flowers ii modified flowers and sexual reproduction
TRANSCRIPT
Flowers II
Modified Flowers and Sexual Reproduction
Bracts
Additional floral structures outside the calyx
May be leaflike or petal-like
The showy white or pink "petals" of dogwood are bracts
Tepals
Sepals are brightly colored and identical to the petals
Complete and Incomplete flowers
Flowers containing all four floral appendages are known as complete flowers
Flowers lacking any of the four floral structures are known as incomplete flowers
Flowers of grasses possess neither sepals nor petals
Perfect and Imperfect Flowers
Perfect flowers have both stamens and carpels
Imperfect flowers lack either stamens or carpels
*Unisexual flowers*Male flowers called staminate*Female flowers called pistillate or
carpellate
Plants with Unisexual Flowers
Monoecious plants have both male and female flowers on a single individual
Dioecious plants are either male or female with only unisexual flowers on a single individual
Ovary Position
Superior ovary - sepals, petals, and stamens are inserted beneath the ovary
Inferior ovary - sepals, petals and stamens are inserted above the ovary
Flower symmetry
Regular flower displays radial symmetry
Irregular flowers display bilateral symmetry
Inflorescence
Flowers grouped in clustersSometimes what appears as a single
flower is actually an inflorescence Sunflower, daisies, and dogwood flower
common examplesThe arrangement of flowers in the
inflorescence varied with many patterns possible: spike, umbel, head, and catkin
Inflorescence Types
Spike Umbel Catkin Head
Sexual Reproductionin Flowers
Meiosis occurs:
Stamens in pollen chambers of anther
Carpels in developing ovules in ovary
Pollen development -1
Microspore mother cells become distinct in the pollen chambers
Each MMC undergoes meiosis to produce 4 microspores
Each microspore develops into a pollen grain, the male gametophyte
Pollen chamber in anther
Pollen development - 2
Microspore nucleus undergoes mitosis to produce
generative nucleus tube nucleus
Microspore wall modified into pollen wall
When mature pollen are released from the anthers
Pollen wall
Intine - inner layerExine - outer layerExine may be ornamented with
spines, ridges, or pores
Pollen - Male gametophyte
Tube nucleus
Generative nucleus
Exine
Intine
Pollen Types
Ragweed Thistle Oak
Ovule
One or more ovules develop within the ovary
Ovule is surrounded by integumentsOpening in integuments is known as
the micropyle
Ovule development - 1
One cell becomes distinct as a megaspore mother cell
The megaspore mother cell undergoes meiosis to produce four megaspores
Three degenerate leaving one surviving megaspore
Ovule development - 2
Surviving megaspore undergoes three mitotic divisions to producing 8 nuclei
These 8 nuclei are distributed with 3 near the micropyle end of the ovule, 3 at the opposite end and 2 (polar nuclei) in the center
One of the nuclei at the micropyle end is the egg
This mature female gametophyte is often called the embryo sac
Ovule - Female Gametophyte
Polar nuclei
Integuments
Egg
Micropyle
Pollination
Transfer of pollen from the anther to the stigma
Self-pollination - same flowerCross- pollination - from one flower
to anotherPollen transfer occurs mainly by
animals or wind
Animal Pollinated Flowers
Flowers brightly colored and fragrant
Essential oils attract Nectar producedColor patterns may
be nectar guidesPollen larger, sticky,
and not abundant
Wind Pollinated Flowers
Flowers small and inconspicuous often an inflorescence
Often lacking sepals and petals; no nectar
Pollen small, dry, light, and abundant One ragweed plant
can release one billion pollen grains (1 million tons/yr in NA)
StigmaStigma
OvaryOvary
POLLEN
Study of pollen called palynology has applications in many diverse fields: petroleum geology, anthropology, archeology, criminology, and medicine
When pollen is released by wind-pollinated plants, only a very tiny percentage reaches the stigma - remainder settles back to earth.
Pollen tube growth
Pollen grain germinates on compatible stigma
Pollen tube begins growing down into the style towards the ovary
Generative nucleus divides mitotically producing two non-motile sperm.
Pollen tube continues to grow until it reaches the micropyle of an ovule
Pollen tube growth
Pollen
Pollen tube
Ovary
Ovule
Pollen tube growth
Pollen
Pollen tube
Ovary
Ovule
**
Sperm
Double fertilization
A distinctive feature of angiosperms Both sperm involved in fertilization. One sperm fertilizes the egg to
produce a zygote Second sperm fuses with the two
polar nuclei producing the primary endosperm nucleus which develops into endosperm
Fertilization
Polar nuclei
Egg
* *
Pollen tubeSperm
Double fertilization
Sperm + Egg -----> Zygote
Sperm + 2 polar nuclei ------> Primary
Endosperm Nucleus
Endosperm
A nutritive tissue for the developing embryo.
Major food source for the human population
Endosperm reserves in wheat, rice, and corn are especially important food sources
Following fertilization
Sepals, petals, and stamens drop off Ovary greatly expands becoming a
fruitEach fertilized ovule becomes a
seedInteguments of the ovule develop
into the seed coat
Summary
1. Meiosis reduces the number of chromosomes from diploid to haploid
2. The flower is the unique reproductive structure of angiosperms
3. Pollination is the transfer of pollen from anther to stigma occurring through the action of wind or animals
4. In angiosperms reproduction is accomplished through the process of double fertilization.