Reproductive Strategies

    A. A life cycle is full sequence from fertilization and formation of a zygote to gamete formation once again.

        1. In contrast to animals with one type of adult generation, flowering plants exhibit an alternation of
           generations life cycle that includes a diploid and a haploid generation.
        2. Sporophyte is diploid generation in an alternation of generations life cycle.
            a. Sporophyte produces haploid spores by meiotic division.
            b. Spores develop into a haploid gametophyte.
        3. Gametophyte is haploid generation in an alternation of generations life cycle.
            a. Gametophyte produces haploid gametes by mitotic division; gametes fuse to form diploid zygote.
            b. Zygote undergoes mitotic cell division to develop into the sporophyte.
        4. In flowering plants, diploid sporophyte is dominant (longer lasting); it is what we commonly recognize.
        5. Sporophyte is generation that contains vascular tissue and has other adaptations suitable to living on
            land, including production of flowers.
        6. A flower produces two types of spores, microspores and megaspores.
            a. A microspore is a plant spore that develops into a microgametophyte.
                1) Microgametophyte is pollen grain; wind or animals carry it to megagametophyte.
                2) When mature, its non-flagellated sperm cells travel down pollen tube to megagametophyte.
            b. Megaspore is a plant spore that develops into a megagametophyte, the embryo sac which remains
                within a sporophyte plant.
        7. Alternation of generations is modified so sperm does not require water to reach an egg, which
            nonflowering plants may require.
            a. Gametophytes are microscopic and dependent upon the sporophyte.
            b. Microgametophyte is transported to megagametophyte by wind or an animal; water is not needed.
            c. Pollen tube carries a sperm to an egg; again no outside water is needed.
            d. Following fertilization, diploid zygote develops into an embryo within a seed enclosed by a fruit.

    B. The Sporophyte

        1. A flower is the reproductive organ of a flowering plant; it develops within a bud.
        2. Shoot apical meristem stops forming leaves to form flowers; axillary buds can become flowers directly.
        3. Flower structures are modified leaves attached to a stem tip (receptacle).
        4. Monocot flower parts are in threes or multiples; dicot flower parts are in fours or fives or multiples.
        5. Sepals are leaf-like, usually green; this outermost whorl protects the bud as flower develops within.
        6. Petals are interior to sepals; coloration accounts for attractiveness of many flowers.
            a. Size, shape, and color of a flower are attractive to a specific pollinator.
            b. Wind-pollinated flowers often have no petals at all.
        7. Pistil is vase-like structure located at center of a flower; it contains one or more carpels.
            a. Pistil may be simple or compound.
            b. Simple pistil contains one carpel; compound pistil contains multiple carpels which are often fused.
        8. Carpelsare reproductive units of flowers and have three parts.
            a. Stigma is an enlarged sticky knob on end of a style; stigma serves to receive pollen grains.
            b. Style is a slender stalk that connects stigma with the ovary.
            c. Ovary is enlarged base of a carpel that contains a number of ovules.
        9. Grouped about a pistil are stamens, stalked structures that have two parts.
            a. Anther is a sac-like container within which pollen grains develop.
            b. Filament is a slender stalk that supports the anther.
        10. Not all flowers have sepals, petals, stamens, and a pistil.
            a. Complete flowers have sepals, petals, stamens, and a pistil; incomplete flowers do not.
            b. Perfect flowers have both stamens and a pistil.
            c. Staminate flowers have only stamens.
            d. Pistillate flowers have only pistils.
        11. If staminate and pistillate flowers are on same plant, the plant is monoecious.
        12. If staminate and pistillate flowers are on different plants, the plant is dioecious.
        13. It is not strictly correct to call a pistil the female part of a flower, or a stamen the male part; these
            organs do not produce gametes.
        14. The pistil and stamen produce megaspores and microspores that mature to produce eggs and sperm.

