PLANT REPRODUCTION – CHAPTER 38 Study guide questions 1. Know the floral organs – be able to label Fig. 38.2. 2. Know the terms relating to floral diversity. 3. Know the structure of the stamen and carpel. 4. Know how a pollen grain develops from a microsporocyte. 5. Know how the embryo sac develops from the megasporocyte. Know the names and positions of the 8 nuclei in the embryo sac. 6. Be able to describe pollination and fertilization. 7. What is double fertilization? 8. Briefly describe two ways in which self-fertilization is prevented. 9. Be able to answer short questions from the notes about development of seed from ovule after fertilization (endosperm development; development of embryo; structure of mature seed). 10. Be able to answer short questions from the notes about fruit development and types of fruits (use lab write-up as well). 11. What are the advantages of asexual reproduction? 12. State one advantage and one biological disadvantage of genetic engineering. Plants reproduce sexually and asexually. 1. Sexual Reproduction Flowers – specialized shoots. Each is made up of floral organs arranged in four whorls, all attached at the receptacle: i. Sepals – enclose and protect bud; usually green and leaf-like ii. Petals – often brightly colored to attract pollinators; collectively called the corolla iii. Stamens – male reproductive organs iv. Carpel – female reproductive organ Figure 38.2 Floral diversity o Complete – have all four floral parts o Incomplete – do not have all four floral parts o Perfect – have both male and female organs o Imperfect – have either male or female organs but not both o Monoecious – one house - have male and female flowers on the same individual o Dioecious – two houses – have male and female flowers on separate individuals Stamen: filament and anther. o Anther is divided into chambers, the pollen sacs o The pollen sacs are the male sporangia o Inside the sporangia are many diploid cells called microsporocytes o Each microsporocyte divides by meiosis to produce 4 haploid microspores o A microspore divides by mitosis to produce two cells: a generative cell and a tube cell. o [The generative cell will eventually produce 2 sperm.] o [The tube cell will form the pollen tube.] o The tube cell and generative cell are enclosed in a thick, resistant wall with a pattern that is unique to the plant species. o Tube cell + generative cell + wall = pollen grain (immature, microscopic male gametophyte). o Only when the pollen lands on a stigma does the generative cell divide by mitosis to form 2 haploid sperm cells. The carpel is made up of the stigma, style and ovary o Stigma – a sticky landing platform for pollen o Style – a “neck” for pollen tube to grow through o Ovary: Has chamber(s) containing ovule(s) Each ovule contains a single sporangium. One cell of the sporangium is a diploid cell, the megasporocyte The megasporocyte divides by meiosis to produce 4 haploid cells Only one haploid cell survives and is known as the megaspore The megaspore divides three times by mitosis to give 8 nuclei These 8 nuclei are then surrounded by cell membranes to form a multicellular female gametophyte, the embryo sac (microscopic, protected by female sporophyte) The egg cell (female gamete) is at one end, flanked by two synergid cells (which attract and guide the pollen tube to the egg cell) There are two polar nuclei in the middle At the other end are three antipodal cells whose function is not known So, ovule = embryo sac + integuments (from sporangium and wall of ovule). Micropyle – gap in integuments for pollen tube. The ovule will eventually become a seed Figure 38.4 Pollination and fertilization: o Pollen grain (male gametophyte) reaches stigma by wind or pollinator o Absorbs moisture and produces a pollen tube o Pollen tube ovary o Generative cell 2 sperm o Pollen tube enters ovary and grows through micropyle to reach the embyo sac o The 2 sperm nuclei are discharged o One sperm fertilizes the egg zygote o The other sperm combines with the 2 polar nuclei to form a triploid nucleus in the center of the large central cell of the embryo sac. This will grow into the endosperm – the nutritive tissue of the seed. o The union of 2 sperm cells with different nuclei in the female gametophyte is known as double fertilization. It is an economic measure – only if the egg is fertilized will the endosperm develop Figure 38.9 Life cycle of plants: o Iincludes sporophyte and gametophyte generations o The sporophyte is dominant. o The sporophyte produces haploid spores by meiosis. o These divide by mitosis to give the haploid male and female gametophytes. o The male gametophyte is the pollen grain. o The female gametopyte is the embryo sac. o The gametophytes produce haploid gametes o Fertilization results in a diploid zygote which develops into the sporophyte embryo within the ovule which is in the ovary. o The ovule becomes