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35
Reproduction in Flowering Plants
Lecture Outline
I.
The flowering plant life cycle
A. Angiosperms alternate between a conspicuous sporophyte and a reduced
gametophyte
1. Alternation of generation is seen in most plants
a) The haploid gametophyte stage is multicellular and produces gametes
by mitosis
b) The diploid sporophyte stage is multicellular and produces spores by
meiosis
2. In flowering plants, the sporophyte generation is independent; the
gametophyte generation is nutritionally dependent on the sporophyte
B. Flowers are involved in sexual reproduction
1. Flowers are reproductive shoots typically composed of sepals, petals,
stamens, and carpels arranged at the end of a stalk
2. Sepals and petals are sterile modified leaves
a) Sepals protect the floral bud and are known collectively as the calyx
b) Some sepals, such as those on a lily, are almost identical to the petals
c) The petals are often adapted to attract pollinators and are collectively
known as the corolla
3. The stamens are the male reproductive organs, composed of a filament that
supports the anther, where pollen is formed
a) For sexual reproduction to occur, pollen must be transferred to the
female portion of the flower, and the pollen grain grows a pollen tube
to the ovule
4. Carpels are located in the center of the flower and may be single or several
in number
a) The pistil is the term for the female parts of the flower
b) The pistil is composed of the stigma, the style, and one or more carpels
(1) A pistil that consists of a single carpel is a simple pistil
(2) A pistil that consists of fused carpels is a compound pistil
c) Ovules are located in the carpels
C. Female gametophytes are produced in the ovary, male gametophytes in the
anther
1. In each ovule, a megasporocyte divides by meiosis forming four
megaspores
a) Three of the megaspores disintegrate, and the remaining megaspore
divides mitotically to produce the embryo sac, the female
gametophyte
b)
The embryo sac typically contains eight nuclei in seven cells
(1) There are six of these cells, including the egg; each contains one
nucleus
(2) The central cell contains two polar nuclei
2.
Pollen sacs within the anther contain microsporocytes, each of which
divides by meiosis forming four microspores
a) Each microspore divides mitotically to produce a pollen grain, the
immature male gametophyte
b) Pollen grain contains a tube cell and a generative cell
c) The pollen grain becomes mature when the generative cell divides to
form two sperm cells
II.
Pollination
A. Pollination is the transfer of pollen to the stigma of the pistil
1. Self-pollination may occur if the pollen arrives at the stigma of the same
flower, or another flower on the same plant
2. Cross-pollination occurs when the pollen arrives at the stigma of a flower
of another plant of the same species
3. Pollination may be accomplished by animals, wind, or water
B. Many plants have mechanisms to prevent self-pollination
1. Self-pollination results in inbreeding, which is deleterious
2. Some species have separate male and female individuals
3. Some species produce pollen at a time when the stigma is not receptive
C. Flowering plants and their animal pollinators have coevolved
1. Coevolution of animal pollinators and flowering plants has resulted in
flowers that attract or reward the animal
2. Plants pollinated by insects (particularly bees) are often blue or yellow, as
they are able to see colors in that wavelength
a) Insects often can see in the ultraviolet range, so flowers may have
markings in this range
b) Bee’s purple is the ultraviolet color seen by insects
3. Insects have a well-developed sense of smell, so insect-pollinated flowers
are often strongly odorous
4. Flowers pollinated by hummingbirds are typically red, lack odor, and
produce much nectar
5. Bat-pollinated flowers are often white, bloom at night, smell like ripe or
rotten fruit, and produce much nectar
6. Animal pollinators have also adapted, both behaviorally and
morphologically, to the plants on which they feed
D. Some flowering plants depend on wind to disperse pollen
1. Flowers that are wind-pollinated produce many small flowers, little nectar
or odor, but large amounts of pollen
III.
Fertilization is followed by seed and fruit development
A. After pollination, one of the two cells in the pollen grain grows a pollen tube
down the style and into an ovule in the ovary
1.
The other cell in the pollen grain divides, and the two sperm that are
produced travel down to the ovule through the pollen tube
B. A unique double fertilization process occurs in flowering plants
1. One sperm unites with the egg to form the zygote
2. A second sperm unites with the two polar nuclei to form a cell that will
produce the endosperm, which is a (typically) triploid nutritive tissue
3. This double fertilization is known only in angiosperms as Ephedra and
Gnetum
4. Following fertilization, the ovule develops into a seed and the ovary
develops into a fruit
C. Embryonic development in seeds is orderly and predictable
1. The stored energy in seeds may provide nutrition to the germinating plant
embryo; but seeds are also eaten by a variety of animals
2. The first division of the egg establishes polarity of the embryo
a) The basal cell forms the suspensor that anchors the embryo and aids in
nutrient uptake from the endosperm
b) The apical cell develops into the embryo
(1) The proembryo is the chain of cells produced first
(2) The globular embryo is the ball of cells that forms next, and
differentiation begins in this stage
(3) When the cotyledons develop, there are two lobes, and this is
called the heart stage (not seen in monocots)
(4) The torpedo stage is characterized by elongation of the cotyledons
D. The mature seed contains an embryonic plant and storage materials
