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Chapter 9: Flowers and Reproduction (Flower Structure and Cross…
Chapter 9: Flowers and Reproduction
Concepts
Reproduction
Generating new individuals that are genetically different from the parents
Species survive better, than genetically homogeneous species
Producing offspring with identical copies of the parental genes
Seeds
: produced by sexual reproduction, often have a means of long-distance dispersal
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Ex: Bamboo fruits and seed are carried by winds.
Ex; Strawberries are eaten, and the seeds later defecated.
Seeds may become widely scattered and germinate in numerous diverse sites.
Each site differing from the others by:
soil conditions
exposure to predators
microclimate
exposure to pathogens
New plants produced asexually are usually not capable of long-distance dispersal.
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Examples that become established in new plants by the same microhabitat as their parent:
rhizomes
plantlets
runners
Fruit Types and Seed Dispersal
True Fruits and Accessory Fruits
simple fruit
: due to the fruit developing from a single ovary or the fused ovaries of one flower
aggregrate fruit
: results if the separate carpels of one gynoecium fuse during development; such as raspberries
accessory fruit
: (false fruit) used if any nonovarian tissue is present
multiple fruit
: if during development all of the individual fruits of an inflorescence fuse into one fruit
true fruit
: used to refer to fruits containing only ovarian tissue
Apples develop from inferior ovaries
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The bulk of fruit is enlarged bases of sepals and petals.
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Only the innermost part is true fruit derived from carpels.
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Classification of Fruit Types
In one method, emphasis is placed on whether the fruit is
dry
or
fleshy
.
A dry fruit is one that is not typically eaten by the natural seed-distributing animals.
A further classification of dry fruits emphasizes fruit opening:
Dehiscent fruits
break open and release seeds
Whereas
indehiscent fruits
do not.
During germination, the embryo absorbs water, swells, and bursts the weak fruit.
Those fruits are caryopses.
Fleshy fruits are those that are eaten during the natural seed distribution process.
For the most part, fleshy are indehiscent.
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Inflorescences and Pollination
If many flowers are grouped together, an
inflorescence
, they give a collective visual signal to pollinators:
One small flower may be overlooked, but not a hundred close together.
In an inflorescence, the plant is able to control accurately the timing of the...
maturation
opening of the flowers
initiation
When grouped into inflorescences, two basic arrangements occur:
determinate inflorescences
Has only a limited potential for growth because the inflorescence apex is converted to a flower.
Ends its possibilities for continued growth.
Typically, but not always, the terminal flower opens first
Then the lower ones open successively.
In the simplest type, below the terminal flower is a bract with an axillary.
Also may have a bract and axillary flower, and so on.
indeterminate inflorescences
The lowest or outermost flowers open first.
Even while these flowers are open, new flowers are still being initiated at the apex.
Flower Structure and Cross-Pollination
Monoecious and Dioecious Species
essential organs
: they produce the critically important spores
If either organ is absent, sexual reproduction is dramatically affected.
Flowers that lack either or both essential organs are not only incomplete but also
imperfect flowers
.
If a flower has both, it is a
perfect flower
even though it may lack either sepals or petals or both.
Sepals and petals do not produce spores and are considered
nonessential organs
.
A species must have both types of organs.
Carpels involved in egg production.
Stamens produce pollen that results in sperm production.
Plants that have perfect flowers satisfy this requirement.
A large number of combinations exist:
dioecy
: a species may have individuals that produce only staminate flowers and others that produce only carpellate flowers
Species (not the flower or the plant) said to be
dioecious
Ex: Marijuana, dates, willows, and papaya
Life cycle consists of four types of plants:
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microgametophytes
megagametophytes
staminate sporophytes
carpellate sporophytes
Dioecy is an extreme adaptation that ensures cross-pollination
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A plant that produces only one type of spore cannot pollinate itself.
monoecy
: the condition of having staminate flowers located on the same plant as carpellate flowers
Monoecious
examples:
cattails
corn
ears are clusters of fertilized carpellate flowers
tassels bear numerous staminate flowers
Animal-Pollinated Flowers
Insects probably visited all kinds of flowers, not recognizing the different species.
As a result, pollen often landed on the stigma of the wrong species, where it is useless.
Mutations in plants that increased a plant's distinctiveness, it's recognizably by an insect.
Became selectively advantageous
Rewards for insect
Sugary nectar
Protein rich-pollen and stamen
Ability to recognize the most nutritious flowers from a distance while flying.
