LABORATORY EXERCISE #11

KINGDOM PLANTAE

 

THE VASCULAR  PLANTS  - II

 

Introduction

 

Reproduction in the seed plants has been profoundly influenced by the requirements of a terrestrial existence.  For the most part, motile gametes are not involved; consequently, water is not required for fertilization.  Rather, the wind or some animal moves the pollen from the male parts to the female.  Not only is the sporophyte dominant, but the gametophyte has been reduced to a tiny cluster of cells which is totally dependent upon the sporophyte for its survival.  Additionally, the embryo is surrounded by a layer of nourishing tissue and a protective covering to form the seed.  Since the seed is typically too large to be disseminated by the wind, other methods of dispersal have evolved in the seed plants.

 

Gymnosperm reproduction

 

Reproduction in gymnosperms will be exemplified by the conifers; specifically, the pine.  Pine reproduction is visually summarized on page 314 of your textbook.  The reproductive structures in this group are cones, of which there are two types; male and female.  The tiny male cones form clusters on the lower branches of the tree (sporophyte) in early Spring.  Each cone consists of a series of small scales arranged in whorls (circles) around a central axis.  Each scale develops two pouches on its lower surface, within which a group of diploid (2n) cells undergoes meiosis to form haploid (n) microspores.  These microspores quickly develop into winged pollen grains which constitute the two-celled male gametophyte.  The male cones last only a few days.

 

The female cones, much larger than the male, are borne singly, higher on the tree and remain on the tree two or more years.  These cones have a construction similar to that of the male, but the gametophyte develops on the top of the scales.  The haploid gametophyte forms after meiosis of a megaspore mother cell.  One or two archegonia form at one end of the gametophyte, each containing a single egg.

 

In late Spring, the pollen sacks split, releasing clouds of yellow pollen into the air.  Pollen grains are carried by the wind to the immature female cone, where they become trapped in a film of sticky fluid.  As the fluid dries, the pollen is carried downward between the cone scales until it lies near the axis, close to the female gametophyte.  At this point the pollen "germinates" to form a tube which grows through the tissues of the female gametophyte to the archegonium. A sperm nucleus is discharged into the archegonium, fertilizing the enclosed egg and forming a zygote, the first cell of the sporophyte.

The cells of the female gametophyte proliferate to form a nutritional base for the developing embryo.  This tissue, together with the embryo and the surrounding covering, forms the seed.  Late in the second year of development, the cone scales separate and the winged seeds are released to begin a new generation of pine trees.

 

 

ACTIVITIES

 

a. Observe the herbarium specimens of pine, as well as the several specimens of pine cones provided.  With the aid of the photographs on pages 55 and 56 in Perry and Morton, be able to identify staminate (male) cones and both immature and mature ovulate (female) cones.

 

b. Select a slide of the Pinus staminate cone and observe under low power.  Be able to identify the pollen sacs, the axis of the cone and the pollen grains.  Increase the magnification and note the winged pollen grains.  Figures 55c and 55d in the Photo Atlas will be of some aid in this activity.

 

c. Now procure a slide of the Pinus ovulate cone.  Under low power, and with the help of Figures 56c and d, locate the developing seeds (ovules) near the axis of the cone.  Find a good section of an ovule and try to identify the integuments (which will become the seed coat) and the micropyle, the opening through which the pollen tube will grow.  You may also be able to find the megaspore mother cell or even an egg, depending upon the maturity of the megasporangium.

 

 

 

Angiosperm reproduction

 

Reproduction in angiosperms involves special structures called flowers.  While the flowers of the various species differ - sometimes radically - a "typical" flower is made up of several whorls of parts, all emanating from the receptacle, the swollen tip of the flower stem.  Examine the flower on page 406 in Starr as you read the following description. The outermost whorl is made up of the sepals.  In some flowers the sepals are brightly colored, but in others they are green and serve to protect the flower before the "bud" opens.  Interior to the sepals are the petals.  These are often brightly-colored and "showy".

 

The next whorl of flower parts are the male reproductive organs.  These are termed stamens and are formed of a stalk, the filament, and the pollen-producing portion or anther.

 

The central portion of the flower is occupied by the female organ.  This is termed the pistil and it is composed of the swollen base or ovary, in which the seeds develop, a slender extension of the ovary termed the style, and the sticky tip of the style (stigma).

Flowers which have all of the above parts are termed perfect flowers.  Many flowers lack some organs and are therefore imperfect.  Some imperfect flowers may be of one sex only, others may have brightly colored sepals and no petals.  Often wind-pollinated flowers have no colorful parts at all!  In still other cases, flower parts of the same or different whorls maybe fused.  On close inspection the pistil, for example, may prove to be compound, indicating a fusion of several simple pistils.

 

 

ACTIVITIES

 

d. Select a flower from the Gladiolus and examine it carefully.  Is it a monocot or a dicot?  (Consult last week's lab if you do not remember how to make this determination.)  Use the photographs on page 60 in your Photo Atlas to assist you in identifying the various parts of the Gladiolus.  Note that both the sepals and petals are colored and are partially fused.  You may use a razor blade to make a careful dissection.  Is the pistil simple or compound?  Look at the stigma for a hint.  Carefully cut across the ovary to allow you to see what is inside.  Were you right?  How many parts were fused to form this ovary?  Note the small white ovules inside.