1. The mature seed is protected by a seed coat derived from the integuments
and is enclosed within a fruit
a) The mature embryo has an embryonic root (radicle), shoot, and one (in
monocots) or two (in dicots) cotyledons (seed leaves)
b) The hypocotyl is a part of the shoot that connects the radicle to the
cotyledons
c) The plumule is the shoot apex above the attachment point of the
cotyledons
2. Seed size is related to survival in a particular environment
a) Plants that grow in widely scattered open sites usually produce small
seeds because they can more easily be dispersed
b) Plants adapted to densely vegetated areas such as forests usually
produce large seeds, which increase the chances for survival in the
shade
E. Fruits are mature, ripened ovaries
1. A fruit contains at least one seed
2. Simple fruits develop from a single pistil, which may be composed of only
one or more than one carpel
a) Simple fruits can be fleshy or dry
b)
A berry is a simple fleshy fruit that has few to many seeds
(1) Tomatos, blueberrys, and grapes are berries
(2) Strawberries, raspberries, and mulberries are not berries
c) A drupe is a simple fleshy fruit with a hard pit covering a single seed
(1) Almonds, avocados, peaches, and olives are drupes
d) Follicles are simple dry fruits that split on one side at maturity
(1) A milkweed pod is a follicle
e) A legume is a simple dry fruit that splits on two sides at maturity
(1) Pea pods and green beans are legumes
f) A capsule is a simple dry fruit that splits on multiple seams at
maturity
(1) Poppy and cotton plants produce capsules
g) Caryopses, or grains, are simple dry fruits that contain a single seed
and do not split at maturity
(1) The seed coat is fused to the fruit wall, and so the caryopsis is
more than just a seed
(2) Corn and wheat produce fruits that are caryopses
h) An achene is the same as a grain, but the seed coat is not fused to the
fruit wall
(1) Sunflowers produce achenes
i) A true nut is a simple dry fruit that has a hard fruit wall that does not
split at maturity
(1) Nuts are relatively large and typically form from a compound
pistil
(2) Chestnuts, acorns, and hazelnuts (filberts) are nuts
(3) Peanuts and Brazil nuts are not nuts
3. Aggregate fruits are formed from one flower with separate carpels, which
fuse at maturity
a) Raspberries and blackberries are aggregate fruits
4. Multiple fruits are formed from carpels from many flowers that fuse at
maturity
a) Pineapples and figs are multiple fruits
5. Accessory fruits are formed by ovary tissue and other parts of the flower
a) Strawberries include the receptacle
b) Apples and pears include the receptacle and part of the calyx
F. Seed dispersal is highly varied
1. Seeds may be dispersed by animals who eat the fruit or who accidentally
carry the fruit distances attached to their fur
2. Some seeds disperse by explosive dehiscence (splitting open)
3. Tumbleweeds disperse seeds by the entire plant rolling through the
countryside
4. Plants that produce winged fruits disperse by the wind
5.
6.
Animals like squirrels and some birds disperse seeds by "squirreling" them
away
Some fruits disperse by water, like the coconut
IV.
Asexual reproduction in flowering plants may involve modified stems, leaves, or
roots
A. A rhizome is an underground horizontal stem
1. Rhizomes may be adapted for starch storage
2. The presence of leaves, nodes, etc. indicates that the rhizome is a stem
3. Branching of rhizomes may result in two distinct plants
B. Tubers are enlarged underground stems that may also break apart in asexual
reproduction
1. Potatoes form tubers
C. Bulbs are short underground buds to which fleshy leaves are attached
1. The leaves store starches or sugars
2. Daughter bulbs may form new plants
3. Lilies and onions form from bulbs
D. Corms are underground stems with papery leaves (scales)
1. Crocuses and cyclamens form corms
2. Lateral buds form new corms that produce new plants
E. Stolons are horizontal, above-ground stems that have long internodes
1. Buds develop that give rise to new plants
2. Strawberries form stolons
F. Some plants produce new plantlets along leaf margins
G. Suckers are aboveground stems that develop from roots and may become an
independent plant
H. Apomixis is the production of seeds without the sexual process
1. An embryo that results from a diploid cell in the ovule is a clone of the
parent plant
2. Apomixis is seen in dandelions, blackberries, citrus trees, and garlic
V.
Sexual and asexual reproduction have different functions
A. Sexual reproduction results in offspring with different combinations of genes
than the parents
1. Sexual reproduction is advantageous in varying environments
2. Asexual reproduction preserves the parental genotype
B. Sexual reproduction has some disadvantages
1. Sexual reproduction requires both male and female flowers, and is often
accompanied by high death rates of the offspring, because of the genetic
variation
Research and Discussion Topics

Investigate the structure of the grains. We rely on the grains heavily for food, and
we process them in various ways. What parts of the seed are removed in the
processing of grains like wheat and rice? How does this affect their nutritive value
to humans or livestock who consume them?

Describe the mechanisms and characteristics of plants that are pollinated by flies,
bees, moths, hummingbirds, and butterflies. What characteristics differentiate
flowers pollinated by various types of pollinators?

Living in freshwater poses a unique problem for plants to disperse their pollen.
What adaptations do freshwater plants have for pollination?

Relatively few higher plants are marine. How do marine plants primarily
reproduce?
Investigate other types of fruits, such as the pepo, samara, schizocarp, silicle, and silique.
What types of plants produce these?