    C. The Gametophytes

        1. The ovary in a carpel contains one or more ovules.
        2. Ovules has center mass of parenchyma cells covered by integument with one opening (micropyle).
        3. One parenchyma cell enlarges to become megasporocyte.
            a. Megasporocyte undergoes meiotic cell division to produce four haploid megaspores.
            b. Three megaspores disintegrate; one megaspore nucleus divides mitotically into eight nuclei.
            c. When cell walls form around the nuclei later, there are seven cells, one of which is binucleate.
            d. Megagametophyte (embryo sac) consists of seven cells:
                1) one egg cell,
                2) two synergid cells,
                3) one central cell with two polar nuclei, and
                4) three antipodal cells.
        4. An anther has four pollen sacs; each contains many microsporocytes.
            a. Microsporocytes are microspore mother cells.
            b. Microsporocytes undergo meiotic cell division to produce four haploid microspores.
            c. Each microspore divides mitotically forming two cells: a tube cell and a generative cell.
            d. This immature microgametophyte is a pollen grain.
            e. Eventually each generative cell divides mitotically to form two sperm.
            f. Walls separating pollen sacs break down and pollen grains are released.
        5. Pollen grains are windblown or carried by various kinds of animals to stigma of a pistil.

    D. Pollination and Fertilization

        1. Pollination and fertilization are separate events.
        2. Pollination is strictly transfer of pollen from anther to stigma of a pistil.
            a. Pollination occurs by wind or with assistance of particular animal pollinators
            b. Self-pollination is transfer of pollen from anther to stigma of the same plant.
            c. Cross pollination is transfer of pollen from anther of one plant to stigma of another plant; it is
                evolutionarily advantageous because of genetic recombination resulting in new and varied plants.
        3. Fertilization (Fig. 39.3b)
            a. Fertilization is fusion of nuclei, as when sperm nucleus and egg nucleus fuse.
            b. When a pollen grain lands on a stigma, it germinates, forming a pollen tube.
            c. A germinated pollen grain, containing a tube cell and two sperm, is mature microgametophyte.
            d. As a pollen tube grows, it passes between cells of stigma and style to reach micropyle of an ovule.
            e. Pollen tube grows through micropyle and releases both sperm cells into the ovule.
            f. One sperm nucleus unites with the egg nucleus, forming a 2n zygote.
            g. The other sperm nucleus migrates and unites with polar nuclei of central cell, forming 3n
                endosperm nucleus.
            h. This is double fertilization.
        4. Zygote divides mitotically to become the embryo; endosperm cell divides mitotically to become endosperm.
            a. Embryo, in most plants, is a young sporophyte.
            b. Endosperm is tissue that will nourish embryo and seedling as they undergo development.

28.2. Seed Development

    A. Stages in Development of Dicot Embryo

        1. After double fertilization, the ovule develops into a seed.
        2. The endosperm nucleus divides to produce a mass of endosperm surrounding embryo.
        3. The single-celled zygote also divides, forming two parts: embryo and suspensor, which anchors
            embryo and transfers nutrients to it from the sporophyte plant.

    B. Embryonic Development

        1. After differentiation into embryo and suspensor, one or two cotyledons develop.
            a. Cotyledon (seed leaf) provides nutrient for a developing plant before it photosynthesizes.
            b. Dicot embryos develop two cotyledons; monocot embryos develop only one cotyledon.
            c. During development of a monocot embryo, the cotyledon rarely stores food; rather, it absorbs food
                molecules from endosperm and passes them to embryo.
            d. During development of a dicot embryo, cotyledons usually store nutrients the embryo uses, obtaining
                those nutrients from endosperm.
        2. Embryo continues to differentiate into three parts.
            a. Epicotyl is between cotyledons and first leaves; it contributes to shoot development.
            b. Hypocotyl is below cotyledon and contributes to stem development.
            c. Radicle is below hypocotyl and contributes to root development.

28.3. Fruit Types and Seed Dispersal
 
     A. Seeds and Fruits

        1. As a zygote develops into embryo, integuments of the ovule harden and become seed coat.
        2. Ovule matures into the seed, containing sporophyte embryo plus stored food.
        3. Ovary (and sometimes other floral parts) develops into fruit (mature ovary that usually contains seeds).

