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Introduction Gymnosperms and angiosperms are vascular, sporophyte-dominant plants that produce seeds. Although these heterosporous plants still reproduce by spores, seeds are the primary means of reproduction and dispersal. A seed consists of an embryonic sporophyte and food supply surrounded by a protective coat. Gymnosperms and angiosperms are the most numerous and widespread plants today because seeds are reproductively superior to spores. A seed contains a multicellular young plant with embryonic roots, stems, and leaves already formed, whereas a spore is a single cell. Also, seeds contain an abundant food supply while there are few food reserves in spores. Finally, seeds have a protective covering, or seed coat, that helps them survive adverse conditions for a prolonged period of time.

STATION 1: Gymnosperm Survey Gymnosperms are the “naked seed” plants, producing seeds that are either totally exposed or on the scales of cones (rather than being enclosed by a fleshy fruit as in the angiosperms). Gymnosperms are classified taxonomically in the following manner: Kingdom Plantae Phylum Coniferophyta - conifers Phylum Ginkgophyta – ginkgoes

Phylum Cycadophyta - cycads Phylum Gnetophyta - gnetophytes Phylum Coniferophyta: Pines The conifers are the most familiar group of gymnosperms and include pines, firs, and spruces. Most conifers are cone-bearing, woody evergreens that bear leaves throughout the year. The leaves of conifers are usually modified into narrow needles with a thick, waxy coat to retain water. Also, most conifers are monoecious, with male pollen cones and female seed cones borne on the same plant. The pine tree has a life cycle typical of conifers. The dominant sporophyte tree bears both male and female cones. The male cones are very small and occur in clusters near the tips of branches; male cones are quite fragile and persist only one season as they grow, produce pollen, and release mature pollen grains into the air. Female cones are larger, and persist for several years

The Seed Plants: Laboratory Gymnosperms & Angiospserms 55

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on the branches of the tree. After several years, winged-seeds are shed from the female cones and dispersed by the wind. Phylum Cycadophyta: “Sago Palms” Cycads are slow-growing, tropical evergreens with palm-like leaves and large seed cones. These so-called “sago palms” are dioecious, meaning that there are separate sexes with female plants bearing seed cones and male plants bearing pollen cones. Popular as ornamentals, many cycads are now endangered due to being over-harvested from the wild.

Male cone w/ pollen

Female cone w/ovules

Sago palm

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Phylum Ginkgophyta: Gingko biloba The only surviving member of this phylum is Gingko biloba, the oldest species of living trees. Like the cycads, Gingko biloba is dioecious, with separate male and female trees. The females bear fleshy seeds that are totally exposed rather than being inside a cone. Unlike evergreens, gingkoes are deciduous and shed their leaves in the fall. Gingko biloba has gained recognition in recent years because there is some evidence that it increases blood flow to the brain, possibly enhancing neurological function. Phylum Gnetophyta: Ephedra Phylum Gnetophyta consists of three diverse genera that share certain characteristics not present in other gymnosperms. Gnetophytes have the same water-conducting cells (vessel elements) present in angiosperms, which are more efficient than the water-conducting cells found in other gymnosperms. Also, some gnetophytes have cones arranged in clusters that give the impression of flower clusters (there is evidence that flowering plants evolved from a gnetophyte) . Phylum Gnetophyta includes Ephedra, a plant that produces the stimulant ephedrine. A. Observe various cones of the genus Pinus.

Contrast the smaller male pollen cones with the larger female cones.

