THE LIFE CYCLE OF ANGIOSPERMS

1. THE LIFE CYCLE OF ANGIOSPERMS Packet #75 Chapter #38

2. INTRODUCTION • Angiosperms are the predominant plant species on the planet. • Unlike gymnosperms, these plants produce seeds that have a covering.

3. LIFE CYCLE OF ANGIOSPERM Figure 38.2 (More Detail)Page 772 • In the angiosperm, the sporophyte generation is dominant • Gametophytes are extremely reduced in size and nutritionally dependent on the sporophyte generation • Flowering plants are hetereosporous

4. FIGURE 38.4THE DEVELOPMENT OF ANGIOSPERM GAMEOPHYTES

5. Pollen sac (microsporangium) (a) Development of a male gametophyte (pollen grain) Each one of the microsporangia contains diploid microsporocytes (microspore mother cells). 1 Micro- sporocyte MEIOSIS Micro- spores (4) Each microsporo- cyte divides by meiosis to produce four haploid microspores, each of which develops into a pollen grain. 2 Each of 4 microspores MITOSIS Generative cell (will form 2 sperm) Male Gametophyte (pollen grain) Nucleus of tube cell KEYto labels 20 m A pollen grain becomes a mature male gametophyte when its generative nucleus divides and forms two sperm. This usually occurs after a pollen grain lands on the stigma of a carpel and the pollen tube begins to grow. (See Figure 38.2b.) 3 Ragweedpollengrain Haploid (2n) Diploid (2n) 75 m Figure 38.4a FIGURE 38.4DEVELOPMENT OF POLLEN GRAINS

6. (b) Development of a female gametophyte (embryo sac) Within the ovule’s megasporangium is a large diploid cell called the megasporocyte (megaspore mother cell). 1 Mega- sporangium Ovule Mega- sporocyte MEIOSIS Integuments Micropyle Surviving megaspore Female gametophyte (embryo sac) The megasporocyte divides by meiosis and gives rise to four haploid cells, but in most species only one of these survives as the megaspore. 2 MITOSIS Antipodel Cells (3) Ovule Polar Nuclei (2) Egg (1) Integuments Synergids (2) Three mitotic divisions of the megaspore form the embryo sac, a multicellular female gametophyte. The ovule now consists of the embryo sac along with the surrounding integuments (protective tissue). 3 Key to labels Embryo sac Figure 38.4b Haploid (2n) 100 m Diploid (2n) FIGURE 38.4DEVELOPMENT OF EMBRYO SACS

7. LIFE CYCLE OF ANGIOSPERM • Each microspore develop into a pollen grain. • Immature male gametophytes are contained within pollen grains. • Each pollen grain has two haploid (n) cells. • One of each four megaspores produced during meiosis develops into an embryo sac (female gametophyte). • Embryo sac contains seven cells with eight nuclei • The egg cell and the central cell, which contains two polar nuclei, participate in fertilization.

8. LIFE CYCLE OF ANGIOSPERM • Pollen released from the anther • Pollen is carried to the sticky stigma • Flowers can self-pollinate or cross-pollinate. • Transfer of pollen from flowers of one plant to flowers of another plant is called cross-pollination

9. LIFE CYCLE OF ANGIOSPERM • Pollen grain germinates after it sticks to the stigma. • Pollen grain now contains the mature male gametophyte and extends a tube down through the style. • After it reaches the ovary, the pollen tube penetrates the microphyle and discharges two sperm cells into the female gametophyte (embryo sac) • Results in double fertilization

10. POLLEN TUBE

11. LIFE CYCLE OF ANGIOSPERM • Double Fertilization • Results in the formation of a diploid zygote and triploid endosperm • One sperm nucleus fuses with the egg to form a diploid zygote. • The other sperm nucleus fuses with the two nuclei if the female gametophyte • Cell is now triploid (3n). • Characteristic of flowering plants.

12. FIGURE 38.6DOUBLE FERTILIZATION • Double fertilization ensures the endosperm will develop only in ovules where the egg has been fertilized • Prevents the angiosperms from wasting resources

13. LIFE CYCLE OF ANGIOSPERM • After double fertilization, the ovule matures into a seed. • The zygote develops into the sporophyte embryo with a rudimentary root and either one or two seed leaves (cotyledons—monocots vs. dicots)

14. LIFE CYCLE OF ANGIOSPERM • Seed consists of the embryo, endosperm, sporangium and a seed coat. • An ovary develops into a fruit as its ovules develop into seeds. • Dispersal occurs via wind or animals • Seed germinates if the environmental conditions are favorable. • Seed coat ruptures, the embryo emerges as a seedling and uses the stored food in the endosperm and cotyledons to begin growth.

15. MECHANISMS TO PREVENT SELF-POLLINATION (SELFING!) Chapter 38

16. INTRODUCTION I • Some flowers self-fertilize but most angiosperms have mechanism that prevent “selfing.” • Dioecious plants • Unisexual • Ackee {Barbados}—Ginnup {Jamacia} • Bisexual flowers • Stamens and carpels mature at different times • Stamens and carpels are arranged in a way that makes self-pollination unlikely

17. INTRODUCTION IIBIOCHEMICAL BLOCKING • AKA Self-incompatibility • Most common method that plants use to prevent “selfing” • Some plants recognize their own pollen and that of closely related individuals • Hmmm! • Biochemical block prevents pollen from completing its development and fertilizing the egg

18. REVIEW Imagine that…plants which are more simplistic organisms appear to have more complex reproductive cycles.

19. REVIEW • Students • List potential examination questions and/or here, and on following slides, based on the packet.