Flowers II Modified Flowers and Sexual Reproduction

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.