B. Observe specimens of cycads, gingkoes, and gnetophytes.

Ginkgo biloba

Ephedra

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Questions: 1. List the characteristics of gymnosperms. 2. List the three components of a seed. 3. Why are seeds reproductively superior to spores? 4. Define:

monoecious - dioecious - deciduous - evergreen – 5. In the space below, list each phylum of gymnosperm, an example and a use of the plant by humans. Phylum Example Use by humans

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STATION 2: Introduction to Angiosperms

Angiosperms, commonly referred to as the flowering plants, are currently the most abundant, diverse and widespread of all terrestrial plants. Members of this group develop a flower for sexual reproduction, producing seeds enclosed within a fruit. Also unique among angiosperms is the process of double fertilization. Flowering plants are sporophyte dominant, with the large, diploid sporophyte being independent of the microscopic gametophytes.

Within Phylum Anthophyta, flowering plants are classified into two clades based upon the number of cotyledons, or embryonic seed leaves, present in the seed. The following table summarizes the major differences between the clades of flowering plants: monocots and eudicots.

Kingdom Plantae Phylum Anthophyta (2 clades: monocots and eudicots)

Characteristic Monocots Eudicots

Embryo One cotyledon Two cotyledons Leaf veins Parallel Reticulated (net-like)

Roots Fibrous roots Taproot (large) presentFloral parts Multiples of three Multiples of four or five

The sporophyte flower usually consists of four whorls of floral organs borne on the expanded end of a called the receptacle. The lowest and outermost structures, called sepals, are often leaflike and serve to protect the flower when it is a bud. Usually sepals are green; however, they may also be brightly colored and resemble petals. Collectively, the sepals of a flower may be referred to as the calyx. The petals lie within the sepal whorl and are typically brightly colored to attract pollinators. The collective term for the petal whorl is the corolla. The stamens, male reproductive floral parts, lie within the petal whorl. Each stamen consists of a slender filament with a pollen-producing anther at the tip. The female counterpart is the pistil. The pistils are the innermost floral structures, each composed of three parts: the stigma, style and ovary. The stigma is the expanded upper portion that secretes a sticky substance to trap and hold the pollen grains. The style is the stalk that extends from the stigma to the ovary, the swollen hollow basal region which will contain the ovules. Ovules will develop into seeds following pollination. The surrounding ovary tissue forms the fruit which encloses the seeds.

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A. Examine the floral specimens provided. Note the variation in size, shape and location of floral parts between flowers.

B. Using the flower model available, label all floral parts. 1) Identify all four floral whorls, the sepals, petals, stamens and pistil. 2) Examine the stamen and associated reproductive structures, including the filament,

anther, pollen grains, pollen tube, tube nucleus and sperm nuclei. 3) Examine the pistil and associated reproductive structures, including the stigma, style,

ovary, ovule, integuments, micropyle, embryo sac, synergids, antipodals, polar nuclei and egg.

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Questions: 1. List the characteristics of angiosperms. 2. What is the function of the petals? Of the sepals? 3. Name the region of the pistil where pollen grains land. 4. Where are pollen grain produced? 5. Fill in the following chart:

Characteristic Monocots Eudicots Embryo

Leaf veins Roots

Floral parts 6. List the two parts of a stamen. 7. List the three parts of a pistil. 8. What tissue becomes the fruit? What is the function of this fruit tissue (how does it aid the plant in completing its life cycle)?

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STATION 3: Angiosperm Reproduction

As is observed in all plants, the life cycle of angiosperms involves the alternation of a multicellular haploid gametophyte with a multicellular diploid sporophyte. The sporophyte produces spores by meiosis, each of which develops into a gametophyte. The sporophyte is the dominant generation in the angiosperms and is represented by any mature plant with flowers. The gametophyte lies within the sporophyte flower and is reduced to a pollen grain (that will produce two sperm nuclei) or an embryo sac (that will produce an egg) within an ovule.

Development of the Microgametophyte (Pollen Grain) The development of the microgametophyte (or pollen grain) occurs within pollen sacs of the anther. Microspore mother cells (microsporocytes) located in the pollen sacs undergo meiosis to produce four haploid microspores. Each microspore divides mitotically to produce an immature pollen grain containing two nuclei, the tube nucleus and the generative nucleus. The pollen grain is considered mature once the generative nucleus divides to form two sperm cells. View the prepared slide of the anther.