    B. Types of Fruits

        1. As fruit develops from an ovary, ovary wall thickens to become pericarp.
        2. Simple fruit develops from an individual ovary, either simple or compound.
        3. Fleshy fruit has a fleshy pericarp (e.g., peach, plum, olive, grape, tomato, apple, and pear).
        4. Compound fruit develops from a group of individual ovaries (e.g. apple, tomato).
        5. Dry fruit pericarp is dry (e.g., milkweed, pea, bean, lentil, poppy, sunflower, acorn, rice, and barley).
            a. Aggregate fruit develops from ovaries from single flower (e.g., blackberry), while an aggregate fruit
                where each ovary becomes a one-seeded fruit is called an achene (e.g., strawberry).
            b. A multiple fruit develops from ovaries from separate flowers fused together (e.g., pineapple).

    C. Seed Dispersal

        1. For plants to be widely distributed, seeds have to be dispersed away from parent plant.
        2. Hooks and spines of clover, bur, and cocklebur attach to fur of animals.
        3. Birds and mammals eat fruits, including seeds, and defecate them at a distance.
        4. Squirrels and other animals gather seeds and fruits and bury them some distance away.
        5. A coconut floats hundreds of kilometers; some plant seeds have trapped air or inflated sacs.
        6. Woolly hairs, plumes, and wings disperse by wind.
        7. Touch-me-not has seed pods that swell as they mature and burst, hurling their ripe seeds.

    D. Seed Germination

        1. Some seeds do not germinate until they have been dormant for a period of time.
            a. Seed dormancy is a time during which no growth occurs even though conditions are favorable.
            b. In temperate zones, seeds may have to be exposed to cold weather before dormancy is broken.
            c. In deserts, germination requires adequate moisture; this ensures that seeds do not germinate until
                a favorable growing season has arrived.
        2. Germination takes place if there is sufficient water, warmth, and oxygen to sustain growth.
            a. Regulation of germination involves both growth inhibitors and growth simulators.
                1) Fleshy fruits contain inhibitors; germination does not occur until seeds are removed and washed.
                2) Growth stimulators are present in seeds of some temperate zone woody plants.
            b. Mechanical action may also be required to facilitate germination.
                1) Water, bacterial action, and fire act on seed coat, allowing it to become permeable to water.
                2) Water uptake causes seed coat to burst.
        3. Germination in Dicots
            a. Prior to germination, embryo consists of the following:
                1) two cotyledons that supply nutrients to embryo and seedling soon shrivel and disappear;
                2) a plumule-a rudimentary plant consists of an epicotyl bearing young leaves;
                3) hypocotyl, which becomes the stem; and
                4) radicle, which develops into roots.
            b. As dicot seedling emerges, the shoot is hook-shaped to protect delicate plumule.
            c. As seed germinates in darkness, it etiolates; stem increases in length and leaves remain small.
            d. Phytochrome pigment sensitive to red and far-red light induces normal growth in light.
        4. Germination in Monocots
            a. Endosperm is food-storage tissue; cotyledon does not have a storage role.
            b. Monocot "seed" is actually the fruit; outer covering is the pericarp.
            c. Prior to germination, embryo consists of one cotyledon, a plumule, and a radicle.
            d. Plumule and radicle are enclosed in protective sheaths: coleoptile and coleorhiza, respectively.
            e. Plumule and radicle burst through these coverings when germination occurs.

28.4. Asexual Reproduction in Plants

    A. Means of Asexual Propagation

        1. Plants contain non-differentiated meristem tissue and reproduce asexually by vegetative propagation.
        2. In asexual reproduction, offspring arise from a single parent and inherit genome of that parent only.
        3. Vegetative propagation utilizes meristematic tissue of a parent plant.
            a. Nodes of stolons will produce strawberry plants.
            b. Violet plants grow from nodes of rhizomes.
            c. Each eye of a potato plant tuber is a bud that produces a new plant.
            d. Sweet potatoes can be propagated from modified roots.
            e. Many trees can be started from small "suckers."
        4. Stem cuttings have long been used to propagate a wide array of plants (e.g. sugarcane, pineapple).
        5. Discovery that auxin will cause roots to develop has expanded ability to use stem cuttings.