 

e. Carefully remove a flower from the potted geranium or other plant supplied by your instructor.  Perhaps the photos on page 61 of Perry and Morton will be of help.  Identify and count the sepals and petals.  Is this a monocot or a dicot?  Is this a perfect flower?  Identify the remaining flower parts.  Dissect the ovary to see if it is simple or compound.

 

 

 

As illustrated in Figure 24.6, page 408-409 of your textbook, the development of fruit begins with the distribution of pollen, the male gametophyte, which formed by meiosis of some diploid cells in the anthers.  Although it is possible for some flowers to pollinate themselves, cross-pollination is generally the rule.  In plants which lack brightly-colored and/or "smelly" flowers, pollen is distributed by the wind.  Bright colors, sweet nectars and strong odors in flowers indicate that their pollen is distributed by insects or other animals.

 

However it is transferred, when the pollen reaches the stigma of the pistil, it adheres and germinates.  The pollen tube grows down the style to the ovary and eventually reaches the ovule into which two sperm nuclei are discharged.

 

Meanwhile, within the ovule, meiosis of a megaspore mother cell and subsequent mitotic divisions lead to the development of a female gametophyte, the tissues of which enclose three special nuclei, an egg nucleus and two polar nuclei.

 

In order for an ovule to develop into a seed, an event unique to the angiosperms must occur.  This is termed double fertilization.  As in all sexual organisms, the egg fuses with a sperm nucleus to form a diploid (2n) zygote which will grow and differentiate to form the embryo.  At the same time, the remaining sperm nucleus fuses with both polar nuclei to form a triploid (3n) primary endosperm nucleus.  From this nucleus is derived the endosperm, a body composed mostly of starch which furnishes the nutritional material necessary to sustain the embryo until it germinates and puts its photosynthetic machinery to work.  The seed, then, is composed of the embryo, the endosperm and the seed coat.

 

The seeds of the angiosperm are retained within the protective confines of the ovary as they develop.  The ovary itself grows larger and thicker and often differentiates specialized tissues to become a fruit.  Biologically, a fruit is simply a ripened ovary.  Fruits protect the seeds and/or aid in distributing them.  Biologically, a vegetable is an edible part of a plant NOT associated with the ovary.  Students are then often surprised to find that tomatoes, cucumbers and squash are technically fruits and not vegetables!  Table 24.1, page 411 of Starr, describes the most common types of fruits.

 

ACTIVITIES

 

f.  Consider the collection of fruits provided and identify those representing the following categories:

 

1.  Dry fruits are those which are dry at maturity.  Included among the dry fruits are:

 

         (a)  The legume consists of an elongated simple pistil which splits open at two points when dry.  Beans and peas are legumes (See Figure 66b in Perry and Morton).

 

         (b) Nuts are dry fruits in which special basal leaves termed bracts partially or totally enclose the ovary.  Acorns, hickory nuts and walnuts are examples.

 

         (c)  In a grain, the wall of the simple ovary is fused with the single seed within it.  Corn, wheat, rice, etc. are grains.

 

         (d)  The winged fruits of the maple are called samaras.  The wings "helicopter" the seed away from the parent plant.  See Figure 24.8, page 411 in Starr.

 

     2.  In fleshy fruits, the ovary wall is thick and generally nutritious.  Its function is apparently to entice animals into eating the fruit and its enclosed seeds.  The seeds pass through the animal undamaged and are therefore disseminated some distance from the parent plant.  Common fleshy fruits include:

 

         (a)  The berry is derived from a compound pistil.  It seems odd to most students that the tomato is a berry!  Examine Photo Atlas Figures 68a and b, then consider the cross-section of the tomato provided.  How many pistils are fused to form this fruit?

 

         (b)  The pome, of which the apple is an example, is termed an accessory fruit because the bulk of the fruit is not made of the ovary wall, but of an accessory structure.  In the case of the apple, the receptacle overgrows the ovary, which we call the core.  Look at both apple dissections.  Examine the longitudinal section, using Figures 68c and d as a guide.  Identify all parts shown on the photos.  Look at the cross-section and see if you can tell if the apple is a dicot or monocot.

 

g.  The lima bean will be examined as a representative dicot seed.  When these seeds are mature, the nutritional material of the endosperm has been absorbed and incorporated into two thickened

 or seed leaves.  When we eat legumes, of which the bean is one, we are consuming mostly cotyledons.  Select one of the softened lima beans and care fully separate it into two halves by slicing the seed coat.  Now examine the Plastomounts of the germinating bean seed.  Use Figures 67a-c in your Photo Atlas to aid you in identifying the various parts.  Why do we consider the bean a dicot?

 

h.  The grasses are monocots and we will use the seed of a large grass, corn, to illustrate the monocot seed.  Examine the corn seed and note the presence of the embryo or "germ".  Most of the corn seed is endosperm and when we buy corn meal it has usually been "de-germed".  Using a razor blade, cut the corn seed in half, being careful to bisect the "germ".  Now add a drop of iodine stain to the cut portion and observe what happens.  Iodine reacts with starch to create a dark blue to black color.  What part of the corn seed stained in this way?  Use Figure 67d to identify the parts of the corn seed.  Note that there is only one cotyledon, the scutellum.

 

 

 

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