Kingdom Plantae, Part II - Gymnosperms and Angiosperms I. Introduction Reproduction in the seed plants (Gymnosperms and Angiosperms) has been greatly influenced by the requirements of a terrestrial existence. For the most part, motile gametes are not involved. Thus, unlike bryophytes and ferns, water is not required for fertilization. Rather, the wind or some animal moves the pollen from the male parts to the female. Furthermore, not only is the sporophyte the dominant generation in seed plants, 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 dispersed by wind, other methods of dispersal have evolved in the seed plants. II. Objectives: Upon completion of this lab, students should be able to: 1) Identify the gametophyte and sporophyte generation of all plants studied. 2) Identify the basic structural characteristics of each plant group studied. 3) Describe the structure, function, and chromosome number (n or 2n) for the gametophyte and sporophyte generations of all plants studied. 4) Describe the life history and reproductive structures of Gymnosperms, and Angiosperms 5) Describe the relationship between green algae, the bryophytes, and vascular plants (ferns, Gymnosperms, and Angiosperms) III. Gymnosperm Reproduction Today, we will study reproduction in gymnosperms by focusing on the conifers (cone producing Gymnosperms), specifically, the pine. Figure 1 provides and overview of the pine lifecycle. The reproductive structures in this group are cones, of which there are two types: staminate (male) and ovulate (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. 1
Figure 1. Gymnosperm (Pinus) lifecycle. In late Spring, the pollen sacks on the male cones 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 pollen 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 generation. 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: 1) Observe the preserved specimens of pine cones provided. Be able to identify staminate (male) cones and ovulate (female) cones. 2
2) 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. Use the space below to sketch what you see. 3) Now examine a slide of the Pinus ovulate cone. Under low power, 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. Use the space below to sketch what you see. 3
IV. Angiosperm Reproduction Reproduction in angiosperms involves special structures called flowers. While the flowers of the various species differ, 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 in Figure 2 below (or the flower model on the side table) 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 brightlycolored 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 anther (pollen-producing structure). 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. Figure 2. Flower structure. 4
ACTIVITIES: 1) Select a flower from the side table and examine it carefully. What color are the sepals? The petals? 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. How many parts were fused to form this ovary? Note the small white ovules inside. Identify the remaining flower parts. 2) Select a different flower from the side table and examine it carefully. Identify and count the sepals and petals. Is this a perfect flower? Identify the remaining flower parts. Dissect the ovary to see if it is simple or compound. What color are the sepals? The petals? Use a razor blade to make a careful dissection. Is the pistil simple or compound? Carefully cut across the ovary to allow you to see what is inside. How many parts were fused to form this ovary? Identify the remaining flower parts. Why do you think many Angiosperms produce brightly colored flowers? Why do you think some Angiosperms produce uncolored flowers? 5
As illustrated in Figure 3, the development of a fruit in an Angiosperm begins with the release of pollen, the male gametophyte. Pollen grains are formed by meiosis of diploid cells called microspores in the anthers. Although it is possible for some flowers to pollinate themselves, cross-pollination (pollinating other flowers) 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. Figure 3. Angiosperm lifecycle. However pollen is transferred, when it reaches the stigma of the pistil of another flower, it adheres and germinates. A 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. 6
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 embryonic plant. At the same time, the remaining sperm nucleus fuses with both polar nuclei to form a triploid (3n) endosperm. 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 (for nourishment) and the seed coat (for protection). The seeds of the angiosperm are retained within the protective confines of the plant ovary as they develop. The ovary grows larger and thicker and ultimately becomes 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! ACTIVITIES: 1) Obtain a prepared slide of an anther cross section, showing young sporogenous tissue. Sketch and label the entire structure below. Be able to identify the walls of the anther and the microspores contained within. What is the ploidy of the walls of the anther? What is the ploidy of the microspores inside the anther? 7
