization.

Question 17:  How many stamens are located in your flower?

Question 18:  By what means may pollination be accomplished in angiosperms?

        4.  Remove the pistil by cutting it free just below the base.  The female organ (pistil) of the flower contains the ovary, the enlarged portion located at its base.  Extending from the ovary is the slender-stalked style.  The tip of the style is the stigma; it is here that pollen is trapped during pollination by a sticky substance produced by the stigma.  Make a series of thin cross sections through the ovary.  Note the ovary is hollow and that the cavity contains small, nearly spherical bodies called ovules.  Ovules contain megasporangia, which will produce the female gametophyte.  Observe figures 8 & 9.  If fertilized, each ovule will produce a seed.  The ovary of some flowers may have more than one cavity (compound ovary).  In aggregate flowers, there is more than one pistil; and in multiple flowers, several clusters of flowers are attached to one receptacle.  Such features of the pistil are useful in identification and classification of fruits.

Question 19:  How many ovules (chambers) are there in the ovary of your flower?

Question 20:  Is the flower you examined a monocot or dicot?

    C.  Pollination and Fertilization
Sexual reproduction of seed plants depends on pollination and fertilization.  A basic description of these events is given in the text.  After discussing in lecture and with aid of the text, answer the following questions.

Question 21:  Where on the pistil are pollen grains deposited?

Question 22:  How do sperm nuclei reach the egg?

Question 23:  What is meant by double fertilization, and what is the final result.

    D.  Seeds
Recall that in the seed plants fertilization results in the formation of an embryo and the seed.  A seed consists of the embryo, some nutrient materials in the form of an endosperm, a seed coat for covering, a micropyle, an opening through which the pollen tube entered the ovule, a scar region (hilum), where the seed was attached to the seed stalk, and one or more seed leaves called the cotyledons.  These nourish the growing embryonic plant until the plant develops its first leaves.

ACTIVITIES:
        1.  Obtain a presoaked monocot seed (corn) and a dicot seed (bean).  With a razor blade, cut the corn seed lengthwise, and stain with iodine.  Observe with the help of figure 10.  Loosen and remove the seed coat of the bean seed.  With the help of figure 11, observe the external features.

        2.  Segregate the two cotyledons to expose the embryo and its parts.  Observe with the help of figure 13.  Compare the two seeds in regard to the number of cotyledons, size of the embryo, and presence of endosperm.

    E.  Seed Germination
When conditions are favorable, mature seeds will germinate to produce a new sporophyte generation.  When a seed germinates, the embryo enlarges, the seed coats burst, and the young plant emerges.  Time permitting, your instructor with illustrations from the text will illustrate the following germination processes.

    Concerning dicots, epigean germination is exhibited, in which the seed covers are pushed above the ground.  Hypogean germination involves monocots, in which the seed cover remains beneath the ground.

    F.  Fruits
Initiated by the developing seed, the ovary of a flower undergoes changes which result in the formation of the fruit.  A fruit is a ripened ovary, together with any accessory flower parts that may be associated with the ovary.  Fruits protect the seed and aid in the dispersal of seeds.  There are many different types of fruits.  See figure 12.

CLASSIFICATION SUMMARY OF FRUITS

I.  Simple fruits, derived from a single ovary
    A.  Fleshy fruits: pome, drupe, berry, pepo
    B.  Dry fruits
        1.  Dehiscent (splitting open when ripe):  legume, follicle, capsule, silique
        2.  Indehiscent (not splitting when ripe):  achene, caryopsis (grain), nut

II.  Aggregate fruits, derived from a number of ovaries belonging to a single flower and located on a single receptacle:  strawberry, rasberry, blackberry

III.  Multiple fruits, derived from the ovaries of several flowers more or less united into one mass:  mulberry, pineapple, fig, osage orange (horse apple)

Question 24:  Utilizing help from your instructor and text, define the following:
            a.  carp -

            b.  endocarp -

            c.  mesocarp -

            d.  exocarp -

 
Figure 6:  Pine life cycle

Figure 7:  Typical flower.  Label the following parts:      

 
Figure 8:  Angiosperm life cycle

Figure 9:  Angiosperm life cycle

Figure 10:  Monocot seed (corn)

Figure 11:  Dicot seed (lima bean)

Figure 12a:  Fruit diversity

Figure 12b:  Fruit diversity