34.1. Life
Cycle of Flowering Plants
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.
(Fig. 39.1)
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). (Fig. 39.2)
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. (Fig. 39.3a)
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
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. (Fig.
39.5)
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.
34.2.
Development of the Embryo
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.
34.3. Fruits
and Seeds
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.
34.4.
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. (Fig. 39.10)
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. (Fig. 39.11)
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.