the seed Figure 38.1 Preventing self-fertilization (and maximizing genetic variation) [Read p 788-9]. o Bisexual flowers – timing of maturation of stamens and carpels differs; or structural arrangement prevents self-fertilization o Self-incompatibility – biochemical block prevents pollen from completing its development. S-genes play a role (if pollen grain S allele matches S allele of stigma, it fails to grow). Pollen grain or stigma may initiate block and molecular mechanisms are varied. Development of seed from ovule after fertilization o Endosperm development After double fertilization, the triploid nucleus in the central cell of the endosperm divides to form a multinucleate “super-cell” This large, multinucleate cell has a milky consistency It becomes partitioned by cytokinesis Cell walls form The endosperm stores nutrients for the developing embryo and often the seedling after germination. In many dicots e.g. bean seeds, the endosperm nutrients are all exported to the cotyledons as the seed develops (the mature seed therefore lacks endosperm). o Embryo development [follow Figure 38.10] Zygote terminal cell and basal cell Basal cell suspensor (for anchorage to parent and transfer of nutrients from parent plant/occasionally endosperm to embryo Terminal cell proembryo Proembryo differentiates into cotyledon(s) [1 cotyledon in monocots; 2 in dicots] Embryo elongates Apical meristem of embryonic shoot is between the cotyledons Root apex with its meristem is at the opposite end Meristems that will give rise to the three tissue systems (dermal, ground and vascular) are also present o Structure of mature seed The embryo with enlarged cotyledons or endosperm stops growing until the seed germinates Dehydration occurs leaving only a small amount of water in the seed The integuments of the ovule form a protective seed coat E.g. the common bean: (Figure 38.11 a) – note cotyledons – fleshy with food absorbed from endosperm; hypocotyl (below where cotyledons are attached to each other) and the embryonic root or radicle; epicotyl (above cotyledon attachment point) with its plumule – shoot tip and two tiny leaves. Compare with the castor bean (Fig. 38.11b) and corn (Fig. 38.11c) and description in text, p 791-2). Development of fruit o Fruits protect the dormant seeds and aid in their dispersal. o A fruit develops from the ovary when fertilization has occurred (hormonally triggered). o The wall of ovary thickens to form the pericarp (the thickened wall of the fruit) o The floral parts either wither and fall off or become part of what we refer to as the fruit Fleshy part of apple is actually the swollen receptacle (core is from ovary) o As the fruit ripens, the seeds complete their development o Ripening depends on mode of seed dispersal Dry fruits (e.g. soybean pod) age causing fruit tissues to open and release the seeds. In fleshy fruits, ripening is under hormonal control – sugars are mobilized and color changes to attract animals that will help to disperse the seeds. o Fruits are modified to enhance seed dispersal – wing(s), burrs, edible o The fruit can be classified according to its developmental origin: Simple – one ovary; one flower e.g. cherry Aggregate – many ovaries; one flower e.g. raspberry Multiple – many ovaries; many clustered flowers e.g. pineapple [Each group has sub-categories – you’ll be looking at some of these in the lab]. Seed germination o Seed dormancy functions in increasing the chance that a seedling will germinate when the time and place are optimal. o Can be days decades or longer o Dormancy can be broken by sufficient moisture, intense heat (fire), intense exposure to cold, light, chemical breakdown of coat (in animal’s digestive tract). o When moisture is absorbed, the seed expands and the the seed coat ruptures. Stored nutrients are mobilized and transferred to growing regions of embryo. o Radicle (embryonic root) emerges first o Shoot tip breaks through soil surface by one of three mechanisms: Hook in hypocotyls straightens, pulling cotyledons from soil (e.g. in beans) Hook on epicotyl straightens (e.g. peas) Shoot grows up through protective tube of coleoptile (e.g. corn and other grasses). 2. Asexual reproduction Also known as vegetative reproduction. Results in clones. Two ways: fragmentation and apomixis. Advantages of sexual reproduction: o Genetic variation o Production of seed which can be dispersed to new location/be dormant until conditions improve Advantages of asexual reproduction: o Rapid cloning to exploit suitable environment o Fragments of parent plant more hardy than seedling Methods of asexually reproducing plants are important in agriculture/commercially: o Use of cuttings; grafting o In vitro cloning – tissue culture (can apply biotechnology procedures to tissue culture methods). 3. Plant biotechnology [read p 797-9 in your text].
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