Flowers II Modified Flowers and Sexual Reproduction Bracts

Flowers II Modified Flowers and Sexual Reproduction

Bracts z Additional floral structures outside the calyx z May be leaflike or petal-like z The showy white or pink "petals" of dogwood are bracts

Tepals z Sepals are brightly colored and identical to the petals

Complete and Incomplete flowers z. Flowers containing all four floral appendages are known as complete flowers z. Flowers lacking any of the four floral structures are known as incomplete flowers z. Flowers of grasses possess neither sepals nor petals

Perfect and Imperfect Flowers z. Perfect flowers have both stamens and carpels z. Imperfect flowers lack either stamens or carpels * Unisexual flowers * Male flowers called staminate * Female flowers called pistillate or carpellate

Plants with Unisexual Flowers z. Monoecious plants have both male and female flowers on a single individual z. Dioecious plants are either male or female with only unisexual flowers on a single individual

Ovary Position z. Superior ovary - sepals, petals, and stamens are inserted beneath the ovary z. Inferior ovary - sepals, petals and stamens are inserted above the ovary

Flower symmetry z. Regular flower displays radial symmetry z. Irregular flowers display bilateral symmetry

Inflorescence z. Flowers grouped in clusters z. Sometimes what appears as a single flower is actually an inflorescence z. Sunflower, daisies, and dogwood flower common examples z. The arrangement of flowers in the inflorescence varied with many patterns possible: spike, umbel, head, and catkin

Inflorescence Types Spike Umbel Catkin Head

Sexual Reproduction in Flowers

Meiosis occurs: z. Stamens y in pollen chambers of anther z. Carpels y in developing ovules in ovary

Pollen development -1 z. Microspore mother cells become distinct in the pollen chambers z. Each MMC undergoes meiosis to produce 4 microspores z. Each microspore develops into a pollen grain, the male gametophyte

Pollen chamber in anther

Pollen development - 2 z. Microspore nucleus undergoes mitosis to produce xgenerative nucleus xtube nucleus z. Microspore wall modified into pollen wall z. When mature pollen are released from the anthers

Pollen wall z. Intine - inner layer z. Exine - outer layer z. Exine may be ornamented with spines, ridges, or pores

Pollen - Male gametophyte Tube nucleus Exine Intine Generative nucleus

Pollen Types Ragweed Thistle Oak

Ovule z. One or more ovules develop within the ovary z. Ovule is surrounded by integuments z. Opening in integuments is known as the micropyle

Ovule development - 1 z. One cell becomes distinct as a megaspore mother cell z. The megaspore mother cell undergoes meiosis to produce four megaspores z. Three degenerate leaving one surviving megaspore

Ovule development - 2 z. Surviving megaspore undergoes three mitotic divisions to producing 8 nuclei z. 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 z. One of the nuclei at the micropyle end is the egg z. This mature female gametophyte is often called the embryo sac

Ovule - Female Gametophyte Polar nuclei Integuments Egg Micropyle

Pollination z. Transfer of pollen from the anther to the stigma z. Self-pollination - same flower z. Cross- pollination - from one flower to another z. Pollen transfer occurs mainly by animals or wind

Animal Pollinated Flowers z Flowers brightly colored and fragrant z Essential oils attract z Nectar produced z Color patterns may be nectar guides z Pollen larger, sticky, and not abundant

Wind Pollinated Flowers z Flowers small and inconspicuous often an inflorescence z Often lacking sepals and petals; no nectar Stigma z Pollen small, dry, light, and abundant Ovary y. One ragweed plant can release one billion pollen grains (1 million tons/yr in NA)

POLLEN z. Study of pollen called palynology has applications in many diverse fields: petroleum geology, anthropology, archeology, criminology, and medicine z. When pollen is released by wind-pollinated plants, only a very tiny percentage reaches the stigma - remainder settles back to earth.

Pollen tube growth z. Pollen grain germinates on compatible stigma z. Pollen tube begins growing down into the style towards the ovary z. Generative nucleus divides mitotically producing two non-motile sperm. z. Pollen tube continues to grow until it reaches the micropyle of an ovule

Pollen tube growth Pollen tube Ovary Ovule

Pollen tube growth Pollen Sperm * * Pollen tube Ovary Ovule

Double fertilization z. A distinctive feature of angiosperms z. Both sperm involved in fertilization. z. One sperm fertilizes the egg to produce a zygote z Second sperm fuses with the two polar nuclei producing the primary endosperm nucleus which develops into endosperm

Fertilization Polar nuclei Egg Pollen tube Sperm * *

Double fertilization z. Sperm + Egg -----> Zygote z. Sperm + 2 polar nuclei ------> Primary Endosperm Nucleus

Endosperm z. A nutritive tissue for the developing embryo. z. Major food source for the human population z. Endosperm reserves in wheat, rice, and corn are especially important food sources

Following fertilization z. Sepals, petals, and stamens drop off z. Ovary greatly expands becoming a fruit z. Each fertilized ovule becomes a seed z. Integuments of the ovule develop into the seed coat

Summary z 1. Meiosis reduces the number of chromosomes from diploid to haploid z 2. The flower is the unique reproductive structure of angiosperms z 3. Pollination is the transfer of pollen from anther to stigma occurring through the action of wind or animals z 4. In angiosperms reproduction is accomplished through the process of double fertilization.