Flowers II

1. Flowers II Modified Flowers and Sexual Reproduction

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

3. Tepals • Sepals are brightly colored and identical to the petals

4. Complete and Incomplete flowers • Flowers containing all four floral appendages are known ascompleteflowers • Flowers lacking any of the four floral structures are known as incompleteflowers • Flowers of grasses possess neither sepals nor petals

5. Perfect and Imperfect Flowers • Perfect flowershave both stamens and carpels • Imperfect flowerslack either stamens or carpels • Unisexual flowers • Male flowers called staminate • Female flowers called pistillate or carpellate

6. Plants with Unisexual Flowers • Monoecious plantshave both male and female flowers on a single individual • Dioecious plantsare either male or female with only unisexual flowers on a single individual

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

8. Flower symmetry • Regular flowerdisplays radial symmetry • Irregular flowers display bilateral symmetry

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

10. Inflorescence Types Spike Umbel Catkin Head

11. Sexual Reproductionin Flowers

12. Meiosis occurs: • Stamens • in pollen chambers of anther • Carpels • in developing ovules in ovary

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

14. Pollen chamber in anther

15. 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

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

17. Pollen - Male gametophyte Tube nucleus Exine Intine Generative nucleus

18. Pollen Types Ragweed Thistle Oak

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

20. 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

21. 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

22. Ovule - Female Gametophyte Polar nuclei Integuments Egg Micropyle

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

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

25. 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) Stigma Ovary

26. 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.

27. 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

28. Pollen tube growth Pollen Pollen tube Ovary Ovule

29. Pollen tube growth Pollen Pollen tube Sperm * * Ovary Ovule

30. 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

31. Fertilization Polar nuclei Egg Pollen tube Sperm * *

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

33. 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

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

35. 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.