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1. Sexual reproduction
- flowers attract pollinators that help bring sperm (n) and egg (n) together
to form zygote (diploid, 2n) [What does "n" stand for?]
- sperm, egg = gametes (two sets of genetic instructions)
- female structures house the embryo during development
2. Asexual reproduction
- "cloning" --> offspring produced by mitosis and are genetically
identical to parent
3. Alternation of stages [Fig. 31.2a]
- sporophyte = diploid stage; develops from zygote by mitosis; familiar adult
plant: roots, stems, leaves
- sporophyte produces flowers for sexual reproduction; flowers contain male
and/or female structures
(- sporophyte may also be produced asexually from runners, cuttings, underground
stems)
- gametophyte = small haploid stage; male and female gametophytes develop from
haploid spores into multicellular, male and female haploid forms (very different
from animals)
- gametophytes develop inside flowers by mitosis of haploid cells; male gametophyte
gives rise to haploid sperm; female gametophyte gives rise to haploid eggs
1. Components of flowers
- specialized parts grow from modified end of floral shoot = receptacle; cells
divide and differentiate into nonfertile parts (petals and sepals) and fertile
parts (stamens and carpels)
- sepals = outermost green, leaflike parts; petals = colored (sometime smelly)
parts located between reproductive parts and sepals; petals and sepals consist
of ground tissue, vascular tissues and epidermis
- stamens = male reproductive parts (filament (stalk) + anther); haploid pollen
grains develop from walled spores inside pollen sacs in anthers (pollen grains
= immature gametophyte; each grain contains two haploid cells, one of which
will give rise to two sperm cells)
- carpels = female reproductive parts at center of flower (ovary + stigma (ovary
at base of carpel, stigma at top (usually on a slender style));
- ovary is where female gametophyte, eggs develop, and where eggs are fertilized
and seeds mature [most of the action is in the ovary]
2. Perfect vs. imperfect flowers
- "perfect" flowers have both male and female parts; "imperfect"
flowers have either male or female parts (oaks); some plants have male and female
flowers on separate individual plants (willows)
1. Microspores --> Pollen grains
- inside anthers, each diploid "mother" cell divides by meiosis and
cytoplasmic division to form four, haploid microspores (with hard walls)
- each microspore will divide to form pollen grains (each grain contains two
haploid cells)
- one cell in each pollen grain will produce the sperm (two of them); the other
will form the pollen tube; [depending on the plant species the pollen grain
will be either a mature gametophyte (containing sperm cells) or an immature
gametophyte (sperm not formed until grain lands on stigma and starts growing
toward ovule)]
2. Megaspores --> Eggs
- in carpel, tissue mass forms ovule (= potential seed) inclosed by "integuments"
- diploid "mother" cell divides by meiosis and cytoplasmic division
to produce four haploid megaspores; only one of these will survive, the other
three disintegrate
- remaining megaspore undergoes mitosis three times without cytoplasmic division!
to produce a single cell with eight nuclei
- cytoplasmic division takes place to produce the mature, female gametophyte:
six cells with one nucleus each + one cell with two nuclei = embryo sac inside
ovule
- one of the six cells will become the fertilized egg; the cell with two nuclei
will become the endosperm (nutrition for the embryo)
3. Pollination --> Fertilization
- pollination = transfer of pollen to surface of stigma by wind, insects, birds,
etc. [read Commentary, pp. 520-521]
- germination = formation of pollen tube [How?] that grows down to and penetrates
ovule; two sperm cells pass down pollen tube and are released into ovule
- double fertilization = (a) fusion of one sperm cell nucleus and one egg cell
nucleus to form the diploid (2n) zygote; (b) the second sperm cell joins with
the diploid endosperm mother cell and the three nuclei fuse --> triploid
(3n) primary endosperm cell
1. Zygote --> Embryo sporophyte
- initially, zygote is attached to parent plant and derives nutrition by direct
transfer during differentiation of embryonic tissues
- cotyledons (= seed leaves) develop as part of all flowering plant embryos;
dicots have two, monocots have one
- cotyledons in many plants absorb endosperm and function in food storage -->
large cotyledons; in other plants (wheat, corn) cotyledons produce enzymes for
transferring stored food from endosperm to seedling
2. Seed and fruit formation
- from zygote to embryo, parent plant supplies nutrition; food reserves accumulate
in expanding endosperm or in cotyledons; eventually, connection between ovule
and ovary wall is broken
- mature ovule's integuments thicken into seed coat; seed = embryo + food reserves
+seed coat
- fruit = mature ovary w/ seeds (ovules) inside; may or may not have modified
floral structures incorporated into it
-fruits have evolved to take advantage of air currents,
water currents and animals for dispersal of seeds
- fruits function in seed protection, dispersal
- dispersal strategies and forms include "wings," hooks, hairs, sticky
surfaces and incorporation in animal digestive wastes
1. Asexual reproduction in nature
- vegetative growth modes include reproduction on modified stems: runners, rhizomes,
corms, tubers, and bulbs
2. Induced propagation
- vegetative propagation from "cuttings" (fragment of shoot system)
or from a tissue or organ (leaf) that is separated from parent plant (jade plant,
African violet)
- tissue culture propagation in which whole plants can form from a group of
cells