As a result, many lines of insects and flowers underwent
coevolution
.
A flower becoming adapted for visitation by a particular insect and the insect for efficient exploitation of the flower.
Has also occurred between flowers and birds and between flowers and bats.
Ex: flower color, size, shape, fragrances, and so on
Shape of the flower is particularly important, as a pollinator actually makes contact with it.
Most flowers are radially symmetrical
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These flowers (and stems and roots) are
actinomorphic
or
regular
.
All insects, birds, and bats are bilaterally symmetrical.
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In many species, flowers and pollinators have coevolved in such a way that the flowers are now also bilaterally symmetrical--
zygomorphic
.
Stigma and Pollen Incompatibility
compatibility barriers
: chemical reactions between pollen and carpels that prevent pollen growth
Incompatibility system
EX: A pollen tube grows, the stigma and style test proteins on the tube's surface.
If one of these proteins is produced by a gene that matches an incompatibility gene in the carpel-bearing plant, the stigma and style block any further growth of the pollen tube.
In self-pollination, all pollen tubes genes match those of the stigma and style, and blocking occurs:
The plant cannot pollinate itself.
Wind-Pollinated Flowers
A totally distinct set of modifications is adaptive.
Mutations that prevent the formation of petals are selectively advantageous.
Energy saved can be used elsewhere in plant.
Sepals are often reduced or absent, and the ovaries need no special protection.
The entire flower may be tiny.
Zygomorphy provides no selective advantage.
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Pollen is released to the wind.
The chance of any particular pollen grain landing on a compatible stigma is small.
Large, feathery stigmas are adaptive by increasing the area that can catch pollen grains.
Huge numbers of grains must be produced.
Produce up to several thousand small flowers
Stamen and Style Maturation Times
Self-fertilization in flowers that have both stamens and carpels is prevented if anthers and stigmas mature at different times.
In many species, anthers release pollen while stigma tissues are immature and unreceptive
The style may not have elongated yet.
Stigmas may be near the base of the flower.
Anthers are at the top, elevated by elongated filaments.
Exposed pollen lives only briefly
Susceptible to desiccation in dry air
Damage to its DNA by ultraviolet light
When the stigma and style become mature, there may be no living pollen left in the flower.
All pollination is effected by younger flowers just opening their anthers.
Not a very effective means of ensuring cross-pollination.
Ovary Position
Ovaries and ovules must be well protected from pollinators.
Paradoxically, a flower must bring a hungry animal to within millimeters of protein-rich ovules in order to effect pollination.
Adaptations that maximize the separation are long styles and stamen filaments.
Burying the ovaries deep within the flower provides further protection.
inferior ovary
: an ovary located below the sepals, petals, and stamens; those appendages are
epigynous
superior ovary
: an ovary located above the sepals, petals, and stamens; those appendages are
hypogynous
half-inferior
: an ovary that is partly inferior, such that the sepals, petals, and stamens appear attached to its side; those appendages are
perigynous
Cross-Pollination
self-pollination
: pollination of a carpel by pollen from the same flower or another flower
cross-pollination
: the pollination of a carpel by pollen from a different individual
Sexual Reproduction
Embryo and Seed Development
Gametophytes
Microgametophyte
The pollen grains of gymnosperms and angiosperms.
The male gametophyte that develops from the microspores of heterosporous plants.
Megagametophyte
In angiosperms, the megagametophyte is the embryo sac.
The female gametophyte that arises from a megaspore of a heterosporous plant.
Flower Structure
Petals
Stamens
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Sepals
Carpels
Fruit Development
Plant Life Cycle
Fertilization
Asexual Reproduction
Fragmentation
: a large spreading of vining plant grows to several meters in length, and individual parts become self-sufficent by establishing adventitious roots
If middle portions of the plant die, the ends become separated and act as individuals.
Certain modifications improve the efficiency of fragmentation.
Ex: cacti
Branches poorly attached to trunk
Plant breaks apart easily
Parts form roots and become independent
Ex: members of saxifrage, grass, and pineapple families
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Plantlets formed where flowers would be expected.
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Look like small bulbs and called bulbils.
Ex: willows and many thistles
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Adventitious buds
Form on roots, grow into plants
May grow out even while the parent plant is still alive
A small cluster of trees may in fact consist of just a single individual.
A grove of aspens covers several acres in Utah and have been discovered to be a single plant.