    B. Tissue Culture of Plants

        1. In 1902, German botanist Gottleib Haberlandt suggested producing entire plants from tissues.
        2. Tissue culture is process of growing tissue artificially in a liquid culture medium.
        3. Haberlandt stated plant cells were totipotent; each cell has full genetic potential of the organism.
        4. In 1958, Cornell botanist F.C. Steward grew a complete carrot plant from a tiny piece of phloem.
        5. When cultured cells are provided with sugars, minerals, vitamins, and cytokinin, the undifferentiated
            cells divide and initially form a callus, an aggregation of undifferentiated cells.
        6. The callus then differentiates into shoot and roots and develops into complete plants.
        7. Micropropagation is a commercial method of producing thousands to millions of identical seedlings,
            by tissue culture in limited space.
        8. Meristem culture micropropagates many new shoots from a single shoot apex culture in a medium with
            correct proportions of auxin and cytokinin.
            a. Since shoots are genetically identical, adult plants that develop are clonal plants.
            b. Clonal plants have same genome and display same traits.
            c. Meristem culture generates meristem that is virus-free; plants produced are also virus-free.
        9. Entire plants can be grown from single plant cells.
            a. Enzymes can digest cell walls and produce naked plant cells called protoplasts.
            b. Protoplasts regenerate a cell wall and begin cell division.
            c. Clumps of cells can be manipulated to form somatic embryos.
            d. Somatic embryos encapsulated in a hydrated gel ("artificial seeds") can be shipped anywhere.
            e. Somatic embryos are cultured by the millions in large tanks (bioreactors).
            f. Plants generated from somatic embryos vary because of mutations; these somaclonal variations may
                produce new traits.
        10. Anther culture cultures mature anthers in a medium of vitamins and growth regulators.
            a. Haploid tube cells within a pollen grain divide, producing proembryos made of up to 40 cells.
            b. Finally, pollen grains rupture, releasing haploid embryos.
                1) The researcher can then generate a haploid plant.
                2) Chemical agents are added to encourage chromosomal doubling; resulting plants are diploid and
                    homozygous for all alleles.
        11. Cell suspension culture uses rapidly growing calluses cut into small pieces and shaken in a
            liquid nutrient medium.
            a. Single cells or small clumps form a suspension of cells; all produce same chemicals as the plant.
            b. This technique is a more efficient way of producing chemicals used in drugs, cosmetics, and
                agricultural applications than farming plants simply to acquire chemicals they produce.

    C. Genetic Engineering of Plants

        1. Traditionally hybridization (crossing different varieties or species) was used to produce new plants.
        2. Transgenic plants carry foreign genes introduced into their cells.
        3. Genetic engineering alters genes of organisms so they have new and different traits.
        4. Protoplasts in particular lend themselves to direct genetic engineering in tissue culture.
            a. High voltage electric pulses create pores in plasma membrane so new DNA can be introduced.
            b. When genes for production of firefly enzyme luciferinase were inserted into tobacco protoplasts,
                adult plants glowed when sprayed with the substrate luciferin.
            c. Foreign DNA can be inserted into a plasmid of Agrobacterium; this bacterium infects plant cells and
                can be used to deliver the recombinant DNA to target cells.
            d. John Sanford and Theodore Klein of Cornell University developed a particle gun to bombard plant
                cells in culture with DNA coated metal particles; later, adult plants are generated.
        5. Crops have been engineered to resist frost, fungal and viral infections, insect predation, and herbicides.
        6. Future crops could have higher protein content and require less water and fertilizer.
        7. Sequencing the genomes of a dicot Arabidopsis thaliana and rice will give a blueprint to the genes of
            other monocots and dicots.
        8. Production of salt-, drought-, and cold-tolerant crops may provide enough food for future populations.
        9. Soybeans have been altered to produce healthier fatty acids, and commercial products.
        10. Genetic engineering is attempting to improve efficiency of RuBP carboxylase and introduce C4
            photosynthesis to rice.
        11. Single-gene transfers allow plants to produce human hormones.
            a. Corn has made antibodies to deliver radioisotopes to tumor cells.
            b. Soybeans make an antibody to treat genital herpes.
            c. Researchers can introduce a human gene into tobacco plants using tobacco mosaic virus.
            d. Tobacco plants produced antigens to treat nonHodgkin's lymphoma.