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PLANT REPRODUCTION – CHAPTER 36

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					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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