What will the microspores become when they mature? How will they accomplish this (type of cell division)? 2) Obtain a prepared slide of an anther cross section, showing mature pollen grains. Sketch and label the entire structure below. Be able to identify the walls of the anther and the pollen grains contained within. What is the ploidy of the walls of the anther? What is the ploidy of the pollen grains inside the anther? How would you describe the structure of a pollen grain? Are there any "breaks" in the wall of the anther? Why would this occur? What will happen to the pollen grains once they are released from the anther? What will the pollen grains become? 8
3) Obtain a prepared slide of pollen tubes. Sketch and label the entire structure below. What is the function of a pollen tube? Where does a pollen tube form? Obtain prepared slides of the Lilium ovary showing ovules at the uninucleate, 2 nucleate, 4 nucleate, and 8 nucleate stages. Sketch and label the structures below. Make sure to identify the micropyle, megasporangium, egg cell, and the central cell with polar nuclei. 9
Depending on the species, there may be from one to several hundred ovules produced in the ovary of each pistil. A micropyle is present at one end of the ovule. The micropyle is a small canal in the tissue surrounding the ovule, through which the pollen tube usually enters the ovule on the way to the egg cell. Water also enters the seed via the micropyle prior to germination. In the majority of angiosperms a single cell in the ovule called the megasporocyte (or megaspore mother cell), enlarges and divides by meiosis to produce four haploid megaspore cells, three of which degenerate. The megaspore mother cell is what you observed as it divided from a single cell (uninucleate stage) to an 8 nucleate stage. The surviving megaspore enlarges and by means of three successive mitotic divisions gives rise to an eight nucleate megagametophyte (also referred to as an embryo sac). At the 8 nucleate stage, thin walls develop around six of the nuclei. At the micropyle end of the megagametophyte an egg cell is flanked by two other cells. At the opposite end of the megagametophyte are three other cells. The large central region of the megagametophyte is referred to as the central cell, which is composed of one cell that contains two polar nuclei. Thus, a mature megagametophyte contains 7 cells, including one egg cell and one central cell that contains 2 nuclei. What is the ploidy of the megasporocyte? What will the megasporocyte become? How will it accomplish this (type of cell division)? 10
What is the polidy of the megaspore mother cell? What type of cell division allows the megaspore mother cell to divide from a uninuceate stage to an 8 nucleate stage? Describe the structure of a mature megagametophyte. How many cells is a megagametophyte composed of? How many nuclei? What is another name for a mature megagametophyte? Which cell of the megagametophyte will fuse with a sperm cell to form a zygote? Which cell of the megagametophyte will fuse with a sperm cell to form the endosperm? Description of fertilization in Angiosperms: The pollen tube grows up the micropyle of the ovule, through the megasporangium and penetrates the megagametophyte. The tip of the tube bursts and the sperm are released. A unique feature of the angiosperms is the process of double fertilization. One sperm fuses with the egg to produce the zygote, the other sperm migrates to the central cell and fuses with the two polar nuclei to produce the endosperm. With double fertilization a number of processes are initiated: The zygote develops into an embryo; the tissue surrounding the megagametophyte develops into a seedcoat; the ovary develops into a fruit; and the primary endosperm nucleus divides to form endosperm. The endosperm functions as the nutritive tissue for the developing embryo and, in many cases, as the food reserve for the mature embryo during seed germination. The Angiosperm Fruit: After fertilization the ovary component of the pistil matures into a fruit. Additional parts of the flower (e.g. the receptacle) may also contribute to the formation of the fruit (e.g., the green leafy part of a strawberry). The different types of angiosperm fruits are far too numerous and varied to be examined in any detail in this course. In general fruits serve two important functions: 1) to protect the seeds during their maturation and 2) to effectively disperse the mature seeds. The chief agents in seed and fruit dispersal are wind, water and animals. What differentiates a fruit from a vegetable? Can you list at least one fruit specialized in seed dispersal via wind? Water? Animals? 11
V. Review Questions 1) How are Gymnosperms and Angiosperms better able to reproduce in terrestrial environments than ferns and bryophytes? 2) How are Angiosperms "more advanced" with regards to reproduction than Gymnosperms? 3) How are the sperm cells of Gymnosperms and Angiosperms different than ferns and bryophytes? Why? 4) What is the evolutionary significance of flowers? 5) What is the evolutionary significance of fruits? 6) Recall that as you progress from bryophytes to ferns to seed plants, the major evolutionary change is a severe reduction in the gametophyte generation. Can you think of one major reason for this shift? 12