Compare with the illustration below, noting the pollen sac and pollen grains. Development of the Megagametophyte (Embryo Sac) Each ovary contains a cavity that contains one to many ovules depending upon the plant species. Penetrating through the integuments, the outermost layers of the ovule, is a small opening, the micropyle, through which the pollen tube later grows following pollination. The development of the megagametophyte (or embryo sac) occurs within the ovule of the ovary. The megaspore mother cells (megasporocytes) located within each ovule undergo meiosis to produce four haploid cells. Three of these cells usually disintegrate without further development, leaving one remaining megaspore to produce the embryo sac (female gametophyte). After three successive mitotic divisions of the megaspore, a mature embryo sac

Anther x.s.

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consists of seven cells with eight haploid nuclei (three antipodal cells, two synergid cells, one egg cell and two polar nuclei). View the prepared slide of embryo sac.

Compare with the illustration below, noting the ovule, integuments, micropyle, synergids, antipodals, egg and polar nuclei

Pollination and Fertilization Pollen released from the pollen sacs may be carried by wind, water and animals (insects, bats, humans and birds) to the stigma of the same or neighboring flower. The transfer of pollen from the anther to the stigma is pollination. Once pollination has occurred, the pollen grain germinates with the tube nucleus forming a pollen tube. As the tube develops down the style and into the ovary, the tube nucleus remains in the tip of the pollen tube to govern its development. The generative nucleus moves into the pollen tube and divides to form two cells, the sperm.

View the prepared slide of the pollen grains and developing pollen tubes.

Compare with the illustration below, noting the pollen sac and pollen grains. The two sperm will be visible within the tube.

Embryo sac within ovule containing 8 nuclei

Pollination & germination of pollen grains

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The pollen tube enters the ovule through the micropyle. As the tube nucleus dies, the two sperm cells enter the embryo sac. One sperm unites with the egg, resulting in a diploid zygote. The other sperm fuses with the polar nuclei in the central cell to form the triploid (3n) endosperm, a nutritive tissue that supports the growing embryo. This process that results in a diploid zygote and a triploid endosperm is called double fertilization and is characteristic of all angiosperms. View the prepared slide the embryo with endosperm.

Compare with the illustration below, identifying the embryo, endosperm & integuments. Embryo Development and Seed Formation After fertilization, the sepals, petals, stamen, stigma, and style dry up and fall off. The zygote undergoes a series of mitotic divisions forming a multicellular embryo located at the base of the embryo sac near the micropyle. Simultaneously, the endosperm undergoes a series of divisions filling the space around the embryo. The ovule becomes the seed, consisting of a dormant sporophyte embryo and packaged food supply (stored in either the cotyledons or endosperm). The integuments grow and harden to form a tough, outer covering of the seed known as the seed coat. The ovary usually enlarges and becomes the fruit. However, in some plants, the fruit is formed from other plant tissues in addition to the ovary. For example, the fleshy portion of an apple is composed of both the ovary and receptacle.

Developing embryo with surrounding endosperm tissue

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Questions: 1. Name the two nuclei found in a pollen grain. What does each produce? 2. List the two separate fertilization events that occur in flowering plants. 3. Following fertilization, the ovule develops into the ____________, the integuments form the

____________________ and the ovary matures into the ____________________.

4. What is the function of the micropyle?

5. Which cells undergo meiosis in the pollen sacs?

6. Which cells undergo meiosis in the ovules?

7. Describe two reasons why Angiosperms are more successful as a group than Gymnosperms.

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Label the following on the life cycle of a flowering plant below:

stamen, pistil, ovule, anther, pollen grain, pollen tube, embryo sac, integument, micropyle, seed

coat, seed, new sporophyte plant, meiosis, double fertilization, megaspores, microspore,

endosperm (3n), zygote (2n)